Interdisciplinary Seminar in Nonlinear Science Archive

Date: June 02, 2006 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: White: Mechanisms and Consequences of Coherent Activity... Description: Interdisciplinary Seminar in Nonlinear Science Title: Mechanisms and Consequences of Coherent Activity in the Hippocampal Formation Speaker: Prof. John White, Boston University Special Note: Refreshments will be served prior to the talk in Tech M416. Abstract: The hippocampal formation is crucial for remembering episodes in one’s life, and evidence suggests that synchronous activity throughout the hippocampus is essential for the mnemonic functions of this brain structure. We have studied the mechanisms of synchronization using electrophysiological and computational methods. More recently, we have exploited methods for introducing real-time control in cellular electrophysiology. These techniques allow us to “knock in” virtual ion channels that can be controlled with great mathematical precision, and to immerse biological neurons in real-time, virtual neuronal networks. These manipulations allow us to test computationally-based hypotheses in living cells. From this work, I will discuss which properties of single cells seem crucial for coherent activity in the hippocampal formation. I will also discuss work on the consequences of precise spike timing in neuronal network function. Selected related papers: 1. Netoff TI, Banks MI, Dorval AD, Acker CD, Haas JS, Kopell N, and White JA (2005) Synchronization in hybrid neuronal networks of the hippocampal formation. Journal of Neurophysiology 93: 1197-1208. 2. Dorval AD and White JA (2005) Channel noise is essential for perithreshold oscillations in entorhinal stellate neurons. Journal of Neuroscience 25: 10025-10028. 3. Zhou YD, Acker CD, Netoff TI, Sen K, and White JA (2005) Increasing calcium transients by broadening postsynaptic action potentials enhances timing-dependent synaptic depression. Proceedings of the National Academy of Sciences USA 102: 19121-19125. Contact: Mary Catsicopoulos 847/491-5586 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 23, 2006 Time: 4:00 PM - 5:00 PM Location: Tech M416 See the Evanston Campus Map Title: Childress: Paired vortices, voricity growth, and hovering... Description: Interdisciplinary Seminar in Nonlinear Science Title: Paired vortices, voricity growth, and hovering flight Speaker: Prof. Stephen Childress, Courant Institute, NYU Special Note: Refreshments will be provided at 3:30pm. This even is co-sponsored by the ESAM Department. Abstract: . Contact: Mary Catsicopoulos 847/491-5586 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 22, 2006 Time: 4:00 PM - 5:00 PM Location: Tech M416 See the Evanston Campus Map Title: Childress: Flapping flight as bifurcation in Reynolds number Description: Interdisciplinary Seminar in Nonlinear Science Title: Flapping flight as bifurcation in Reynolds number Speaker: Prof. Stephen Childress, Courant Institute, NYU Special Note: A reception will follow the talk at 5pm. This even is co-sponsored by the ESAM Dept. Abstract: . Contact: Mary Catsicopoulos 847/491-5586 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 19, 2006 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Duplantier: Brownian Motion Description: Interdisciplinary Seminar in Nonlinear Science Title: Brownian Motion Speaker: Prof. Bertrand Duplantier, Theoretical Physics, CEA- Saclay Special Note: Refreshments will be served prior to the talk in TECH M416. Abstract: This talk will give a historical perspective on Brownian motion. Einstein derived in 1905 the diffusion coefficient of Brownian particles suspended in a liquid, actually after a similar derivation by William Sutherland in Melbourne. Later approaches by Einstein include an unpublished one recently discovered that I shall present. Experimental confirmation of the theory came with Perrin, meanwhile Langevin invented stochastic calculus. In mathematics, the Wiener measure on continuous random paths provided a first rigorous approach (with Bachelier in 1900!), as well as the first functional integral! Some present-day applications of Brownian motion to physics and biology will also be discussed. Contact: Mary Catsicopoulos 847/491-5586 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 12, 2006 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Reichhardt: Boolean models for genetic regularity networks Description: Interdisciplinary Seminar in Nonlinear Science Title: Boolean models for genetic regularity networks Speaker: Dr. Cynthia Reichhardt, Los Alamos National Laboratory Special Note: Refreshments will be served prior to the talk in Tech M426. Abstract: The genetic regulatory networks responsible for cell differentiation and cell cycle control can be modeled by Boolean logic networks of varying connectivity, in which each node in the network represents a gene. We describe the underlying motivation for these models, and propose a new method for categorizing the strategy functions assigned to each node based on symmetry properties of Ising hypercubes. Under biologically relevant conditions, interactions between nodes strongly restrict the available phase space of the network, implying that the experimentally observed conservation of wild-type phenotype may arise automatically due to the combinatoric properties of the underlying genetic network. Contact: Mary Catsicopoulos 847/491-5586 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 05, 2006 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Wang: Efficiency of flapping flight Description: Interdisciplinary Seminar in Nonlinear Science Title: Efficiency of flapping flight Speaker: Prof. Jane Wang, Cornell Special Note: Refreshments will be served prior to the talk in Tech M416. Abstract: Flapping and fixed wing provide two means to fly. Birds and insects have evolved to employ the first method and we have succeeded in using the second. We do not know, however, which one is more efficient. In this talk, I will define and discuss the aerodynamic efficiency of flapping flight both in simple models and in insect flight. Contact: Mary Catsicopoulos 847/491-5586 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 28, 2006 Time: 4:00 PM - 5:00 PM Location: Tech M416 See the Evanston Campus Map Title: Huber: Dominos, first-passage problems, and dynamic... Description: Interdisciplinary Seminar in Nonlinear Science Title: Dominos, first-passage problems, and dynamic binding in flagellar motor statistics Speaker: Prof. Greg Huber, University of Connecticut Special Note: Refreshments will be served prior to the talk in Tech M416. Abstract: The flagellar motor of bacteria is perhaps the most elaborate stoichiometric molecular machine engineered by evolution. It harnesses the chemical energy associated with a proton gradient for its rotation (typically hundreds of Hz), while, at longer time scales, stochastically switching between clockwise (CW) and counterclockwise (CCW) rotations. And while the switching of the rotational direction of the flagellar motor plays a crucial role in bacterial chemotaxis, its mechanism remains poorly understood. Here, I'll present recent work on the switch statistics of the flagellar motor of Caulobacter crescentus. I'll show how the interval distribution of the CW and CCW rotation provides insight into the switching, and describe a model that combines first-passage-time theory and dynamic binding to explain the switching mechanism. The model accounts for the motor's dependence on CheYp concentration, and predicts that dynamic binding of CheYp to the switch-complex proteins is the key to the signal amplification of the motor. Contact: Mary Catsicopoulos 847/491-5586 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 14, 2006 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Dobson: Towards Estimating the Risk of Cascading Failure... Description: Interdisciplinary Seminar in Nonlinear Science Title: Towards Estimating the Risk of Cascading Failure Blackouts Speaker: Prof. Ian Dobson, University of Wisconsin - Madison Special Note: Refreshments will be served prior to the talk in M416. Abstract: Large blackouts are usually caused by a complicated sequence of cascading failures. Initial failures weaken the system and cause further failures, which further weaken the system and so on. A spectacular example is the August 2003 blackout in North America that disconnected electric power to 50 million people at a cost estimated in billions of dollars. How can we quantify the risk of such a complicated event on a controlled network with a huge number and variety of components? Probabilistic branching process models of cascading failure and methods of assessing the risk of cascading failure are emerging. In particular, I will discuss methods to estimate the extent to which failures propagate and the probability distribution of blackout sizes based on data from simulations of cascading failure. There is evidence that there is a critical transition in blackout risk as the electric power grid becomes more stressed and the methods also quantify the nearness to this critical transition. The overall research goal is a theory and metrics for cascading failure that can be applied to monitor the risk of blackouts. This is joint work with Kevin Wierzbicki at the University of Wisconsin, Ben Carreras at Oak Ridge National Laboratory, Tennessee and David Newman at the University of Alaska-Fairbanks. Contact: Mary Catsicopoulos 847/491-5586 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 07, 2006 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Segre': Learning about the evolution of biochemical... Description: Interdisciplinary Seminar in Nonlinear Science Title: Learning about the evolution of biochemical networks from topology, fluxes and perturbations Speaker: Professor Daniel Segre', Boston University Special Note: Refreshments will be served prior to the talk in Tech M416. Abstract: We are interested in the evolutionary dynamics of biological networks, in particular in the interplay between response to genetic and environmental perturbations, genomic-level functional organization, and optimal adaptation. By implementing steady state (flux balance) models of whole-cell biochemical networks, and computing epistatic interactions resulting from double gene deletions, one can make new hypothesis about the organization of genes and pathways into hierarchical modules. To go beyond single organisms, and learn about metabolism at the ecosystem level, different approaches may be required. By relying on reaction topology and a network expansion algorithm it is possible to study the effect of key molecules (such as oxygen) on the evolution of life. For more information, see http://prelude.bu.edu . Contact: Mary Catsicopoulos 847/491-5586 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: March 31, 2006 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Wasserman: Statistical Analysis for Network Science Description: Interdisciplinary Seminar in Nonlinear Science Title: Statistical Analysis for Network Science Speaker: Prof. Stanley Wasserman, Indiana University Special Note: Refreshments will be served prior to the talk in M416. Abstract: This talk highlights the wide range of statistical analyses that are part of network science. Of particular importance are the exponential family of random graph distributions, known as p*, and recent work on robustness and resistance of network data when actors and/or relational ties are missing or removed. Contact: Mary Catsicopoulos 847/491-5586 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: March 10, 2006 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Schatz: Homological Characterization of Spiral Defect Chaos Description: Interdisciplinary Seminar in Nonlinear Science Title: Homological Characterization of Spiral Defect Chaos Speaker: Prof. Michael F. Schatz, Center for Nonlinear Science and School of Physics, Georgia Tech Special Note: Refreshments will be served prior to the talk in Tech M416. Abstract: Algebraic topology (homology) is used to analyze the weakly turbulent state of spiral defect chaos in both laboratory experiments and numerical simulations of Rayleigh-Benard convection. The analysis reveals topological asymmetries that arise when non-Boussinesq effects are present. The asymmetries are found in different flow fields in the simulations and are robust to substantial alterations to flow visualization conditions in the experiment. However, the asymmetries are observable using conventional statistical measures. These results suggest homology may provide a new and general approach for connecting spatio-temporal observations of chaotic or turbulent patterns to theoretical models. Contact: Mary Catsicopoulos 847/491-5586 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 17, 2006 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Guimera: Modules and roles: towards a cartography of... Description: Interdisciplinary Seminar in Nonlinear Science Title: Modules and roles: towards a cartography of complex networks Speaker: Dr. Roger Guimera, Northwestern University Special Note: Refreshments will be served prior to the talk in M416. Abstract: In complex systems, individual components interact with each other giving rise to complex networks, which are neither totally regular or totally random. Because of the interplay between network topology and dynamics, it is crucial to characterize the structure of complex networks. The focus of most research on complex networks has been on global network properties. While global properties may sometimes provide useful insights, their relevance hinges strongly on the homogeneity of the networks. However, most real world networks display a marked modular structure, which means that, rather than being homogeneous in their connectivity, nodes tend to establish many more connections with a subset of the nodes in the network than with the remaining nodes. In my talk, I will discuss recent theoretical and computational developments that enable one to uncover the modular structure of complex networks. I will also discuss how, after identifying network modules, one can classify nodes into roles according to their pattern of intra- and inter-module connections. Finally, I will show that understanding the modular structure of biochemical networks sheds light onto their evolution, and has potential applications for the identification of drug targets. Contact: Mary Catsicopoulos 847/491-5586 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 10, 2006 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Dodds: Models of Social and Biological Contagion Description: Interdisciplinary Seminar in Nonlinear Science Title: Models of Social and Biological Contagion Speaker: Dr. P. S. Dodds, Columbia University Special Note: Refreshments will be served prior to the talk in Tech M416. Abstract: I will first discuss extensions of simple models of disease and social influence spreading that we have developed in order to address (1) epidemic size distributions, (2) resurgence of caseload number in epidemics, (3) the effect network structure has on the spread of social influence, and (4) how different response functions affect spread including those leading to chaotic behavior. I will then present a model of contagion that unifies and generalizes existing models of the spread of social influences and infectious diseases. Our generalized model incorporates individual memory of exposure to a contagious entity (e.g., a rumor or disease), variable magnitudes of exposure (dose sizes), and heterogeneity in the susceptibility of individuals. Through analysis of our model, we identify three basic classes of contagion models which we call epidemic threshold, vanishing critical mass, and critical mass respectively. Finally, I will discuss results from Musiclab, from our latest online sociological experiment, which systematically explores how people follow each other in making decisions regarding cultural objects. Contact: Mary Catsicopoulos 847-491-5586 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 03, 2006 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Feres: Random billiards and diffusion Description: Interdisciplinary Seminar in Nonlinear Science Title: Random billiards and diffusion Speaker: Prof. Renato Feres, Washington University, St. Louis Special Note: Refreshments will be served prior to the talk in Tech M416. Abstract: We introduce a class of one-dimensional random dynamical systems derived from billiard maps and study certain random walks on the real line derived from them. We then discuss the interplay between the billiard geometry and stochastic properties of the random system, as well as spectral properties of the associated Markov (scattering) operator. Based on numerical experiments, we also propose a conjectural description of how the billiard geometry affects the diffusion process obtained from the random walk under appropriate scaling limit. Contact: Mary Catsicopoulos 847/491-5586 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: January 06, 2006 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Tang: On Function and Form Description: Interdisciplinary Seminar in Nonlinear Science Title: On Function and Form Speaker: Prof. Chao Tang, UCSF Special Note: Refreshments will be served prior to the talk in Tech M416. Abstract: n the biological world, intimate relations between function and form are well established on the macroscopic and the microscopic scales. Macroscopically, vivid examples are almost everywhere we open our eyes: from the aerodynamic shapes of marine animals, the beautiful colors of flowers, to the extremely diverse life forms to fill in every niche on the planet. Microscopically, we have witnessed the great triumph of the structural biology in the past decades. There is, however, a big gap in between-on the mesoscopic scales. On these scales, to what extend the function of a system (e.g. signal transduction, pattern formation, cell polarization, homeostasis, cell division, bacteria stress response, etc.) and the organization of its parts are related? In this talk, I will present a case study of the segmentation polarity gene network in Drosophila. The function of this network is to stabilize the segmentation pattern of gene expression during the development. We found that although there are numerous networks which can perform the function, the requirement for the function to be robust severely constrains the network's topology. The network selected by nature is among the most robust topological classes. Furthermore, I will show that the knowledge of viable topologies can be used to help identify missing links in the network. A comprehensive understanding of the relationship between function and topology may shed light in the engineering of novel systems. Contact: Mary Catsicopoulos 847/491-5586 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: December 02, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Kanso: An Articulated Fish Swimming with Point Vortices Description: Interdisciplinary Seminar in Nonlinear Science Title: An Articulated Fish Swimming with Point Vortices Speaker: Dr. Eva Kanso, University of Southern California Special Note: Refreshments will be served prior to the seminar in Tech M416. Abstract: Fish and cetaceans move in water with great agility through rhythmic unsteady motions of their body, fins and tail, and it is widely believed based on experimental evidence that fish exploit the unsteadiness in the flow to their advantage. Recently, we considered the problem of swimming in potential flow in an attempt to discern the fundamental principles of fish locomotion. Under these idealized conditions, that is, (in the absence of a vortex shedding mechanism}, we showed that a fish modeled as an articulated system of rigid links can propel and steer itself by only controlling its shape variables (the allowable relative rotations between the links). This result is important because, contrary to a common belief, it demonstrates that the forces and moments applied on the fish body by shed vortices are {not solely} responsible for the net locomotion. The net locomotion in potential flow occurs due to the transfer of momentum between the articulated body and the fluid. We are currently extending this reduced formulation to models that capture the complex interaction with point vortices so as to unravel the role of vorticity in aquatic locomotion and how fish exploit the presence of vortices to their locomotory advantage. Contact: Mary Catsicopoulos 847/491-5586 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: November 18, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Worster: The interior of mushy layers revealed Description: Interdisciplinary Seminar in Nonlinear Science Title: The interior of mushy layers revealed Speaker: Prof. M. Grae Worster, Cambridge, UK Special Note: Refreshments will be served prior to the seminar in Tech M416. Abstract: Mushy layers are two-phase, two-component, reactive porous media that occur commonly during the solidification of multi-component liquids, such as sea water, to form sea ice, or metallic alloys during casting. As melt percolates through the mushy layer, driven by buoyancy forces and transporting heat and solute, the solid phase of the medium evolves by dissolution or precipitation, which in turn changes the permeability of the medium and affects the flow. In particular, fluid-filled channels (chimneys) can form by dissolution and focus the flow into narrow regions. Our understanding of the formation, evolution and consequence of such channels has advanced significantly over the past couple of decades by a combination of laboratory experiments and applied mathematical modelling. Visual observation of the dynamic evolution of these systems has been of utmost importance in these developments, and has recently been augmented by using Magnetic Resonance Imaging (MRI) to reveal the internal microstructure as it changes in time. I shall describe both the theoretical and experimental studies of mushy layers and explain how fluid flow within them affects both their microstructure and the macroscopic development of solidifying systems. Contact: Mary Catsicopoulos 847/491-5586 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: November 11, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Kaper: Dynamics of patterns in activator-inhibitor systems Description: Interdisciplinary Seminar in Nonlinear Science Title: Dynamics of patterns in activator-inhibitor systems Speaker: Prof. Tasso Kaper, Boston University Special Note: Refreshments will be served prior to the talk in Tech M416. Abstract: Bioremediation is a promising technique for cleaning contaminated soil. We study an idealized bioremediation model involving a substrate (contaminant to be removed), electron acceptor (added nutrient), and microorganisms in a one-dimensional soil column. Using analytical and geometric singular perturbation theory, we study traveling waves (TW) corresponding to motion of a biologically active zone, in which the microorganisms consume both substrate and acceptor. We construct them using both matched asymptotic expansions and invariant manifold theory, showing that they lie in the transverse intersection of appropriate stable and unstable manifolds. We also study how the TWs depend on the physical parameters, including the half saturation constants in the reaction kinetics, and we investigate a bifurcation in which these TWs lose stability to time-periodic TWs. Contact: Mary Catsicopoulos 847/491-5586 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: November 04, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Lauder: How fishes swim: experimental hydrodynamics and... Description: Interdisciplinary Seminar in Nonlinear Science Title: How fishes swim: experimental hydrodynamics and the mechanics of flexible propulsors Speaker: George Lauder, Harvard University Abstract: There are 25,000 species of fishes, and a key feature of this remarkable evolutionary diversity is a great variety of propulsive systems used by fishes for maneuvering in the aquatic environment. Fishes have numerous control surfaces (fins) which act to transfer momentum to the surrounding fluid. Fishes are unstable and use several control surfaces simultaneously for propulsion and to maintain body position. In this presentation I will discuss the results of recent experimental kinematic and hydrodynamic studies of fish fin function. Recent high-resolution video analyses of fish fin movements during locomotion show that fins undergo much greater deformations than previously suspected. Experimental work on fin mechanics shows that fishes possess a mechanism for actively adjusting fin surface curvature to modulate locomotor force. Fish fin motion results in the formation of vortex rings of various conformations, and quantification of vortex rings shed into the wake by freely-swimming fishes has proven to be useful for understanding the mechanisms of propulsion. Experimental study of fish propulsion in combination with computational fluid dynamic analysis is providing the basis for the design of biomimetic robotic fin-thrusters for use in low-speed maneuvering underwater vehicles. Contact: Malcolm MacIver 847-491-3540 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 28, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Ruina: Energetics of Walking Description: Interdisciplinary Seminar in Nonlinear Science Title: Energetics of Walking Speaker: Prof. Andy Ruina, Cornell Special Note: Refreshments will be available prior to the talk in Tech M416. Abstract: Although the common approach to both observing and artificially generating terrestrial locomotion of animals and robots is to emphasize control, the approach we consider is to emphasize the mechanics. The over-riding theme is that some aspects of animal motion may be well understood by considering ways to minimize energy use and, similarly, more effective robots may be built if energy use minimization is considered in both the architecture and the actuation. Something to think about before the talk: what muscles do you use to walk at what points in the walking cycle? Why? Additional recent theoretical/simulation results include ideas about how to walk and brachiate (swing from branches) with (in principle) zero energy cost; analogous to the rolling of a rigid wheel. Biography: Andy Ruina runs the Robotics and Human Power Lab and teach mechanics and math classes. His main current research is the mechanics of coordination, particularly legged locomotion. He is interested in classical rigid-body dynamics especially as related to contact (collisions, friction, non-holonomic constraints). He used to work mostly on friction laws, especially in the context earthquake slip (experiments and theory). He also knows more or less about dynamical systems, bicycles, solid mechanics, and fracture. His degrees are from Engineering at Brown U (ScB 76, ScM. 78, Ph.D. 81). He was a National Science Foundation Presidential Young Investigator. Contact: Mary Catsicopoulos 847/491-5586 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 21, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Emonet: Towards a digital bacterium Description: Interdisciplinary Seminar in Nonlinear Science Title: Towards a digital bacterium Speaker: Thierry Emonet, U Chicago Special Note: Refreshments will be served prior to the seminar in Tech M416. Abstract: In recent years, single-cell biology has focused on the relationship between the stochastic nature of molecular interactions and variability of cellular processes. In addressing this problem, most efforts in computational biology have isolated one particular scale of interest, concentrating on either intracellular, cellular or population dynamics. Our long-term goal is to develop a modular computation framework able to cross scales and related stochastic events at the intracellular level to the behavior of a single cell and ultimately to the dynamics of a population of cells. In this talk I will relate our first steps towards that goal. As a test-bed for our approach we are using bacterial chemotaxis. E. coli bacteria can sense their environment and use that information to control their flagellar motors and move closer to sources of nutrients. To understand how a single E. coli processes information we decomposed the chemotaxis pathway into simpler information processing units. We then modeled each unit analytically and/or numerically, and tested model predictions with data from measurements in single cells. Finally, we constructed a digital bacterium equipped with the necessary modules to perform chemotaxis: receptors, adaptation module, intracellular signal carriers (response regulator), motors and flagella. Digital chemotaxis assays consisting of more than 1000 independent digital cells swimming in a 3D environment reproduced experimental data from both single cells and bacterial populations. Our analysis sheds new light on the relationship between behavioral variability and adaptation in a single cell. Contact: Mary Catsicopoulos 847/491-5586 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 07, 2005 Time: 4:00 PM - 5:00 PM Location: Tech L211 See the Evanston Campus Map Title: Dickinson: How Flies Fly Description: Interdisciplinary Seminar in Nonlinear Science Title: How Flies Fly Speaker: Michael Dickinson, California Institute of Technology Special Note: Please note unusual time. Abstract: A central feature in the natural history and behavior of most insects is the ability to fly through their environment in search of food, shelter, and mates. What physiological and mechanical specializations enable insects to fly stably and orient toward attractive objects? The goal of the research in my laboratory is to reverse engineer the flight control system of a fruit fly, and thus determine the means by which the brain and body function collectively to control the animal's trajectory through space. Like all forms of locomotion, flight behavior results from a complex set of interactions, not just within circuits in the brain, but among neurons, muscles, skeletal elements, and physical process within the external world. To control flight, the fly's nervous system must generate a code of motor information that plays out through a small but complicated set of power and steering muscles. These muscles induce microscopic oscillations in an external skeleton that drive the wings back and forth, producing a time-variant pattern of aerodynamic forces that the fly modulates to steer and maneuver through the air. The animal's motion through space alters the stream of information that runs through an array of visual, chemical, and mechanical sensors, which collectively provide feedback to stabilize flight and orient the animal towards specific targets. This research illustrates how processes within the physical world function with neural and mechanical features of an organism's design function to generate complex behavior. Contact: Malcolm MacIver 847/ 491-3540 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 07, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Couzin: Collective animal behavior Description: Interdisciplinary Seminar in Nonlinear Science Title: Collective animal behavior Speaker: Iain Couzin, Oxford University Abstract: Collective organization is everywhere, both around us and within us. Our brains are composed of billions of interconnected cells communicating with chemical and electrical signals. Our bodies are formed from clustering, communicating cells, and we ourselves are integrated in our own collective human society. Elsewhere in the natural world hundreds of thousands of blind army ants coordinate a massive raid across the rainforest floor, a flock of birds arcs and ripples while descending to roost and a fish school convulses, as if one entity, when attacked by a predator. How can animal groups move in unison? How does individual behaviour produce group dynamics? How do animal societies make informed unanimous decisions? From ant swarms to traffic jams, from consensus decision-making in animal groups to that among collections of neurons, I will discuss how, and why, coordinated collective patterns are generated in biological systems. Contact: Mary Silber 847-491-5586 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: September 30, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Maddocks: Multi-Scale Models of Sequence-Dependent DNA... Description: Interdisciplinary Seminar in Nonlinear Science Title: Multi-Scale Models of Sequence-Dependent DNA Mechanics Speaker: John Maddocks, Swiss Federal Institute of Technology Abstract: I will survey some continuum mechanics approaches to modelling the sequence-dependent physical properties of DNA fragments at the biologically pertinent length scales of a few tens to a few hundreds of base pairs. Contact: Bill Kath 847-491-8784 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: September 23, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Akhmediev: Dissipative solitons in life and science Description: Interdisciplinary Seminar in Nonlinear Science Title: Dissipative solitons in life and science Speaker: Nail Akhmediev, The Australian National University Abstract: The concept of soliton has been changing over the last decade or two. It changed from being a nonlinear mode of an integrable system to being described as a Hamiltonian soliton and moreover a dissipative soliton. Dissipative solitons form a new paradigm for the investigation of phenomena involving stable structures in nonlinear systems far from equilibrium. This talk is aimed to reflect new items in the general list of solitons and to discuss their common features and differences from the traditional concept of solitons. Contact: Bill Kath 847-491-8784 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: June 10, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Barthelemy: Heterogeneity and predictability of global... Description: Interdisciplinary Seminar in Nonlinear Science Title: Heterogeneity and predictability of global epidemics Speaker: Marc Barthelemy, CEA, Bruyeres-le-Chatel, France Abstract: We investigate the role of the large scale properties of the airline transportation network in determining the global diffusion pattern of emerging disease. We study a stochastic epidemic modeling framework that considers the entire world-wide airline database along with census data for urban populations, and which allows to relate the system complexity to the disease spreading pattern. In particular, we characterize quantitatively the heterogeneity level of the spreading pattern and its predictability in presence of stochastic fluctuations. The effect of the complex airline network topology, traffic flows and population heterogeneity is rationalized by comparison with relevant null hypotheses, demonstrating the role of the different levels of heterogeneity in the formation of the global spatio-temporal spreading pattern. Contact: Luis Amaral 847-491-7850 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: June 03, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Pesch: Rayleigh-Benard convection with modulated... Description: Interdisciplinary Seminar in Nonlinear Science Title: Rayleigh-Benard convection with modulated acceleration Speaker: Werner Pesch, University of Bayreuth, Germany Abstract: Rayleigh-Benard convection with modulated acceleration Rayleigh-Benard convection (RBC) of a horizontal fluid layer driven by a temperature gradient, is one of the best studied paradigms of pattern formation in nonequilibrium systems. Time-periodic modulation of the acceleration (gravity force) by periodic vibration of the standard convection cell provides an interesting extension that allows the study of parametrically driven instabilities in this classic systems. We present a theoretical analysis of various competing pattern-forming mechanisms based on the underlying Boussinesq equations. Recent experiments of the case of vertical vibration have revealed a host of interesting patterns, for instance superlattices of stars on a square backbone. They exist near a codimension-2 point, where a harmonic response of the system switches to a subharmonic one, and are organized by a new four-mode resonance mechanism. The case of horizontal shaking is more involved since already the basic state is associated with a time-periodic shear flow. We will address briely the resulting competition between buoyancy-driven and shear-flow instabilities. Applying shaking in two perpendicular horizontal directions (rotating oscillatory acceleration) can lead to Kuppers-Lortz like domain chaos scenarios. Contact: Hermann Riecke 847-491-8316 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 20, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Elston: Regulation of MAP kinase activity revealed by... Description: Interdisciplinary Seminar in Nonlinear Science Title: Regulation of MAP kinase activity revealed by mathematical modeling and experimental analysis Speaker: Tim Elston, University of North Carolina, Chapel Hill Abstract: Signaling pathways that allow cells to respond appropriately to different external stimuli often share common components. Therefore an important question in cell signaling is how pathway specificity is achieved. In yeast,two developmental pathways, mating and invasive growth, are regulated by the same MAP kinase cascade. After providing an introduction to these signaling pathways, I will discuss an integrated research approach that combines mathematical modeling with experimental investigations to uncover the mechanism that leads to pathway specificity at the MAP kinase level. Contact: Ted Rieger 847-425-1642 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 13, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Buchler: Chaos in the Music of the Stars Description: Interdisciplinary Seminar in Nonlinear Science Title: Chaos in the Music of the Stars Speaker: Robert Buchler, University of Florida Abstract: In contrast to the well-known classical Cepheid variable stars, which undergo pulsations with clocklike regularity, some large-amplitude pulsating stars exhibit a highly irregular oscillatory behavior. The nature of these irregular pulsations remained a mystery for a long time. We analyze the observational data with a general method of modern nonlinear dynamics, namely a flow reconstruction in phase space. The underlying dynamics is shown to be chaotic, and of a surprisingly low dimension of 4. We also present the results of numerical hydrodynamic simulations that show cascades of period doubling and chaos in models for these stars. We conclude with the physical insight one can gain from this analysis. Contact: Fred Rasio 847-467-3419 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 06, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Chialvo: Critical Brain Networks Description: Interdisciplinary Seminar in Nonlinear Science Title: Critical Brain Networks Speaker: Dante R. Chialvo, Physiology, Feinberg School of Medicine, Northwestern University Abstract: Highly correlated brain dynamics produces synchronized states with no behavioral value, while weakly correlated dynamics prevents information flow. In this talk we argue in favor of the idea that the working brain stays at an intermediate (critical) regime characterized by power-law relations. We discuss recent results describing the traffic between brain regions that continuously creates and reshapes complex functional networks of correlated dynamics (Phys. Rev. Lett. 94, 018102, 2005 and Physica A 340, 756, 2004). One aspect of this work relates to the analysis of avalanches of neuronal activity in cortical cultures. Another aspect involves functional magnetic resonance imaging, used to extract functional networks connecting correlated human brain sites. These results will be discussed in the context of the above idea of brain states as being on a permanent critical balance between miserable failure on one side and useless explosion on the other. Contact: Hermann Riecke 847-491-8316 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 29, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Roman Rafikov Coagulation in Astrophysical Systems Description: Interdisciplinary Seminar in Nonlinear Science Title: Coagulation Processes in Astrophysical Systems Speaker: Roman Rafikov, Institute for Advanced Study, Princeton Abstract:Coagulation is a process of assemblage of many small integral units into bigger entities. This phenomenon is ubiquitous in many physical systems and is used to describe processes of gelation (chemistry), aerosol formation (atmospheric sciences), formation of planets (astrophysics), large scale clustering of matter in the Universe (cosmology), and so on. I will describe several particular manifestations of coagulation in astrophysics and will demonstrate that, in many situations where the mutual gravity of mergingobjects is important, the effect of the so-called gravitational focusing causes coagulation to proceed in a very distinct manner. I will touch upon the analytical apparatus used to describe the process of coagulation, in particular in systems with focusing. Finally, I will describe in some detail the coupling of coagulation and gravitational dynamics emerging in studies of terrestrial planet formation. Contact: Fred Rasio 847-467-3419 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 08, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Setayeshgar: Physical Limits to Biochemical Signaling Description: Interdisciplinary Seminar in Nonlinear Science Title: Physical Limits to Biochemical Signaling Speaker: Sima Setayeshgar, Indiana University Abstract: Many biochemical processes in living cells that regulate the cell's functions are inherently stochastic, as the relevant molecules are present in small copy numbers making their fluctuations significant. Twenty-five years ago, Berg and Purcell showed that bacterial chemotaxis, where a single celled organism must respond to small changes in concentration of chemicals outside the cell, is limited directly by molecule counting noise, and that aspects of the bacteria's behavioral and computational strategies must be chosen to minimize the effects of this noise. The chemotaxis paradigm can be generalized to other biochemical mechanisms governing important functions of the cell, from translating external signals to appropriate internal signaling molecules, replicating DNA, reading genes and driving metabolism to triggering cell death. How reliably can these tasks be carried out in the presence of inherent fluctuations in the numbers of the key players? Specifically, how accurately can a biological sensor, which in turn controls downstream biochemical events, measure the concentration of an internal or external signaling molecule? We revisit and generalize the classic results of Berg and Purcell from a statistical mechanics point of view. Given improved experimental methods allowing accurate measurements, it is now possible to compare the actual performance of biochemical signaling systems within the cell with the corresponding counting noise limits. We compare this limit with the physical performance of several examples for which experimental data is available, an important one being the binding of transcription factors to genes. Contact: Hermann Riecke 847-491-3904 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 06, 2005 Time: 5:00 PM - 6:00 PM Location: Pancoe 1-401 (2000 Campus Drive, Evanston Campus) See the Evanston Campus Map Title: Daniel Stouffer, Postdoctoral Fellow, Chemical... Description: Evolution Discussion Group Title: A Complex Systems Approach to the Study of Natural Ecosystems Speaker: Daniel Stouffer, Postdoctoral Fellow, Chemical Engineering and NSF /IGERT Dynamics of Complex Systems , Northwestern University Special Note: Note unusual time and location Abstract: TBA Contact: Teresa Horton 847-467-1686 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 01, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Levchenko: Quantification of NF-kappaB: marriage of... Description: Interdisciplinary Seminar in Nonlinear Science Title: Quantification of NF-kappaB: marriage of genetics, biochemistry and mathematical modeling Speaker: Andre Levchenko, Johns Hopkins University Abstract: The NF-kappaB pathway is at the heart of cellular and physiological stress responses. Its activation underlies the life and death decisions on the level of a single cell, being vastly important in understanding of a myriad of diseases, including cancer and auto-immune syndromes. The dynamics of activation of this pathway displays many idiosyncratic and functionally significant characteristics, which were recently addressed by us through iterative and integrative use of a variety of computational and experimental techniques. Among the insights gained was a much deeper understanding of gene regulation by this pathway and its modular organization. In my presentation, I will illustrate the amazing power of computational analysis when it is tightly linked with experimental research, in understanding of this and other signaling phenomena. Contact: Ted Rieger 874-425-1642 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: March 11, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Alber: On Multiscale Approaches to Three-Dimensional... Description: Interdisciplinary Seminar in Nonlinear Science Title: On Multiscale Approaches to Three-Dimensional Modeling of Morphogenesis Speaker: Mark Alber, University of Notre Dame Abstract: In this talk the foundation of a unified, object-oriented, three-dimensional biomodeling environment is presented, which allows one to integrate multiple submodels at scales from subcellular to those of tissues and organs. Our current implementation combines a modified discrete model from statistical mechanics, the Cellular Potts Model, with a continuum reaction-diffusion model and a state automaton with well-defined conditions for cell differentiation transitions to model genetic regulation. This environment allows us to rapidly and compactly create computational models of a class of complex developmental phenomena. To illustrate model development, we simulate a simplified version of the formation of the skeletal pattern in a growing embryonic vertebrate limb. We will also describe the first three-dimensional model of cell aggregation in Myxobacteria based on a short-range cell-cell communication. Myxobacteria are a model system for studying cell-cell interaction and organization preceeding differentiation. When starved, tens of thousands of Myxobacteria cells change their motility, align, stream and form aggregates, which then develop into fruiting bodies. While cell aggregation has canonically been modeled as the result of chemotaxis, growing evidence shows that Myxobacteria organization depends on a contact-mediated cell signaling mechanism. We demonstrate that the same 3D discrete stochastic model simulates for different values of parameters all different stages of cell aggregation: the formation of a traffic jam, which then triggers formation of an aggregation center culminating in a fruiting body formation. Contact: William L. Kath 847-491-3904 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: March 04, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Grunbaum: Resolving individual-level behaviors underlying distinct group search strategies in fish. Description: Interdisciplinary Seminar in Nonlinear Science Title: Resolving individual-level behaviors underlying distinct group search strategies in fish. Speaker: Daniel Grunbaum, University of Washington Abstract: The difficulty of designing effective control algorithms for coordinated group activities by autonomous agents has motivated interest in possible analogies with fish schools and other natural social groups. These are of interest because they appear to operate under analogous constraints on locomotion and information exchange but nonetheless perform certain functions well, such as coordinated movement, foraging, and predator avoidance. However, few tools are available to deduce underlying behavioral algorithms from observations of natural social groups. We have been investigating how emergent characteristics of fish schools vary in response to environmental conditions and physiological states of members. After introducing some biological perspectives on grouping, I will present evidence that, in at least some fish schools, individuals are capable of multiple modes of interacting with neighbors that result in distinct group characteristics. Furthermore, individuals within a group appear to have a consensus behavioral mode expressed at a given time, suggesting that direct or indirect cues of modes adopted by neighbors are intrinsic to schooling dynamics. Interactions with neighbors occur on at least two levels: collective choice of group characteristics, and movement responses to neighbors to implement that choice. Contact: Kevin Lynch 847-491-7545 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 25, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Topaz: Social biological organisms: aggregation patterns... Description: Interdisciplinary Seminar in Nonlinear Science Title: Social biological organisms: aggregation patterns and dynamics Speaker: Chad Topaz, UCLA Abstract: Biological aggregations such as insect swarms, bird flocks, and fish schools are arguably some of the most common and least understood patterns in nature. These groups are thought to arise from "social forces" acting on individual organisms, including attraction (for protection and mate choice) and dispersion (for collision avoidance). In this talk, I will discuss recent work on two models which elucidates the connection between individuals' rules for movement and the macroscopic properties of aggregations. In the first model, movement rules of incompressible form provide a mechanism for the formation of vortex-like swarms similar to ant mills. In the second model, long-range attraction and short-range dispersal lead to population "clumps" with characteristics seen widely in natural aggregations, namely sharp edges and nearly constant internal population density. For both models, the mathematical goal is to find compactly supported solutions of nonlinear partial integrodifferential conservation equations. Contact: Mary Silber 847-491-3904 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 18, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Murray: Cooperative Control of Multi-Vehicle Systems Description: Focus Topic Title: Cooperative Control of Multi-Vehicle Systems Speaker: Richard Murray, Control and Dynamical Systems, California Institute of Technology Abstract: Increases in fast and inexpensive wireless communications have enabled a new generation of robotic applications in which a collection of vehicles perform a cooperative task in dynamic, uncertain, and even adversarial environments. While conventional control theory is well suited to design control laws for the individual vehicles, the tools for designing the interconnected control laws required for the complex tasks required in applications is considerably less well developed. In this talk I will describe three sets of results that make inroads into this class of problems: formation stability using graph Laplacians, distributed optimization for cooperative control, and distributed estimation across networks. These results are being tested on a new experimental platform being jointly developed at Cornell and Caltech that allows two teams of 6-10 robots each and up to two human operators to compete in a game of capture the flag. Contact: Kevin Lynch 847-467-5451 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 11, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Doiron: Delay-Induced Neural Oscillations in... Description: Interdisciplinary Seminar in Nonlinear Science Title: Delay-Induced Neural Oscillations in Communicating Fish Speaker: Brent Doiron, Center for Neural Science, NYU Abstract: Closed-loop feedback architectures between separate neuronal assemblies are commonplace in the brain. These architectures allow self-interaction for a population of neurons, often delayed due to the finite velocity of axon transmission. The extensive feedback anatomy present in the sensory processing centers of weakly electric fish makes them ideal for the study of how delayed feedback dynamics influence sensory processing. We show that a delayed inhibitory feedback is responsible for an oscillatory burst discharge in primary electrosensory neurons. Interestingly, this response is contingent on driving stimuli that have large spatial extent, high frequency components, and significant spatial correlations. For electric fish the only natural stimuli that satisfies all of the above criteria is communication signals between fish. Using simplified neural networks and linear response theory this selective response is understood as a delay induced oscillation that resonates with the high frequency component of stimuli. Furthermore, the overall amplitude is scaled by the degree of spatial correlation in the stimuli. Thus, neural delayed feedback interactions impart a selective response to important natural stimuli for these animals. Contact: William L. Kath 847-491-3904 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 04, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Blasdel: Optical Imaging of Non-Vascular Intrinsic... Description: Interdisciplinary Seminar in Nonlinear Science Title: Optical Imaging of non-vascular Intrinsic Signals in Monkey Visual Cortex Speaker: Gary Blasdel, Northwestern University Abstract: Not available Contact: William L. Kath 847-491-3904 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: January 14, 2005 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Booth: Mean theta phase of model CA1 pyramidal cell... Description: Interdisciplinary Seminar in Nonlinear Science Title: Mean theta phase of model CA1 pyramidal cell firing changes with location of synaptic stimuli Speaker: Victoria Booth, University of Michigan Abstract: In recordings from rat hippocampal CA1 place cells during learning and subsequent sleep episodes, Poe et al (2000) observed changes in place cell activity during REM reactivation that correlated with experience and followed the course of memory consolidation. The mean phase of place cell firing in relation to the theta rhythm membrane oscillation reversed during REM reactivation over the course of several days as the animal became familiar with the environment. Cells with place fields in an initially novel environment switched from firing near the theta rhythm peaks to firing near the theta troughs during REM while maintaining their theta peak activity during waking exploration. Theta trough firing during REM may uniquely facilitate depotentiation of intra-hippocampal synapses which are associated with now familiar, cortically-consolidated memories and allow for learning of new information and the integration of novel information with old memories. In a computational modeling study, we investigated neural mechanisms underlying experience-dependent theta phase reversal by incorporating the differential in the phase of the theta rhythm drive at the two excitatory afferent pathways to CA1 and testing model CA1 cell responses to inputs at these two paths. We will discuss simulation results that show that firing in response to proximal, Schaffer collateral synaptic input occurs preferentially near the peaks of the theta membrane oscillation while firing in response to distal dendritic, temporo-ammonic synaptic input occurs near the theta troughs. These results support the hypothesis that the gradual shift to theta trough firing would result from a growing potentiation of synapses in the direct, temporo-ammonic pat Contact: William L. Kath 847-491-3904 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: December 03, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Klavins: Self-Organizing Robotic Systems Description: Interdisciplinary Seminar in Nonlinear Science Title: Self-Organizing Robotic Systems Speaker: Eric Klavins, University of Washington Abstract: The fundamental problem of designing self-organizing systems is to understand how local interactions between components give rise to global properties. This problem must be solved if we are to engineer predictable and reliable large-scale systems consisting of vast numbers of parts (e.g.micro-robots, cells or molecules). In this talk I will describe a formal approach to modeling and designing self-organizing systems based on graph rewriting. The approach allows us to describe massively parallel algorithms for self-assembly, self-replication, distributed locomotion, and other decentralized processes, and to rigorously prove that they work. I will illustrate the approach by showing how it can be used in robotics and in MEMs self-assembly. Contact: Kevin Lynch 847-467-5451 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: November 19, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Nadal: Neural computation in the cerebellum (and... Description: Interdisciplinary Seminar in Nonlinear Science Title: Neural computation in the cerebellum (and elsewhere): what silent synapses tell us Speaker: Jean-Pierre Nadal, ENS, Paris Abstract: The cerebellum is a part of the brain involved in the control of coordinated movements. It has a very specific architecture organized around the Purkinje cells, each one receiving some 150,000 synaptic inputs from granule cells. These granule cell to Purkinje cell synapses are thought to store learned information that enables the cerebellum to perform motor control. Recordings of unitary connections in rat adult cerebellar slices have revealed an unexpected property: a large majority (80%) of these synapses appear to be electrically silent - the responding synaptic weights are zero. In order to investigate the principles governing synaptic weights, we have made use of the (classical) analogy between the Purkinje cell and the perceptron, a prototypical learning machine that learns input-output associations. We have derived the perceptron weights distribution, and compared it with the empirical data. The experimental and theoretical distributions agree closely, and the theoretical analysis reveals a computational role for silent synapses: they are required for maximizing storage capacity. I will present and explain this apparently paradoxical result, and discuss under what conditions silent synapses can be expected to be found in other parts of the brain. Contact: Sara Solla 847-467-5080 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: November 12, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Hartmann: Hardware Models for Studying Sensory Coding in... Description: Interdisciplinary Seminar in Nonlinear Science Title: Hardware Models for Studying Sensory Coding in the Nervous System Speaker: Mitra Hartmann, Northwestern University Abstract: Rats use rhythmic (5-12 Hz) movements of their whiskers to explore the environment and to extract information about 3-dimensional object features, including size, shape, and texture. Our laboratory uses the rat whisker system as a model to study how information is encoded and represented in the brain. We have constructed robotic whisker arrays that shed light on the following questions: (1) How is object distance encoded by the rat whisker system? (2) How might information be represented in the first stages of sensory processing (the trigeminal ganglion)? What does a “receptive field” mean at these stages? (3) How might periodic movement aid in sensory acquisition? Finally, I will describe how our whisker arrays may be used to enable more autonomous robots. Contact: William L. Kath 847-491-3904 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: November 05, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Bullo: Motion Coordination for Multi-Agent Networks:... Description: Interdisciplinary Seminar in Nonlinear Science Title: Motion Coordination for Multi-Agent Networks: Algorithmic Design and Theoretical Framework Speaker: Francesco Bullo, University of California at Santa Barbara Abstract: This talk describes recent progress on motion coordination problems for multi-agent systems. The work is motivated by emerging applications in active sensor networks and autonomous robotic systems. We present algorithms for deployment, rendezvous and task allocation. The resulting closed-loop behaviors are adaptive, spatially distributed, and verifiably correct. We also discuss some novel tools in control theory (advanced LaSalle stability analysis), geometric optimization (task encoding methods), and computational geometry (proximity graphs and spatially distributed maps). Contact: Kevin Lynch 847-467-5451 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 29, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Howell: Spacecraft Mission Design in Multi-Body Regimes... Description: Interdisciplinary Seminar in Nonlinear Science Title: Spacecraft Mission Design in Multi-Body Regimes and the Application of Dynamical Systems Speaker: Kathleen C. Howell, Purdue University Abstract: A number of space missions have recently been proposed that aim to take advantage of the growing scientific interest in the regions of space near the libration points in the Sun - Earth/Moon system. The renewed interest in the Moon has also heightened activity in projects that involve transfers from Earth-Moon to Sun-Earth libration points. However, accomplishment of many short- and long-term science and exploration goals require innovative spacecraft trajectory concepts, which, in turn, implies that more efficient design techniques for analysis and new philosophies for design must be considered; a more complete understanding of the solution space is imperative. Baseline concepts for any of the various missions must exploit the rich dynamics in these regions of space. One goal of these investigations, then, is a clearer understanding of the fundamental dynamics associated with the trajectory design problem in multi-body regimes, where qualitative information is needed concerning sets of solutions and their evolution. Nonlinear dynamical systems theory is a key component in progress toward that objective. Invariant manifolds have already served as a guide to the geometry of the phase space in the threebody problem and as a basis for the generation of natural pathways near the libration points. But, the primary objective is to use this information for trajectory design. The GENESIS mission first demonstrated that these ideas could be used for design of a trajectory. The stable and unstable manifolds served to generate various solution arcs and establish trajectory options; the methodology was then very successfully applied to this challenging Earth return mission. Contact: William Kath 847-491-3904 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 29, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Howell: Spacecraft Mission Design in Multi-Body Regimes Description: Interdisciplinary Seminar in Nonlinear Science Title: Spacecraft Mission Design in Multi-Body Regimes Speaker: Professor Kathleen C. Howell, Purdue Universit;y Abstract: A number of space missions have recently been proposed that aim to take advantage of the growing scientific interest in the regions of space near the libration points in the Sun - Earth/Moon system. The renewed interest in the Moon has also heightened activity in projects that involve transfers from Earth-Moon to Sun-Earth libration points. However, accomplishment of many short- and long-term science and exploration goals require innovative spacecraft trajectory concepts, which, in turn, implies that more efficient design techniques for analysis and new philosophies for design must be considered. Baseline concepts for any of the various missions must exploit the rich dynamics in these regions of space. One goal of these investigations, then, is a clearer understanding of the fundamental dynamics associated with the trajectory design problem in multi-body regimes, where qualitative information is needed concerning sets of solutions and their evolution. Nonlinear dynamical systems theory is a key component in progress toward that objective. Invariant manifolds have already served as a guide to the geometry of the phase space in the three-body problem and as a basis for the generation of natural pathways near the libration points. But, the primary objective is to use this information for trajectory design. The GENESIS mission first demonstrated that these ideas could be used for design of a trajectory. Besides the baseline design, the geometrical insight also supports maneuver analysis studies as well as stationkeeping. The next step, then, is to continue development in the four-body problem and develop capabilities for quick and efficient design in the Sun-Earth-Moon system. Contact: Bill Kath 847-491-8784 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 22, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Marko: Micromechanical study of proteins interacting with... Description: Interdisciplinary Seminar in Nonlinear Science Title: Micromechanical study of proteins interacting with single DNA molecules Speaker: John F. Marko, University of Illinois at Chicago Abstract: I will discuss the use of micromechanical assays - essentially measurements of elasticity - as methods to study how DNA is organized, ultimately into chromosomes. I will discuss studies at three scales of complexity. First, results of single-DNA micromanipulation experiments to study proteins which compact DNA by bending it will be presented. Next, I will show how similar techniques can be applied to the study of the dynamics of assembly of chromatin fiber onto a single DNA molecule. Finally I will describe experiments which probe the internal organization of entire mitotic chromosomes isolated from dividing cells. Contact: Hermann Riecke 847-491-8316 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 15, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Leonard: Collective Motion and Mobile Sensor Networks Description: Interdisciplinary Seminar in Nonlinear Science Title: Collective Motion and Mobile Sensor Networks Speaker: Naomi Ehrich Leonard, Princeton University Abstract: Collectives in biology perform remarkably well. Fish successfully forage for food and evade predators by swimming in a school, even though each individual fish is relatively simple. In engineered settings, it can be desirable to emulate nature and enable collective behavior of groups of relatively simple autonomous agents. Our application is to an autonomous, mobile sampling network, consisting of sensor-equipped robotic vehicles, used to aid in observing and predicting physical and biological dynamics in the oceans. In this talk I will describe recent work, with Derek Paley and Rodolphe Sepulchre, on collective motion of a planar group of self-propelled particles with steering control. The linear momentum of the group is a key control parameter; it is maximal in the case of parallel motions of the group and minimal in the case of circular motions around a fixed point. We exploit the analogy with phase models of coupled oscillators to stabilize parallel and circular motions. I will discuss further how these stabilized patterns can be used in design of mobile sensor networks for oceanographic sampling. We use objective analysis to design a sensor network that collects the richest information. To motivate I will show results with a fleet of underwater gliders from the Monterey Bay Field Experiment held in August 2003 as part of the Autonomous Ocean Sampling Network project. Contact: Kevin Lynch 847-467-5451 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 08, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Eckhardt: Transition to turbulence in pipe flow:new... Description: Interdisciplinary Seminar in Nonlinear Science Title: Transition to turbulence in pipe flow:new aspects of an old problem Speaker: Bruno Eckhardt, University of Marburg, Germany Abstract: Over a hundred years ago Osborne Reynolds studied the transition to turbulence in pipe flow, and documented its dependence on flow speed and perturbation and the structures of the turbulent flow. Theoretical analyses of the problem are made difficult by the fact that the laminar parabolic flow profile is linearly stable for all Reynolds numbers. A combination of modern experiments and direct numerical simulations, model building and a nonlinear dynamics phenomenology has unravelled some of the mysteries of the transition: there are fractal stability boundaries, chaotic saddles, exponential life time distributions and unstable periodic states. Besides the numerical evidence there now is also experimental support for some of the ideas. Contact: Bill Kath 847-491-8784 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 01, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Riecke: Persistence and Failure in Networks of Excitable... Description: Interdisciplinary Seminar in Nonlinear Science Title: Persistence and Failure in Networks of Excitable Neurons with Local and Non-Local Connectivity Speaker: Hermann Riecke, Northwestern University, Engineering Sciences and Applied Math Abstract: I will talk about an IGERT-project of my former student Alex Roxin that arose from an IGERT-class Sara Solla and I taught on networked dynamical elements. In the absence of any input many neurons are excitable rather than oscillatory. Coupled together they can support the propagation of excitation waves. They typically annihilate upon collisions and in a network with purely local coupling activity does not persist. We introduce non-local "short-cuts," which lead to a small-world-like topology, and find persistent activity and exceedingly long chaotic transients, as well as a failure transition as the density of short-cuts is increased. Contact: Bill Kath 847-491-8784 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 28, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Lega: Instability of local deformations of an elastic... Description: Interdisciplinary Seminar in Nonlinear Science Title: Instability of local deformations of an elastic filament Speaker: Joceline Lega, U. Arizona Abstract: When subject to sufficient twist, an elastic filament kept under tension typically undergoes a writhing bifurcation. Near threshold, the dynamics of the filament may be modeled by two coupled nonlinear Klein-Gordon equations, which are envelope equations for the amplitudes of the local deformations and twist. I will consider the question of the spectral stability of a two-parameter family of pulse-like solutions of these envelope equations. More precisely, I will explain how to obtain a criterion on the speed of propagation of the pulses, which is a necessary and sufficient condition for their spectral stability. This will involve Evans function techniques as well as Hamiltonian methods. I will also discuss the numerical evaluation of the Evans function. This work is joint with Stephane Lafortune and Silvia Madrid-Jaramillo. Contact: Mary Silber 847-491-3345 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 21, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Bassler: The Evolution of Developmental Canalization in... Description: Interdisciplinary Seminar in Nonlinear Science Title: The Evolution of Developmental Canalization in Networks of Competing Boolean Nodes Speaker: Kevin Bassler, University of Houston Abstract: Canalization, which describes a robustness of phenotype expression against either genetic or environmental change, has long been recognized as an important property of evolution in developmental biology. This is because it results in a buffering of genetic variation that stabilizes the evolutionary process. However, the mechanism responsible for evolution of canalization has remained controversial. Recently, we have discovered a novel mechanism for the evolution of canalization in developmental systems that is free of the problems associated with previously proposed mechanisms. The mechanism is based on a frustrated competition between genes and is demonstrated in a simple model of a genetic regulatory network. The model network evolves to a highly canalized steady state in which two competing dynamical effects balance. It will also be shown that the evolved steady state is a critical state that is fundamentally different than the critical states of random networks. Contact: Monica Olvera de la Cruz 847-491-7801 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 14, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Gollub: Internal Structure of Granular Shear Flows* Description: Interdisciplinary Seminar in Nonlinear Science Title: Internal Structure of Granular Shear Flows* Speaker: Jerry Gollub, Haverford College and University of Pennsylvania Abstract: The response of granular material to imposed shear is a key to understanding granular dynamics. The internal structure (e.g. ordered or random) of the material can strongly affect the shear response of these materials. In this work, we use a novel imaging technique and sensitive measurement of granular volume and forces to investigate the interior of granular shear flows with a narrow particle size distribution. We find that shearing can trigger crystallization of the material, and an accompanying step compaction event. The velocity profiles vary over more than five orders of magnitude (as a function of position). The internal flow profiles are radically different in the ordered and disordered internal states. Furtheremore, boundary conditions can have a profound influence on the entire packing. We find that continuously sheared flow can exhibit non-unique final states even under identical boundary conditions. In some cases both ordered and disordered states coexist, with the disordered state being metastable. Our experiments raise interesting theoretical questions about how prior history is recorded in the internal structure of granular packings, affecting their instantaneous rheology and long-term evolution in response to shear. *Work done with JC Tsai, and supported by the NSF, Division of Materials Research. Contact: Richard Lueptow 847-491-4265 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 07, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Behringer: Science in the Sandbox: Forces and... Description: Interdisciplinary Seminar in Nonlinear Science Title: Science in the Sandbox: Forces and Flucutations in Granular Materials Speaker: Robert Behringer, Duke University Abstract: Granular materials present a host of scientific and technological challenges. The handling of granular materials involves vasts amount of resources, and covers a substantial range of industrial sectors. From the point of view of basic physics, our understanding of granular materials falls substantially behind our understanding of conventional materials, e.g. ordinary solids, liquids and gases. However, granular materials exhibit similarities to all of these more conventional molecular phases. I will begin with a brief introduction to the different granular phases.I will then focus on some of the key issues associated with the understanding of dense granular materials. These include the suprisingly strong character of force fluctuations, and the way in hich forces are transmitted within dense granular systems. Recent experiments point to a statistical description of granular materials and provide tests of a spectrum of new granular models. Contact: Rich Lueptow 847-491-4265 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 30, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Mollenauer: Ultra-Long-Haul Dense WDM with... Description: Interdisciplinary Seminar in Nonlinear Science Title: Ultra-Long-Haul Dense WDM with Dispersion-Managed Solitons: The paradox of best performance from this most nonlinear of transmission modes Speaker: Dr. Linn F. Mollenauer, Bell Labs & Lucent Technologies Abstract: Dense WDM with dispersion-managed solitons tends to suffer from only one serious non-linear penalty, viz., the jitter in pulse arrival times resulting from XPM (cross-phase modulation) during inter-channel collisions. With the recent invention of periodic-group-delay-complemented dispersion compensation, however, even that penalty has been reduced to near insignificance, thus enabling the experimental demonstration, in this laboratory, of Tbit/s-level dense WDM at 10G per channel over the record-breaking distance of at least 18,000 km. This new limit is set primarily by the growth of spontaneous emission noise. Yet the nonlinear phase shift in such DMS transmission grows at the rate of at least 1 rad/Mm, in stark contrast to virtually all other rival modes, where it is purposely made to grow at much slower rates. In this talk, in addition to sketching the new PGD-complemented dispersion compensation and its first experimental test, I shall try to examine this apparent paradox of superior performance from the greatest nonlinearity. Contact: Prem Kumar 847-491-4128 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 23, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Ghrist: Optimal coordination in robotics Description: Interdisciplinary Seminar in Nonlinear Science Title: Optimal coordination in robotics Speaker: Robert Ghrist, University of Illinois Abstract: Optimizing the motion of a single robot is a well-studied problem. More challenging is the optimization of multiple robots sharing a common environment. In such a setting, each robot has its own notion of what optimal means: we consider vector-valued (or Pareto) optimization for coordination. In Pareto optimization, it is very common to have an infinite number of distinct optimal classes. In contrast, we demonstrate that a very large class of robotic coordination problems admits a finite bound on the number of optima. Most surprisingly, this phenomenon is intimately related to geometric properties of a certain coordination space. Contact: Mary Silber 847-491-8782 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 16, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Saffman: Spatiotemporal dynamics of intracavity second... Description: Interdisciplinary Seminar in Nonlinear Science Title: Spatiotemporal dynamics of intracavity second harmonic generation: Speaker: Mark Saffman, University of Wisconsin Abstract: I will talk about general features of pattern formation in nonlinear optical systems, and show how optics provides the possibility of selecting patterns by control of external boundary conditions. Recent work in intracavity second harmonic generation where patterns have both classical and quantum features will then be discussed. Contact: Prem Kumar 847-491-4128 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 09, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Brenner: Packing spheres with capillary forces Description: Interdisciplinary Seminar in Nonlinear Science Title: Packing spheres with capillary forces Speaker: Michael Brenner, Harvard University Abstract: Recent experiments (V. Manoharan et. al., Science, 301, 483 (2003)) have demonstrated that the evaporating liquid droplets coated with N colloidal particles leads unique packings. For N less than 12 the packings are identical to those which minimize the second moment of the particle distribution. We discuss both numerical simulations, and a theoretical framework for understanding these results. We argue that although the packings that are created are largely determined by geometrical constraints. Contact: Hermann Riecke 847-491-8316 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 02, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Cundiff: Pulse dynamics in modelocked lasers Description: Interdisciplinary Seminar in Nonlinear Science Title: Pulse dynamics in modelocked lasers Speaker: Steven T. Cundiff, JILA, NIST & University of Colorado Abstract: The pulse circulating in a modelocked laser undergoes complex evolution due to the interplay of dispersion, nonlinearity, and possibly birefringence. There are close analogies to dispersion managed solitons in optical communications. Since the pulse is sampled once per round trip, it is possible to observe the dynamics in great detail over very long distances. In this talk, I will present several examples how this can be exploited. The first is the occurrence of temporal vector solitons and axis instabilities in a modelocked fiber laser. The second is the observation of transient instabilities in a modelocked Ti:sapphire laser. The final topic will be the dynamics of the carrier-envelope phase, a recent area of significant interest in ultrafast optics. Contact: William L. Kath 847-491-8784 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: March 16, 2004 Time: 2:00 PM - 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Grigoriev: Chaotic mixing in microdroplets: theory and... Description: Interdisciplinary Seminar in Nonlinear Science Title: Chaotic mixing in microdroplets: theory and experiment Speaker: Roman Grigoriev, Georgia Tech Special Note: Note unusual day - TUESDAY Abstract: Liquids do not mix easily in microfluidic systems, which are being developed into labs-on-a-chip that promise revolutionary applications in biotechnology, chemistry and medicine. Recent studies have suggested that microfluidic stirring via chaotic advection can achieve the efficient mixing required in typical uses. For devices based on continuous flow through microchannels, strategies for inducing chaotic mixing by altering device geometries have been proposed. I will describe a general methodology for introducing chaotic mixing in discrete volume (microdroplet) systems, which allow miniaturization of many standard laboratory protocols that are difficult to realize with continuous flow. The mixing properties of the flows in microdroplets are governed by their symmetries, which give rise to invariant surfaces serving as barriers to transport.Complete three-dimensional mixing by chaotic advection requires destruction of all flow invariants. As an illustration of this idea, I will demonstrate that complete mixing can be obtained in a time-dependent flow produced by motion of a microdroplet along a two-dimensional path and describe the experiments that optically manipulate and mix microdroplets. Contact: Hermann Riecke 847-491-8316 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: March 12, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Mark Newman: Community structure in complex networks Description: Interdisciplinary Seminar in Nonlinear Science Title: Community structure in complex networks Speaker: Mark Newman, University Of Michigan Abstract: Many networks, including technological, social, and biological networks, divide naturally into communities. For instance, communities on the world wide web might represent groups of pages on related topics, communities in a biochemical network might represent functional groups. Traditional methods for detecting community structure in networks, like spectral partitioning and hierarchical clustering based on structural equivalence measures, give poor results when applied to such real-world problems. I will present number of new algorithms and methods for detecting community structure, particularly methods based on betweenness measures and methods based on modularity optimization. Examples of the application of these methods to a variety of network datasets will be given, including collaboration networks, acquaintance networks, and food webs. Contact: Hermann Riecke 847 491-5396 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: March 05, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Alex Gaeta: Nonlinear Wave Collapse of Ultrashort Laser Pulses Description: Interdisciplinary Seminar in Nonlinear Science Title: Nonlinear Wave Collapse of Ultrashort Laser Pulses Speaker: Alex Gaeta, Cornell University Abstract:Optical systems allow for the exploration of a wide range of nonlinear dynamical behavior. In this talk I will describe our theoretical and experimental studies on the propagation of ultrashort laser pulses in transparent media under conditions in which the power of the pulse is near or above the threshold power for catastrophic collapse. Such a system is nominally described by the 3-D nonlinear Schrodinger equation and can exhibit complex dynamics including self-similar collapse, temporal pulse splitting, optical shock-wave formation, supercontinuum generation, and multiple collapse events. Contact: Prem Kumar 847-491-4128 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 27, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Gunaratne: How to Find out when Bones may Break Description: Interdisciplinary Seminar in Nonlinear Science Title: How to Find out when Bones may Break Speaker: Gemunu Gunaratne, University of Houston Abstract: Osteoporosis is one of the major health problems in western societies. Since therapeutic agents used to reduce bone degradation have adverse side-effects, it is essential to be able to reliably identify subjects requiring therapy. Bone density, which is currently the principal means for detecting osteoporosis, is known to only partially account for bone strength. It has been well-established that factors such as the levels of connectivity and anisotropy of the inner porous segments of bone (trabeculae) play a significant role in determining the strength of vertebral and other bones. Reliability of diagnostic tools can only be enhanced by properly accounting for the consequences of these multiple facets. We have used a mathematical model consisting of disordered cubic networks of struts and nodes to represent porous bone. The freedom to implement independent changes provides means to deduce those factors which cause largest reductions in bone strength. It was found that the principal reason for age-related bone degredation is the increasing inefficiency in load transmission, and that this is caused by longer fractures in a bone sample. Ideas from fracture mechanics and percolation theory can then be used (1) to derive a new expression relating the strength and density of a sample, and (2) to deduce a new diagnstic for osteoporosis. The latter depends on vibrational responses of a bone sample. These conclusions have been confirmed using computer models built from digitized images of bone and/or data from human bone samples. Contact: Hermann Riecke 847-491-8316 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 20, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Ebert: Branching sparks --- the dynamics of electric... Description: Interdisciplinary Seminar in Nonlinear Science Title: Branching sparks --- the dynamics of electric breakdown Speaker: Ute Ebert, CWI Amsterdam Abstract: The initial phase of sparking is determined by so-called streamers. These are weakly ionized channels during their growth period. The growth is characterized by a self-induced enhancement of the electric field at the tip of the discharge channel. Streamers propagate with elocities of the order of 1000 km/sec. Recent ultrafast photography gives a new view on their dynamics. Streamer concepts are also being applied to recently discovered high altitude lightning, so-called red sprites. I will review recent observations and then explain the state of microscopic modelling, computations and theoretical concepts. Basically, already a single discharge channel has a multiscale structure with a thin ionization front surrounding a rather inert body. I will present computational results with adaptive grids, and I will discuss the properties of ionization fronts, moving boundary approximations for these fronts, and solutions of the moving boundary problem with conformal mapping methods. The result is the prediction that streamers in a sufficiently high potential can branch spontaneously due to a Laplacian instability as is also observed in computations. This quantitative prediction has to be confronted with phenomenological models for spark branching of the type of diffusion limited aggregation. Contact: Mary Silber 847-491-8782 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 13, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Solomon: Reaction-advection-diffusion systems with chaotic advection Description: Interdisciplinary Seminar in Nonlinear Science Title: Reaction-advection-diffusion systems with chaotic advection Speaker: Tom Solomon, Bucknell University Abstract: We are conducting experiments on pattern-formation and traveling wave propagation in the Belousov-Zhabotinsky chemical reaction in flows that exhibit Lagrangian chaos (chaotic mixing). Two fluid flows are studied: a blinking vortex flow and an oscillating/drifting vortex chain. For the blinking vortex flow, the chemical patterns that form are compared with fields that describe the chaotic mixing of passive impurities in the same flow. For the oscillating/drifting vortex chain, complicated traveling waves are observed. We discuss these traveling waves in terms of recent theories of chaotic traveling waves. We also discuss the relationship between these experiments and theories discussing the importance of superdiffusive mixing on front propagation in reaction-advection-diffusion systems. Contact: Mary Silber 847-491-5396 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 06, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: R. Morimoto : The Biological Response to the Stress of... Description: Interdisciplinary Seminar in Nonlinear Science Title: The Biological Response to the Stress of Misfolded Proteins: Neurodegenerative Disease and Aging Speaker: R. Morimoto , Northwestern University Abstract: TBA Contact: Hermann Riecke 847-491-8316 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: January 30, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: none: no seminar due to NSF site visit of the IGERT program Description: Interdisciplinary Seminar in Nonlinear Science Title: no seminar due to NSF site visit of the IGERT program Speaker: none, none Abstract: none Contact: Hermann Riecke 847-491-8316 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: January 23, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: McKinstrie: Phase jitter in soliton communication systems Description: Interdisciplinary Seminar in Nonlinear Science Title: Phase jitter in soliton communication systems Speaker: C. McKinstrie, Bell Laboratories, Lucent Technologies Abstract:In differential-phase-shift-keyed (DPSK)communication systems information is encoded using the phase differences between neighboring solitons. For DPSK systems the error-free transmission distance is limited by phase jitter, in which power shifts caused by amplifier nose are converted into phase shifts by self-phase modulation. I will describe how to model the growth of phase jitter in constant-dispersion and dispersion-managed systems. I will also describe how cross-phase modulation augments the growth of phase jitter in wavelength-division-multiplexed systems. Phase jitter in any system can be reduced significantly by in-line phase conjugation and post-transmission nonlinear phase-shift compensation. Phase jitter is governed by a pair of stochastic differential equations (SDEs) for the soliton power and phase perturbations. If the perturbations are small, one can model their growth using linearized SDEs, which produce Gauss probability-density functions (PDFs). However, system failures are caused by large perturbations that occur infrequently. For such perturbations the lin-earized equations (and the associated Gauss PDFs)are not valid. I will describe how to model power and phase jitter analytically, by using methods of statistical physics, and numerically, by making importance-sampled simulations based on the SDEs. Contact: Bill Kath 847-491-8784 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: January 16, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Moehlis: The Response Dynamics of Neural Oscillator... Description: Interdisciplinary Seminar in Nonlinear Science Title: The Response Dynamics of Neural Oscillator Populations Speaker: Jeff Moehlis, University of California, Santa Barbara Abstract: We undertake a probabilistic analysis of the response of repetitively firing neural populations to simple pulselike stimuli. This work is motivated by experimental data which shows that neurons in a region of the brain known as the locus coeruleus (LC) can exhibit distinct firing patterns which are strongly correlated with performance on cognitive tasks. Using a phase oscillator model for the LC neurons, we compute average firing probabilities for a pool of neurons in response to stimuli over many trials. This involves the solution of an advection-diffusion equation, and shows that neural response (1) is elevated in populations with lower baseline firing rates, and (2) decays due to noise and distributions of neuron frequencies. Similar results are obtained for other types of neurons, although the details of the response depend crucially on the type of bifurcation which leads to their repetitive firing. Contact: Mary Silber 847-491-8782 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: January 09, 2004 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Dissipative Solitons, Particle Like Behaviour of Self-Organized Patterns Description: Interdisciplinary Seminar in Nonlinear Science Title: Dissipative solitons in physical systems Speaker: H.-G. Purwins, University of Muenster, Germany Abstract:Spatially extended nonlinear physical, chemical, biological and other dissipative systems exhibit an overwhelming variety of self-organized patterns. The understanding of these patterns is one of the great challenges of modern science. Among the observed structures rather robust well-localized solitary patterns with particle like behaviour are of particular interest. These objects that we refer to as dissipative solitons undergo scattering, formation of “molecules”, generation and annihilation. In addition they can serve as elementary constituents of patterns of higher complexity. Dissipative solitons have been studied experimentally in planar gas-discharge systems in an exemplary way and theoretically in terms of a simple reaction-diffusion model. In this way an understanding of the mechanisms of formation, stabilization, propagation and interaction becomes possible. A mathematical foundation for the particle picture can be given by reducing the model equations to a set of ordinary differential equations containing the centre coordinates of dissipative solitons. It can be demonstrated that dissipative solitons behave similar in very different dissipative systems therefore attracting considerable interest in natural sciences and their application. Contact: Hermann Riecke 847-491-8316 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: December 05, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Mantzaris: A Computational Study of the Signal... Description: Interdisciplinary Seminar in Nonlinear Science Title: A Computational Study of the Signal Transduction Mechanisms and Wave Propagation in Astrocytic Glial Cells Speaker: Nikos V. Mantzaris, Rice University Chemical Engineering Department, ,Houston, TX Abstract: Glial cells of the central nervous system were considered to be passive bystanders in the mammalian brain. However, over the past decade, several neuroscientists have provided strong experimental evidence suggesting a very active role of these non-neuronal cells in information transmission and processing in the mammalian brain. In cell culture, intracellular and intercellular calcium waves have been shown to propagate between astrocytes (a type of glial cells) for hundreds of microns. The mechanism of signal transmission between glial cells has been an issue of great debate amongst neuroscientists. Several studies have suggested a role of gap junctions in mediating intercellular signaling, while recently a role of an extracellular signal has become more appreciated. Experimental studies have revealed many aspects of calcium wave propagation in glial cells. First, it has been shown that IP3-mediated release of Ca2+ from the ER calcium stores results from the binding of various agonists, such as ATP, to P2Y purinergic receptors. IP3 production is a result of PLC activation through a G-protein coupled mechanism. Furthermore, protein kinase C (PKC) has been linked to the adaptation of the P2Y receptors, while ATP release from stimulated glial cells via a calcium-independent mechanism, has recently been reported. Contact: Vasilly Hatzimanikatis 847-491-5357 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: November 21, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Clewley: Multiple scale analysis and local models... Description: Interdisciplinary Seminar in Nonlinear Science Title: Multiple scale analysis and local models generate low-dimensional maps Speaker: Robert Clewley, Boston University Abstract: Systems of oscillators described by ODEs arise commonly in the natural sciences, often with multiple time-scales. State-dependent coupling can add to the complexity, introducing new time-scales. These time-scales may not be explicit in the equations. We consider a computational technique that can be used to reduce the study of such networks, near a known attractor, to a set of low-dimensional approximate models. We find this conceptually related to a center manifold reduction. The technique adds rigor to intuitive reduction techniques that are ubiquitous in modeling high-dimensional coupled systems. In particular, it quantifies the robustness and parametric dependence of coherent temporal activity in more detail than a return-map stability analysis of periodic orbits, because it provides information along the entire length of the trajectory. As an example we describe the analysis of a network of Hodgkin-Huxley equations for biological membrane excitability. Contact: Hermann Riecke 847-491-8316 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: November 14, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Doering: The stochastic Fisher-Kolmogorov-Petrovsky-Piscunov equation Description: Interdisciplinary Seminar in Nonlinear Science Title: The stochastic Fisher-Kolmogorov-Petrovsky-Piscunov equation, Speaker: Charles R. Doering, Department of Mathematics and Michigan Center for Theoretical Physics Abstract: The Fisher-Kolmogorov-Petrovsky-Piscunov (FKPP) pde is a classical model used to describe the evolution of a spatially distributed population with local logistic growth-saturation dynamics and diffusive spreading. It is a `mean-field' model in the sense that all discreteness and noise effects are neglected. In this talk we describe a rigorous connection between a stochastic FKPP pde with a particular form of multiplicative noise and a single species birth- coalescence reaction-diffusion particle system. The correspondence is not in terms of a fluctuating hydrodynamic description for the reaction-diffusion model, but rather via the concept of `duality', an idea that has played a major role in the probabilistic analysis of interacting particle systems in recent decades. The idea of duality will be discussed and used to derive an exact formula for the extinction probability of any initial configuration for the stochastic FKPP equation. Duality will also be used to exploit the connection between the diffusion-limited birth-coalescence process and the strong-noise limit of the stochastic FKPP equation to determine the effect of high noise levels on the propagation speed of a wavefront in this stochastic pde. This is joint work with Carl Mueller (University of Rochester) and Peter Smereka (University of Michigan), Physica A Vol. 325, 243-259 (2003). Contact: Sandip Ghosal 847-467-5990 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: November 07, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: MacIver: Congruence of sensing and locomotor volumes in... Description: Interdisciplinary Seminar in Nonlinear Science Title: Congruence of sensing and locomotor volumes in an omnidirectional animal Speaker: Malcolm MacIver, Northwestern University Abstract: Sensory capabilities of an organism define a spatially restricted volume within which objects can be reliably detected. Motor capabilities independently define a volume within which rapid manoeuvres can be made and target objects can be promptly intercepted. Analysis of reconstructed electrosensory stimuli associated with prey capture behaviour of weakly electric fish reveals an omnidirectional sensing volume surrounding the body. Analysis of the fish's motor capabilities reveals a similarly shaped small-time locomotor volume that reflectsthe unusual manoeuvrability and backwards-swimming capabilities of these animals. The locomotor volume maximally overlaps the prey sensing volume when evaluated over a time interval of approximately 350 ms. We speculate that sensory and locomotor capabilities have coevolved to allow the animal to monitor the volume of space that can be reached within the time it takes for the animal to come to a complete stop. Quantitative analysis of locomotor volumes may provide a generally useful tool for analysing and interpreting sensory function in other organisms, and gives insights into the relationships between the sensing, locomotor, control, and planning aspects of animals and other autonomous agents. Contact: Sara Solla 847-467-5080 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 31, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Luijten: How to Stabilize Colloidal Suspensions? Application of a New Algorithm for Complex Fluids Description: Interdisciplinary Seminar in Nonlinear Science Title: How to Stabilize Colloidal Suspensions? Application of a New Algorithm for Complex Fluids Speaker: Erik Luijten, University of Illinois Abstract: Colloidal particles find widespread application as precursors for nanostructured materials, including advanced coatings, drug carriers, and colloidal crystals. These materials are frequently fabricated from the liquid phase via hierarchical self-assembly processes that are governed by the interactions of the particles, their shape, and their size. The understanding and prediction of phase behavior and stability of a suspension relies on a fundamental knowledge of the effective forces between colloids. Colloidal suspensions are typically stabilized by mitigating the ubiquitous attractive van der Waals forces via Coulombic or steric repulsive forces. These mechanisms, however, pose serious problems for the fabrication of close-packed colloidal crystals: the repulsive forces not only prevent gelation, but also increase the lattice spacing of the sedimented colloids. A lattice spacing larger than 1-2% of the particle diameter will lead to cracking of the crystal upon drying. Recently, an alternative method for tailoring the interactions has been discovered, which relies on the addition of highly-charged nanoparticles. Since a convincing theoretical understanding of this new stabilization method is currently lacking, we have employed computer simulations to clarify the underlying mechanism. The disparity in time and length scales, arising from the presence of micron-sized colloids and nanometer-sized particles, requires the use of new simulational algorithms that have rather striking properties. In this seminar, I will discuss these numerical techniques and the advantages they bring to the simulation of complex fluids in general. Subsequently, I will present our findings for the nanoparticle-colloid mixtures. Contact: Monica Olvera 847-491-7801 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 24, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Makse: Thermodynamics of Jammed Matter Description: Interdisciplinary Seminar in Nonlinear Science Title: Thermodynamics of Jammed Matter Speaker: Herman Makse, City College of the City University of New York Abstract: Jamming refers to a state which emerges when a many-body system is blocked in a static configuration far from equilibrium. The concept of jamming is emerging as a fundamental feature of many diverse systems such as granular materials, emulsions, colloidal suspensions, structural glasses and spin glasses. It has been postulated that the behavior of systems experiencing such a structural arrest can be described by a statistical ensemble at a fixed total volume such that all microscopic jammed states are equally probable and become accessible to one another (the ergodic hypothesis) by the application of tapping or shear, just as thermal systems explore their energy landscape through Brownian motion. Although the idea of a thermodynamic description of granular matter was recognized as attractive, it was not universally accepted because there is no known first principle justification of the statistical ensemble, such as there is for ordinary statistical mechanics of liquids or gases (Liouville's theorem). Here we combine micromechanical modeling, computational simulations and experimental characterization to explore this unifying thermodynamic framework. In particular we present recent results for jammed granular materials and compressed mulsion systems which strongly support the thermodynamic approach. Contact: Luis Amaral 847-491-7850 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 17, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Weiss: Remarkable Dynamics of an Overcompensatory Leslie Description: Interdisciplinary Seminar in Nonlinear Science Title: Remarkable Dynamics of an Overcompensatory Leslie Population Model Speaker: Howard Weiss, Pennsylvania State Abstract: We study the dynamics of an overcompensated Leslie population model where the fertility rates decay exponentially with population size. This nonlinear and noninvertible model has been highlighted by Caswell in his treatise \cite{Cas} on population models. We find a plethora of remarkably complicated dynamical behaviors, many of which have not been previously observed in population models, and which may give rise to new paradigms in population biology and demography. We study the two and three dimensional models and find all codimension one local bifurcations, period doubling cascades, attracting closed curves which bifurcate into strange attractors, multiple co-existing strange attractors with large basins (which cause an intrinsic lack of ``ergodicity"), crises (interior, boundary, and merging -- which can cause a discontinuous large population swing), merging of attractors, phase locking, and transient chaos. We also find remarkable one parameter families which exhibit most of these phenomena. We show that some of the more exotic phenomena arise from homoclinic tangencies. Contact: Keith Burns 847-491-3013 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 10, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Szleifer: Kinetics of protein adsorption: multi-scale... Description: Interdisciplinary Seminar in Nonlinear Science Title: Kinetics of protein adsorption: multi-scale approach for bridging time scales Speaker: Igal Szleifer, Purdue University Abstract: Protein adsorption plays a key role in a variety of biological processes. Control of protein adsorption is of primary importance in the design of biocompatible materials and drug carriers. In this talk we will describe a molecular approach that allows for the systematic study of the kinetics and thermodynamics of the adsorption process. The understanding of the molecular parameters that determine protein adsorption enable the design of surface modifiers that control the adsorption process. The adsorption of proteins is determined by interactions that are much larger than the thermal energy. The time scale for adsorption and desorption can be of the order of minutes to hours. Thus, it is a challenging fundamental problem to study at the molecular level. We will show how to build a multi-scale molecular approach that enables the study of the adsorption/desorption process bridging the gap in time scales from nanoseconds to hours keeping at all approximation levels molecular details of the protein-polymer mixture. We will use the adsorption of proteins on surfaces with grafted polymer as a model system to apply the hierarchy of theoretical approaches. The effect of polymer molecular weight, surface coverage and chemical architecture on the adsorption of the proteins will be discussed in detail. We will show the ability of the polymers to trap the proteins once they are adsorbed resulting in irreversible adsorption due to the very long time scales of desorption. The presentation will end with a discussion of the type of systems where our theory can be applied and the limitations and possible generalization of the multi-scale approach. Contact: Monica Olvera 847-491-7801 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 03, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Wassgren: Granular flow around an immersed cylinder Description: Interdisciplinary Seminar in Nonlinear Science Title: Granular flow around an immersed cylinder Speaker: Carl Wassgren, School of Mechanical Engineering, Purdue University Abstract: Interest in granular material dynamics has increased significantly in recent years. Although many aspects of the governing mechanics of granular flows are now well understood, there are still many areas that have not yet been explored. In particular, the problem of flow around immersed objects still has many unresolved questions. This talk presents recent experimental and computational investigations of dense and dilute granular flows around an immersed cylinder. Measurements of the drag force acting on and the flow field around cylinders are presented. The drag force measurements, when expressed in terms of a drag coefficient, are functions of the Knudsen and Mach numbers for the dilute flow case, and of the Froude number for the dense flow case. The effects of coefficient of restitution and friction coefficient are also presented. The transition between the dilute and dense flow regimes remains a topic of investigation. Contact: Rich Lueptow 847-491-4265 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: September 26, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: H.L.Swinney: Focus Topic: Noise and melting in square... Description: Interdisciplinary Seminar in Nonlinear Science Title: Focus Topic: Noise and melting in square granular patterns Speaker: H.L.Swinney, U. Texas, Austin Abstract: Previous work revealed square standing wave patterns in vertically oscillated granular layers for accelerations greater than about 2.5g, at frequencies typically below 30 Hz. The present work shows that as the transition to square patterns is approached, transient disordered waves with a characteristic length scale emerge and increase in power and coherence. The scaling behavior in the vicinity of the transition agrees with the ift-Hohenberg theory for convection in fluids, but the noise in the granular system is orders of magnitude larger than the thermal noise in a convecting fluid. Above the transition, the square lattice pattern exhibits the normal modes of a harmonically coupled square crystalline lattice. The amplitude of a mode can be further excited either by modulation of the container oscillation frequency or by reduction of the friction between the grains and the plate. For large mode amplitude, the lattice melts in accord with the Lindemann criterion for melting in two dimensions. Contact: R. Lueptow 847-491-4265 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: June 27, 2003 Time: 11:00 AM - 12:00 AM Location: Tech M416 See the Evanston Campus Map Title: Liu: Transition to chaos in continuous-time random... Description: Interdisciplinary Seminar in Nonlinear Science Title: Transition to chaos in continuous-time random dynamical systems Speaker: Zong-Hua Liu, Arizona State U. Special Note: note unusual time Abstract: The problem of noise-induced chaos is fundamental to understanding the interplay between stochasticity and nonlinearity, which is important for a variety of fields. In this talk, we consider situations where in a continuous-time dynamical system, a nonchaotic attractor coexists with a nonattracting chaotic saddle, as in a periodic window. Under the influence of noise, chaos can arise. We investigate the fundamental dynamical mechanism responsible for the transition and obtain a general scaling law for the largest Lyapunov exponent. A striking finding is that the topology of the flow is fundamentally disturbed after the onset of noisy chaos, and we point out that such a disturbance is due to changes in the number of unstable eigendirections along a continuous trajectory under the influence of noise. Contact: Hermann Riecke 847-491-5396 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: June 26, 2003 Time: 11:00 AM - 12:00 AM Location: Tech M416 See the Evanston Campus Map Title: Breban: Competition in Phase Synchronization of Chaotic... Description: Interdisciplinary Seminar in Nonlinear Science Title: Competition in Phase Synchronization of Chaotic Attractors & Indeterminate Saddle-Node Bifurcation Speaker: Romulus Breban, University of Maryland Special Note: Note unusual time Abstract: In this talk two different topics in dynamical systems will be discussed: 1)the competition between two periodic signals in phase synchronization of chaotic attractors when a the system is driven with two frequencies. There are several possible motivations for this study. First, there may be real situations where a chaotic dynamical system simultaneously receives inputs from two distinct periodic systems (e.g., a neuron receiving signals from two other neurons). Second, the study of a signal with two frequencies can be regarded as the next step from the single frequency case in understanding phase synchronization of chaos by signals with nontrivial frequency power spectra. Third, this situation is a generalization of the problem in which two periodic signals compete to entrain a nonlinear periodic oscillator. 2) the scaling properties of the indeterminate saddle-node bifurcation for a slowly swept control parameter and in the presence of noise. We consider systems that depend slowly on time (i.e., they drift) in such a way that the system slowly sweeps through a saddle-node bifurcation. Due to a common type of fractal basin boundary structure, such situations can often be expected to be ``indeterminate'' in the sense that it is fundamentally difficult to predict the eventual fate of an orbit that tracks the pre-bifurcation node attractor. We study the scaling properties of this indeterminacy; specifically, the sensitive dependence of the orbit's final attractor on the sweeping rate, and the scaling with noise amplitude of the final attractor capture probability. Contact: Hermann Riecke 847-491-8316 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: June 06, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Werner: Self-organization in Natural Systems: Permafrost Description: Interdisciplinary Seminar in Nonlinear Science Title: Self-organization and emergent behavior in natural systems: examples from permafrost terrain Speaker: Brad Werner, Scripps Institute of Oceanography Special Note: Focus Topic: Complexity in Environmental Systems Abstract: Landscapes are complicated in that they are composed of a broad range of interacting physical, chemical and biological components. Landscapes are simple in that they occupy a miniscule fraction of possible states. Traditional approaches to modeling landscapes have focused on either detailed descriptions (reductionism) or the essence of landscapes shared by many diverse systems (universality). I will discuss a new approach meant to reconcile the simple and complicated aspects of landscapes, as well as these two traditional methodologies, based on the observation that landscape dynamics breaks up into a hierarchy of behaviors characterized by discrete, separated time scales for reactions to perturbations. I will argue that there is no causal relationship between the dynamics of one level of the hierarchy and another, and therefore that models should be constructed using variables and dynamics at the time scale of the landform of interest. These concepts are illustrated using landforms in polar regions, with a focus on ice-wedge networks and thaw lake terrain. Contact: Aaron Packman 847-491-9902 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 30, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Parker: Nonlinear morphodynamics of erosional and... Description: Focus Topic: Complexity in Environmental Systems Title: Nonlinear morphodynamics of erosional and transportational cyclic steps in rivers Speaker: Gary Parker, University of Minnesota Abstract: TBA Contact: Aaron Packman 847-491-9902 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 23, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Morris: Traveling Waves in Axial Segregation (rescheduled) Description: Focus Topic: Complexity in Granular Systems Title: Traveling Waves in Axial Segregation: The Dynamics of Subsurface Core Speaker: Stephen Morris, University of Toronto Abstract: Axial segregation is a striking phenomenon which is often observed when granular materials of different sizes are rotated in a horizontal cylindrical drum; the grains separate according to their size into bands along the axis of the drum. Recently, a transient travelling wave state has been found to occur prior to axial segregation in a mixture of salt and sand grains. The existence of complex wave-like dynamics leading up to segregation cannot be explained by the simple reverse diffusion model of Savage, or its later elaborations. A new model of the segregation process has been proposed that describes both wave dynamics and axial segregation. The model uses two local variables, the concentration difference of large to small grains and the dynamic angle of repose, the angle which the flowing surface of grains forms. The model takes the form of two coupled nonlinear diffusion equations for the radially averaged quantities and can be plausibly derived from a more primitive three-dimensional transport model. This model predicts that these two axial fields are 90 degrees out of phase during the transient travelling wave state, and are in phase for the axially segregated state. Contact: Rich Lueptow 847-491-4265 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 16, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Dockery: Finger Formation in Biofilm Layers Description: Interdisciplinary Seminar in Nonlinear Science Title: Finger Formation in Biofilm Layers Speaker: Jack Dockery, Montana State University Abstract: A simple single substrate limiting model of a growing biofilm layer is presented. One dimensional moving front solutions are analyzed. Under certain conditions these solutions are shown to be linearly unstable to fingering instabilities. Scaling laws for the biofilm growth rate and length scale are derived. The nonlinear evolution of the fingering instabilities is tracked numerically using a level set method, leading to the observation of mushroom-like structures. Contact: Mary Silber 847 491-8782 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 14, 2003 Time: 4:30 PM - 5:30 PM Location: Tech L361 See the Evanston Campus Map Title: BME Seminar. Fredberg: Are We Built of Glass? Description: Title: Are We Built of Glass? Speaker: Jeffrey Fredberg, PhD, Professor of Bioengineering and Physiology, Department of Environmental Health, Harvard University Special Note: Note Unusual Location Abstract: We measured elastic and frictional moduli of a variety of cell types over a wide range of time scales and using a variety of biological interventions. In all instances elastic stresses dominated at frequencies below 300 Hz, increased only weakly with frequency and followed a power law; no characteristic time scale was evident. Frictional stresses paralleled the elastic behavior at frequencies below 10 Hz but approached a Newtonian viscous behavior at higher frequencies. Surprisingly, all data could be collapsed onto master curves, the existence of which implies that elastic and frictional stresses share a common underlying mechanism. These observations are consistent with the hypothesis that the cytoskeleton of the living cell behaves as a soft glassy material, wherein cytoskeletal proteins modulate cell mechanical properties mainly by changing an effective temperature of the cytoskeletal matrix. If so, then the effective temperature becomes an easily quantified determinant of the ability of the cytoskeleton to deform, flow and reorganize. Taken together, these findings define an integrative empirical framework for studying protein interactions within the complex microenvironment of the cell body, and appear to set limits on what can be predicted about integrated mechanical behavior of the matrix based solely on cytoskeletal constituents considered in isolation. Contact: Stacey Lewis 847-491-8178 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 09, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Westwick: Nonlinear system identification: A tool for... Description: Interdisciplinary Seminar in Nonlinear Science Title: Nonlinear system identification: A tool for modeling complex biosystems Speaker: David Westwick, University of Calgary Abstract: System identification entails the construction of mathematical models of dynamical systems from measurements of their inputs and outputs. Physiological applications of system identification have centered on gaining mechanistic insight from the identified model. Thus, rather than linearizing the model at an operating point, as is often done in control applications, identified models of physiological systems must deal explicitly with their nonlinearities. We will introduce some of the most commonly used models of nonlinear systems: functional expansions such as the Wiener and Volterra series, and block structuredmodels, comprising cascades of linear dynamic elements and memoryless nonlinearities. Recent advances in the identification of both functional expansions and block-structured models will be discussed. First,tests used to determine a block structure will be applied in a statistical framework, using confidence bounds recently derived for least squares estimates of Volterra kernels. Secondly, iterative cross-correlation based methods for fitting block structured models will be replaced with nonlinear optimizations based on a separable least squares decomposition. Finally, this new approach for fitting block structured models will be modified to allow for the use of alternate descriptions of the nonlinear element(s),including piecewise linear functions, neural networks, rational polynomials and cubic splines. Theoretical discussions will be supported by example applications drawn from studies of sensory systems, biomechanics and lung tissue dynamics. Contact: Sara Solla 847-467-5080 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 02, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Alan Hastings-Focus Topic:Complexity in Environmental Systems: Biocomplexity in the Design of Marine Description: Complexity in Environmental Systems Title: Biocomplexity in the Design of Marine Reserves Speaker: Alan Hastings, University of California-Davis Abstract: I will present models for Marine Reserves, which are based on understanding issues of connectivity of coastal marine populations, and integrating issues of both biodiversity and exploitation. The underlying mathematical approaches are based on integro-difference equations, which are used in several ways, and on reaction diffusion equations. Both qualitative and quantitative results will be presented. Contact: Aaron Packman 847-491-9902 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 18, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Benson - Focus Topic: Complexity in Environmental Systems - Aquifers and Streams as Fractal Filters Description: Complexity in Environmental Systems Title: Aquifers and Streams as Fractal Filters: Fractal Calculus in Your Backyard Speaker: Dave Benson, Desert Research Institute Abstract: Fractal calculus, which concerns derivatives of rational order (e.g.,the 1.65th) is a seemingly esoteric topic. Diffusion-like equations with fractional derivatives are the governing equations of particles that do not move in a smooth, Euclidean world. Instead, they move in a rough world that has randomness on all scales. For example, most of the dye molecules placed in a mountain stream will move away quickly, but a small amount will be caught for minutes to days in eddies, while another fraction may move into the relatively motionless water beneath the streambed. It may take months or years before the last molecules are washed away. The same is true for contaminants spilled into aquifers. Generalizations of the classical central limit theorem relegate this behavior to power-law random functions. The nonlinearity is off-putting until we realize that these random motions are described by linear, non-integer order governing equations. The equations are parsimonious and easy to solve, even though they model terrifically complex, multiscale behavior. Contact: Aaron Packman 847-491-9902 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 11, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Aronson: Driven Granular Media: Phase Transitions,... Description: Complexity in Granular Systems Title: Driven Granular Media: Phase Transitions, Patterns and Vortices Speaker: Igor Aronson, Argonne National Laboratory Abstract: Large ensembles of small particles display fascinating collective behavior when they acquire an electric charge and respond to competing long-range electromagnetic and short-range contact forces. Many industrial technologies face the challenge of assembling and separating such single- or multi-component micro and nano- size ensembles. The dynamics of conducting microparticles in strong electric field in the air or in deep vacuum was studied in Refs. [1,2]. Phase transitions and clustering instability of the electrostatically driven granular gas were found. A continuum model for the phase separation and coarsening in was formulated in terms of a Ginzburg-Landau equation subject to conservation of the total number of grains. In the regime of well-developed clusters, the continuum model is used to derive "sharp-interface" equations that govern the dynamics of the inter-phase boundary. The situation is remarkably different when the cell filled with poorly conducting liquid (toluene-ethanol mixute). We have found that metallic particles form a rich variety of phases not observed in air-filled cell. These phases include static precipitates: honeycombs lattices and Wigner crystals; and novel dynamic condensates: toroidal vortices and pulsating rings [3]. The observed phenomena are attributed to interaction between particles and electro-hydrodynamic flows produced by the action of the electric field on ionic charges in the bulk of liquid. Contact: Rich Lueptow 847-491-4265 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 04, 2003 Time: 2:00 PM - 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Rappel: Cellular signaling networks in Dictyostelium... Description: Interdisciplinary Seminar in Nonlinear Science Title: Cellular signaling networks in Dictyostelium discoideum: Speaker: Wouter Rappel, University of California-San Diego Abstract: Chemotaxis refers to the ability of cells to respond to spatial and temporal chemical gradients. It plays an important role in determining the direction of motion of cells in many biological processes including embryogenesis, wound healing and angiogenesis. In this talk will focus on a model system, the amoeboid organism Dictyostelium discoideum, which has emerged as a useful model system for studying chemotactic responses.Using available biological data we have constructed a theoretical model for the initial phase of chemotaxis in Dictyostelium. We will discuss the results of our physics-minded model and will show that it can make specific predictions. We will also present ideas about future experiments that could help our understanding of chemotactic signaling networks. Contact: Hermann Riecke 847-491-8316 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: March 14, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Weeks: The physics of the colloidal glass Description: Interdisciplinary Seminar in Nonlinear Science Title: The physics of the colloidal glass Speaker: Eric Weeks, Emory University Abstract: We study concentrated colloidal suspensions, a model system which has a glass transition. By using a confocal microscope to follow the three-dimensional motion of colloidal particles, we can directly observe the microscopic behavior responsible for the macroscopic viscosity divergence of glasses. We approach the glass transition by increasing the packing fraction of the particles. We find that particle dynamics are heterogeneous in both space and time: particle motion is characterized by a dynamic correlation length scale which grows as the glass transition is approached. Our most recent experiments use small magnetic particles to locally "poke" on the colloidal samples. By using an external magnet, we can pull a magnetic particle at constant force; oscillate a magnetic particle back and forth; and rotate clusters of magnetic particles. The response to these perturbations changes significantly near the glass transition. For example, we study the motion of the colloidal particles surrounding a moving magnetic particle. We find the magnitudes of displacements of the surrounding particles decay exponentially in space, in contrast to the flow expected for a Newtonian fluid. We study how this decay length varies with geometry (translation or rotation), packing fraction, and dimensionless Peclet number (forcing speed). Contact: Paul Umbanhowar 847 467 7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: March 07, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Gilchrist: Mixing and Segregation of Granular Materials... Description: Interdisciplinary Seminar in Nonlinear Science Title: Mixing and Segregation of Granular Materials in Spherical and Cubical Tumblers Speaker: Dr. James Gilchrist, UIUC Abstract: Transport, mixing and segregation of powders are critical phenomena in many industrial processes. There is a direct competition between mixing and segregation in flowing granular materials. Like fluids, it has been shown that chaotic advection in a granular flow - interpreted in a continuum way - can enhance the rate of mixing. Conversely, flowing granular materials will constituent grains differing in physical properties, such as size and density, tend to segregate. Simple protocols and mixer geometries are used to examine this interplay in rotating tumblers. Novel experiments in rocking spherical tumblers examine the pattern formation when axial segregation and chaotic advection coexist. Likewise, a new discontinuous mixing protocol has been developed to shed light on the interplay between radial segregation and chaotic flow in sphere and cube shaped tumblers. Contact: Paul Umbanhowar 847 467 7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 28, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Shen: Granular Fingering Description: Interdisciplinary Seminar in Nonlinear Science Title: Granular Fingering Speaker: Amy Shen, Washington University, St. Louis Abstract: It is well known that the front of a fluid sheet flowing over an inclined surface is unstable to spanwise perturbations. This instability, which manifests itself in the formation of fingers of fairly constant wavelength, is thought to reflect a competition between bulk viscosity and surface tension. Recent experiments have shown that the front of a granular layer flowing over an inclined surface may also develop fingers. Because the materials used in these experiments are cohesionless and, thus, unable to sustain surface tension, this instability is surprising. As an explanation, it has been suggested that frontal finger formation in granular materials is induced by the segregation of coarse, irregularly shaped grains. We present the results of an experiment involving a thin granular layer flowing within a horizontal cylinder that is rotated about its axis. We uncover an assortment of stationary and temporally varying spatial patterns, patterns that bear close resemblance to aeolian ripples and sand dunes. In particular, we find that, even when the medium is well-sieved and consists of nearly uniform spherical grains, its front may develop fingers, suggesting that, in general flow configurations, mechanisms other than grain segregation may drive the instability of a straight front. We develop a system of evolution equations that accounts for the steady motion of grains with the cylinder and for the motion of saltating and reptating grains and find that the model is able to predict some of the qualitative features observed from our rotating cylinder experiment. Contact: Paul Umbanhowar 847-467-7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 21, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Hagen: Drool of lizard, heart of horse...what goes into... Description: Interdisciplinary Seminar in Nonlinear Science Title: Drool of lizard, heart of horse...what goes into fast protein folding? Speaker: Stephen Hagen, University of Florida Abstract: Experimentally observed rates for the spontaneous folding of small proteins span a range of many decades, from very slow (many seconds for some proteins) to very fast (a few microseconds for others). What physical properties of the polypeptide chain ultimately control the speed of protein folding? I will discuss how laser-based optical techniques allow us to trigger and probe fast events in protein folding, and how they provide insight into the physical contraints on this fundamentally important biological process. Contact: Paul Umbanhowar 847-467-7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 14, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Nepomnyashchy: Feedback Control of Morphological Instabilities Description: Interdisciplinary Seminar in Nonlinear Science Title: Feedback Control of Morphological Instabilities Speaker: Alexander A. Nepomnyashchy, Technion Abstract: The morphological instability of a crystal growth is one of the most important types of crystal growth instabilities. The non-uniformity of the crystal composition caused by the morphological instability greatly reduces the crystal quality. In some cases, the development of the morphological instability is not saturated due to the nonlinear factors and leads to the formation of deep cells and dendrites. The present talk is devoted to the problem of the feedback control of morphological instabilities in the course of the directional solidification. The possibility of a feedback control is studied for both monotonic and oscillatory instabilities. Two ways of the feedback control are considered: (1) a local control by means of external heating within the melt in the vicinity of the melt/crystal interface; (2) a global control by adjusting the applied temperature gradient and the pulling speed. A special attention is paid to the case of small segregation coefficient, where the nonlinear instability is subcritical. The development and the suppression of instabilities are studied analytically and simulated numerically. Contact: Alexander Golovin 847 491-5346 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 07, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Mezic:Ergodic theory methods for controllability: group translations, Hamiltonian systems, quantum.. Description: Interdisciplinary Seminar in Nonlinear Science Title: Ergodic theory methods for controllability: group translations, Hamiltonian systems, quantum control Speaker: Igor Mezic, UCSB Abstract: We start from the following simple problem: consider a bead on a circular string immersed in honey. At time 0 the bead is kicked by a hammer and travels by an angle u0 before coming to rest. A different strength of the kick is chosen and hammer applied, so the bead travels by angle u1. How "many" different strengths must one be able to apply in order to travel from any initial to any final position on the circle? The dynamics of this problem is given by &thetan+1=&thetan+un, where &thetan is the position on a circle S1 after n kicks and un &sub U &sub S1. The problem of getting from an initial state to a final state using a sequence of inputs to the system, called the controllability problem, can be considered for translations on compact Abelian groups (not necessarily much more complicated than the example above). This in turn leads us to consider Hamiltonian systems, since dynamics of integrable Hamiltonian systems foliates the phase space into tori. more... Contact: Mary Silber 847 491-8782 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: January 31, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Aldana: Boolean Dynamics of Networks with Scale-free Topology Description: Interdisciplinary Seminar in Nonlinear Science Title: Boolean Dynamics of Networks with Scale-free Topology Speaker: Maximino Aldana, University of Chicago Abstract: I implement the scale-free topology into the Boolean network model proposed by Stuart Kauffman in 1969 to describe generically the dynamics involved in the processes of gene regulation and cell differentiation. In the original Kauffman model, the network topology is homogeneously random. Under such conditions, the parameters of the model have to be fine-tuned in order to achieve the dynamical stability required by living organisms to perform with reliability. Such fine-tuning is contrary to experimental observations. However, when the scale-free topology is implemented into the Kauffman model, stable dynamics are obtained without fine-tuning the parameters of the model. Additionally, the scale-free topology provides the network with both the dynamical stability and the evolvability essential for living organisms to perform with reliability and at the same time to adapt and evolve. From a biological point of view, I hypothesize that the scale-free topology is not only the result of aggregation processes such as preferential attachment; it may also be the result of evolutionary selective processes. Contact: Hermann Riecke 847-491-8316 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: January 24, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Silbert: Gravity-driven dense granular flows Description: Interdisciplinary Seminar in Nonlinear Science Title: Gravity-driven dense granular flows Speaker: Leo Silbert, University of Chicago Abstract: The rheology of dense granular assemblies is studied using large-scale molecular dynamics simulations for the case of gravity driven flows in an inclined plane geometry, i.e. chute flow. Over a wide range of parameter space, a steady state flow regime exists in which the energy input from gravity balances that dissipated from friction and inelastic collisions. The corresponding phase behaviour is discussed. Within the steady state regime the flow velocity depth-profile is seen to depend on how close the system is to jamming and matches several experimental studies. Static granular packings are also discussed. Contact: Paul Umbanhowar 847-467-7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: January 17, 2003 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Borodich: Similarity and Fractals in Contact and Nano... Description: Interdisciplinary Seminar in Nonlinear Science Title: Similarity and Fractals in Contact and Nano Mechanics Speaker: Feodor Borodich, Northwestern University Abstract: It is known that natural processes in a surprisingly large number of cases are, broadly speaking, self-similar on their intermediate stage of development. Two objects are similar if one can be obtained from another by some transformation. Usually, the transformation is the coordinate dilations. For example, two triangles are similar if one can be transformed into another one by magnification (dilation), rotation, and translation. The idea of similarity is undergoing the upsurge of interest due to the introducing the concept of fractals. However, similarity is not just fractals. Fractal concepts, parametric homogeneity and other non-traditional similarity ideas are discussed in detail. Various applications of similarity methods to problems of contact and nano mechanics are considered. These applications include the Hertz type contact problems and nano-indentation tests, models of contact for fractal punches, and recent results concerning abrasion wear of a steel ball by nano-sharp asperities of hard carbon containing coatings. The talk is targeted to non-specialists. All used concepts will be explained. Contact: Paul Umbanhowar 847-467-7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: December 12, 2002 Time: 2:00 PM - 3:00 PM Location: Tech Md416 See the Evanston Campus Map Title: s: a Description: Interdisciplinary Seminar in Nonlinear Science Title: a Speaker: s, d Special Note: f Abstract: a Contact: asdf 111 231-9999 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: December 05, 2002 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Forest: Laminar flows of nematic polymers: molecular theory and averaged descriptions Description: Interdisciplinary Seminar in Nonlinear Science Title: Laminar flows of nematic polymers: molecular theory and averaged descriptions Speaker: Greg Forest, University of North Carolina at Chapel Hill Special Note: Note unusal day (THURSDAY) Abstract: Nematic polymers are pervasive in biological as well as synthetic "soft matter" materials, and are responsible for remarkable material properties ranging from strength, electrical and thermal conductivity, and impermeability. These macromolecules are distinguished by highly anisotropic shapes, so that collectively, above a critical concentration, a spontaneous ordering transition occurs (the isotropic-nematic phase transition). The Tobacco Mosaic Virus and spider silk are rod-like polymers, whereas nano-clays and carbon pitch are platelets. Bulk properties of nematic polymer and nano-composite materials are controlled by features, dynamical and morphological, which are created during flow processing. This lecture summarizes work of our research group on theory, modeling, and simulation of nematic polymers in laminar flows. Theory is based on the Doi kinetic theory for flowing, anisotropic macromolecules with a distortional elasticity potential, along with mesoscopic and continuum approximations. Regarding behavior, we focus first on monodomain bulk attractors in linear flows, which are the precursors to structure formation. We then model onset and evolution of spatial morphology in the flow between shear plates. We report various unexplained phenomena which pose interesting applied mathematical problems and which, we contend, are fundamental to any control strategy for flow-processed macromolecular materials. Our research group includes Qi Wang, Florida State, Ruhai Zhou, UNC-CH, and Hong Zhou, UC-Santa Cruz, together with graduate students Eric Choate, Joohee Lee, and Xiaoyu Zheng at UNC-CH. Contact: Mary Silber 847-491-8782 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: November 22, 2002 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Amaral: Complex Networks in Nature Description: Interdisciplinary Seminar in Nonlinear Science Title: Complex Networks in Nature Speaker: Luis Amaral, Northwestern Abstract: An important but much over-looked aspect in the modeling of biological, technological and social systems is the structure of the interaction networks connecting the units comprising the system. Here, I will review some recent developments in our understanding of complex networks in these areas. In particular I will demonstrate how the ``small-world'' phenomenon occurs and why it is so ubiquitous ["Small-world networks: Evidence for a crossover picture", Phys. Rev. Lett. 82, 3180 (1999)]. I will also present evidence for the possibility to classify small-world networks according to their degree distribution---i.e., the distribution of number of connections of the units comprising the system---and suggest mechanisms for the appearance of such classes ["Classes of small-world networks", Proc. Nat. Acad. Sci. USA 97, 11149-11152 (2000); ``The web of human sexual contacts'', Nature 411, 907 (2001)]. In the rest of my talk I discuss some of my work applying these concepts to a number of different systems including food webs, conformation spaces, etc ["Robust patterns in food-web structure", Phys. Rev. Lett. 88, 228102 (2002); "Small-world networks and the conformation space of a short lattice polymer chain", Europhys. Lett. 55, 594-600 (2001)]. Contact: Paul Umbanhowar 847 467 7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: November 15, 2002 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Zeeman: Modeling the LH surge in the human menstrual cycle. Description: Interdisciplinary Seminar in Nonlinear Science Title: Title: Modeling the LH surge in the human menstrual cycle. Speaker: M.L. Zeeman, University of Texas, San Antonio Abstract: In vertebrates, ovulation is triggered by a surge of luteinizing hormone (LH) from the pituitary. The precise mechanism for initiating the surge in the human menstrual cycle remains a fundamental open question of physiology. Sampling of serum LH on a time scale of minutes reveals pulsatile release from the pituitary in response to pulses of gonadotropin releasing hormone (GnRH) from the hypothalamus. In this talk we discuss the physiological background, and we present a mathematical model of the human pituitary as a damped oscillator driven by the hypothalamic oscillator. The numerically simulated LH surge is consistent with existing data on the time scales of both minutes and days. We use the model to explain the surprising GnRH pulse frequency characteristics required to successfully treat human infertility disorders such as Kallmann's syndrome, and to make new experimental predictions. Contact: Mary Silber 847 491-8782 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: November 08, 2002 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Grosberg: Toy Proteins Functioning Like a Machine Description: Interdisciplinary Seminar in Nonlinear Science Title: Toy Proteins Functioning Like a Machine Speaker: Alexander Grosberg, University of Minnesota Abstract: We design a toy protein mimicking a machine-like function of an enzyme. Using an insight gained by the study of conformation space of compact lattice polymers, we demonstrate the possibility of a large scale conformational rearrangement which occurs (i)without opening a compact state, and (ii) along a linear (one-dimensional) pathway. We also demonstrate the possibility to extend sequence design method such that it yields a "collective funnel" landscape in which the toy protein (computationally) folds into the valley with rearrangement pathway at its bottom. Energies of the states along the pathway can be designed to be about equal, allowing for diffusion along the pathway. They can also be designed to provide for a significant bias in one certain direction, in which case toy protein acts as a spring capable to accumulate energy without dissipation. Together with a toy ligand molecule, our "enzymatic" machine can perform the entire cycle, including conformational relaxation in one direction upon ligand binding and conformational relaxation in the opposite direction upon ligand release. This model, however schematic, should be useful as a test ground for phenomenological theories of machine-like properties of enzymes. Contact: Monica Olvera 847-491-7801 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: November 01, 2002 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Tyson: CyberYeast: Computational Models of Cell Cycle... Description: Interdisciplinary Seminar in Nonlinear Science Title: CyberYeast: Computational Models of Cell Cycle Regulation Speaker: John Tyson, Virginia Tech Abstract: The "last step" of computational molecular biology, to derive the physiological properties of a cell from underlying networks of interacting proteins, is an especially challenging problem that has received little attention so far. For example, the molecular mechanism that controls DNA synthesis and nuclear division is so complex that its behavior cannot be understood by casual, hand-waving arguments. By translating this mechanism into differential equations, we can use the computer to compare models to the observed behavior of cells. A combination of numerical simulation and bifurcation theory provides a powerful tool for understanding the molecular basis of cell division. Contact: Vassily Hatzimanikatis 847 491 5357 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 25, 2002 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Perelson:Modeling Viral Infections in Humans Description: Interdisciplinary Seminar in Nonlinear Science Title:Modeling Viral Infections in Humans Speaker: Alan Perelson, Los Alamos National Laboratory Abstract:Diseases such as HIV, hepatitis C virus, and influenza kill many Americans each year. While the medical community is involved in immediate treatment, there are contributions that can be made by the complex systems and mathematical modeling community. I will review modeling work involving ordinary differential equations that has uncovered important features of each of these diseases and which has had an influence on the medical and biological community. Contact: Vassily Hatzimanikatis 847 491 5357 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 17, 2002 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Luigi Lugiato : Cavity solitons in semiconductor microresonators Description: Interdisciplinary Seminar in Nonlinear Science Title: Cavity solitons in semiconductor microresonators Speaker: Luigi Lugiato , Universitŕ dell'Insubria Special Note: Note unusual day (Thursday) Abstract: Cavity solitons are a special kind of spatial solitons which arise in a dissipative environment. They can be written and erased in arbitrary locations in the section of broad area radiation beams. Their positions and their motion can be appropriately controlled. Such features make them very interesting for possible applications to information processing. The final part of the talk will focus on the experimental observation of cavity solitons in semiconductor microcavities, recently published in Nature. Contact: Prem Kumar 847 491-4128 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 11, 2002 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Frank-Kamenetskii: Biophysics of the DNA molecule Description: Interdisciplinary Seminar in Nonlinear Science Title: Biophysics of the DNA molecule Speaker: Maxim Frank-Kamenetskii, Boston University Abstract: Biophysical studies of DNA and their relevance to DNA functioning are overviewed. Many properties of the DNA molecule can be quantitatively understood on the basis of simplified theoretical models: the elastic rod model, the helix-coil model and the polyelecrtolyte model. The elastic rode model has proved to be especially fruitful in treating the DNA behavior in single-molecule experiments and in studying DNA topological properties. DNA functioning critically depends on the ability of the double helix for looping, which makes it possible for proteins bound to different sites on DNA to interact with each other. Such looping is greatly facilitated by an artificial DNA analog, PNA, which allows directional modulation of the activity of proteins working on DNA. Contact: Sasha Golovin 847-491-5346 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 04, 2002 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Fibich: Self Focusing of Circularly-polarized Beams Description: Interdisciplinary Seminar in Nonlinear Science Title: Self Focusing of Circularly-polarized Beams Speaker: Gadi Fibich, Tel Aviv University Abstract: In this talk we present a systematic study of propagation of circularly polarized beams in a Kerr medium. In contrast to previous studies,vectorial effects and nonparaxiality are not neglected in the derivation. We show that the standard model in the Literature for self-focusing of circularly-polarized beams can lead to completely wrong results, that circular polarization is stable during self-focusing, and that nonparaxiality and vectorial effects arrest collapse. We also show that circularly polarized beams are much less likely to undergo multiple filamentation than linearly-polarized beams. This is a joint work with B. Ilan. Contact: W.L. Kath (847) 491-5585 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: September 27, 2002 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Miller: Predicting muscle activity from neurons recorded in the... Description: Interdisciplinary Seminar in Nonlinear Science Title: Predicting muscle activity from neurons recorded in the central nervous system Speaker: Lee E. Miller, Northwestern University Abstract: Several areas of the cerebral cortex and brainstem contain neurons that project directly to the spinal cord. These projection neurons contact interneurons and motor neurons within the cord that activate muscles. The extent to which these spinal neurons significantly transform the descending command signals has been a topic of debate for many years. In my research I record directly from these CNS command neurons and study their relation to the muscle activity and movements that they presumably control. The three-fold goals are 1) to understand the coordinate system within which these motor command signals are encoded, 2) to determine the mechanisms by which neuronal networks generate these command signals, and 3) to develop methods by which signals like these, recorded from a human patient, could be used to control muscle activity or an external device. In this talk I will concentrate on several studies in which we have shown that the discharge of neurons within both the primary motor cortex and two brainstem regions can be located within a "muscle space" which remains reasonably stable over time. Using this relation, we are developing methods to predict the modulation of muscle activity from neural activity, using both actual data sets and simulated networks of hundreds of neurons. Results suggest that it may be possible to make good predictions of muscle activity from sets of neurons of approximately the size that it is now feasible to record from an experimental animal. Contact: Sara Solla 312-503-1408 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: September 20, 2002 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Genet: Cellular basis for time computations in the cerebellum: multi-threshold model of electric. . Description: Interdisciplinary Seminar in Nonlinear Science Title: Cellular basis for time computations in the cerebellum: multi-threshold model of electric activation in cerebellar Purkinje cell dendrites Speaker: Stephane Genet, Universite Pierre et Marie Curie, Paris Abstract: This seminar will present a system of parabolic, partial differential equations that was derived from biophysical data to investigate the membrane mechanisms endowing Purkinje cell dendrites with the ability to produce fast (1ms) Ca-spikes and prolonged (several hundreds of ms) low amplitude plateau potentials. These mechanisms will be discussed using a phase-plane representation of the spatially homogenous version of the model. Bifurcation of this system will be presented to show how its responses to a brief input can be changed by the level of tonic excitation, together with an analysis of the full model for the propagation of these responses in the dendritic tree. We will show how Purkinje cell dendritic computations may solve the time-problem in the trace paradigm of the classical conditioning of the eyelid by envisioning the background activation of parallel fibers as the physiological counterpart of the tonic excitation in the model. Contact: Sara Solla 847-467-5080 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: June 05, 2002 Time: 2:00 PM - 3:00 PM Location: Tech Institute Room M416 - Note special day (Wednesday) See the Evanston Campus Map Title: Murray: Chaos in the Solar System Description: Focus Topic: Non-linear dynamics of stellar and planetary systems Title: Chaos in the Solar System Speaker: Norm Murray, University of Toronto Abstract: I will give a physical explanation of the resonant structure of planetary systems, and how it leads to the prediction that chaos should be generic in multiplanet systems. Using this picture, I will show how to calculate both Lyapunov time (the time over which information about the detailed dynamical state of the system is lost) and diffusion times for the actions (semimajor axis and angular momentum or eccentricity)of both asteroids and planets. The latter allows for the prediction of collision or ejection times for both asteroids and planets. As an example I will show why the giant planets in our solar system are chaotic, and when we can expect something dramatic to happen. Contact: Fred Rasio 847-467-3419 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 31, 2002 Time: 4:00 PM Location: Tech L211 - Note special time and location See the Evanston Campus Map Title: Chiang: Winding and Unwinding a Clockwork Universe Description: Joint Nonlinear Science Seminar and Physics & Astronomy Department Colloquium Title: Winding and Unwinding a Clockwork Universe Speaker: Eugene Chiang, University of California-Berkeley Abstract: The motions of celestial bodies observe a form and proportion reminiscent of well-tuned clocks. Among the principal mechanical phenomena are "mean motion resonances." These occur when the orbital period of a body divided by the orbital period of another body takes the ratio of small positive integers. Gravitational interactions between resonant bodies are particularly dramatic. The dynamics within mean motion resonances will be discussed in two contexts: (1) the Kuiper Belt of bodies orbiting in the outer Solar System, of which Pluto is the largest known member, and (2) extrasolar planets, whose orbits are incredibly elongated compared to those in our Solar System. These two types of systems may illustrate, respectively, the winding and unwinding of our clockwork universe. Contact: Fred Rasio 847-467-3419 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 24, 2002 Time: 2:00 PM - 3:00 PM Location: Tech Institute - Room 416 See the Evanston Campus Map Title: Ecke: Granular Chains - Geometric Constraints, Entropy, and More Description: IGERT nonlinear seminars Title: Granular Chains - Geometric Constraints, Entropy, and More Speaker: Robert Ecke, Los Alamos National Lab Abstract: Vertically-vibrated chains provide an excellent system for the study of generic features of systems where geometric constraints are dominant such as polymers and proteins. I will discuss experimental and theoretical results on the lifetime of knots and the probability distribution of figure eight structures. The latter results are an interesting example of the application of notions of equilibrium statistical mechanics to nonequilibrium systems. I will also discuss the spontaneous formation of spiral states, the radius of gyration of free chains and loops, and the possibilities in multi-chain states. Contact: Hermann Riecke 847-491-8316 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 17, 2002 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Malhotra: Chaos and Stability in Planetary Systems Description: Focus topic: Non-linear dynamics of stellar and planetary systems Title: Chaos and Stability in Planetary Systems Speaker: Renu Malhotra, University of Arizona Abstract: This talk will review recent advances in understanding the long term dynamics of our Solar system and of planetary configurations in general. The dynamics of our Solar system includes both chaotic and stable motions. The large planets exhibit remarkable stability of their orbital parameters on gigayear timescales, while also possessing all the formal characteristics of a chaotic dynamical system. The small bodies in our Solar system -- asteroids, comets, Kuiper Belt objects, and interplanetary dust particles -- contain a mix of chaotic and stable populations, a characteristic that makes the Solar system as a whole a continously evolving and dynamic system. Early in the history of the Solar system, gravitational interactions between the large planets and the remnant planetesimal disk caused the planets to evolve from a prior more compact and less stable orbital configuration to the current stable-but-weakly-chaotic dynamic state. It is unclear whether the `architecture' of our Solar system should be expected to be typical of planetary systems about Sun-like stars. Recently-discovered extra-solar planetary systems have diverse architectures, generally in great contrast with ours. The details of a planetary system configuration may be critical to the habitability of planets. Contact: Fred Rasio 847-467-3419 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 10, 2002 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Lin: Frequency locking of reaction-diffusion patterns Description: Nonlinear Seminar Title: Frequency locking of reaction-diffusion patterns Speaker: Anna Lin, Duke University Abstract: The resonant response of a single nonlinear oscillator to periodic forcing is well understood, yet little is known about frequency locking in spatially-extended oscillatory systems such as arrays of Josephson junctions or the heart. In these systems the important functions and dynamics often occur far from equilibrium, where they can exhibit spatio-temporal patterns. We use an experimental and a numerical reaction-diffusion system driven far from equilibrium to study the effects of frequency locking on pattern formation. I will introduce a quantitative description of resonant patterns which allows us to identify transitions between pattern states as the forcing strength is varied. The resonant patterns observed in the experiments show qualitative agreement with our numerical model and with an analysis of an amplitude equation, suggesting that they are general features of frequency locking in oscillatory continua. Contact: Paul Umbanhowar 847 467-7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 03, 2002 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Bower: How does the cerebellum compute? Description: Nonlinear Seminar Title: How does the cerebellum compute? Speaker: James Bower, The Research Imaging Center at The University of Texas Health Science Center-San Antonio and the Cajal Neuroscience Research Center at the University of Texas- San Antonio Abstract: The nearly crystalline structure of the mammalian cerebellar cortex has allowed investigators to develop a highly detailed description of its neurons and network architecture. This knowledge, in turn, has substantially influenced speculations concerning cerebellar function. At the core of many of these speculations is the assumption that the more than 150,000 parallel fiber synaptic inputs made on individual cerebellar Purkinje cells provide a direct excitatory drive on the output of these cells. However, experimental evidence that this is not the case has been accumulating for more than 35 years. This presentation will first review the evidence for and against a strong influence of parallel fibers on Purkinje cells, and will then consider the findings of our recent experimental and model-based investigations suggesting that parallel fiber synapses are closely coupled functionally to molecular layer inhibition, and are therefore modulatory in nature. The talk will conclude by considering the consequences of this new interpretation of cerebellar cortical circuitry for current theories of cerebellar function. Contact: Sara Solla 847 467-5080 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 26, 2002 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Othmer: Signal transduction and behavioral response in E.... Description: Nonlinear Seminar Title: Signal transduction and behavioral response in E. Coli: a model system for understanding macroscopic patterns from microscopic rules Speaker: Hans G. Othmer, University of Minnesota Special Note: Focus Topic: Complexity in Biological Systems Abstract: Chemotaxis in the bacterium E. coli is widely-studied because of its accessibility and because it incorporates processes that are important in the response of numerous sensory systems to stimuli: signal detection and transduction, excitation, adaptation, and a change in behavior. Quantitative data on the change in behavior is available for this system, and the major biochemical steps in the signal transduction/processing pathway have been identified. In this talk we will discuss a mathematical model of single cells that can reproduce many of the major features of signal transduction, adaptation and aggregation, and which incorporates the interaction of the chemotactic protein CheYp with the flagellar motor. We shall show the results of Monte Carlo simulations of population behavior and discuss the problem of how to obtain macroscopic equations that incorporate essential features of the microscopic model. Contact: Vasilly Hatzimanikatis 847-491-5357 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 19, 2002 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Merritt: Non-Integrable Galactic Dynamics Description: Nonlinear Seminar Title: Non-Integrable Galactic Dynammics Speaker: David Merritt, Rutgers University Special Note: Focus Topic:"Nonlinear dynamics of stellar and planetary systems" Abstract: Non-Integrable Galactic Dynamics Orbital and self-consistent dynamics of non-integrable galaxy models are reviewed. Topics covered include torus construction; resonances; triaxial systems with central singularities; mixing and collisionless relaxation; and chaos in collisional systems. Contact: Fred Rasio 847-467-3419 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 12, 2002 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: D. C. Heggie: Exponential Divergence Description: Focus Topic: "Nonlinear dynamics of stellar and planetary systems" Title: Exponential Divergence Speaker: Douglas Heggie, University of Edinburgh Abstract: The famous "butterfly effect" is expressed quantitatively by Liapounov exponents, which measure the rate of divergence of neighbouring solutions of a dynamical system. This lecture begins by reviewing the basic notions of this body of theory, and illustrates these in the context where so much non-linear dynamics was first developed: the N-body problem. The distinction between N=2 and N=3 illustrates the link between the Liapounov exponents and integrability. We illustrate the practical limitations imposed by looking at divergence over a finite time, and in the regime where the divergence can no longer be considered as small, where new results will be presented. Finally we consider the implications of exponential divergence in the realm of computer simulation. Contact: Fred Rasio 847-467-3419 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 05, 2002 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Losert: Control of binary alloy crystal growth patterns Description: Nonlinear Seminar Title: Control of binary alloy crystal growth patterns Speaker: Wolfgang Losert, University of Maryland Abstract: Alloys exhibit a large range of complex structures on a micrometer to mm scale, which influence important material properties such as strength or stiffness. Our approach to microstructure control is based on in situ dynamical selection of a pattern during crystallization. Using laser or UV light to locally perturb a transparent model alloy, we can alter the growth dynamics and guide the crystal toward new stable microstructures. For example, during directional solidification of a binary alloy, we can trigger a transition from a stable cellular microstructure to a stable dendritic microstructure. We also use perturbations to quantify, how stable a microstructure is. Theoretical calculations of response functions of the pattern and phase-field model simulations are key to interpreting our results. I will also show in-situ measurements of three-dimensional crystal growth patterns obtained with a two-photon confocal microscope. Contact: Paul Umbanhowar 847 467-7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: March 15, 2002 Time: 2:00 PM Location: Technological Institute M416 See the Evanston Campus Map Title: Tsimring: Noise-induced dynamics in bistable systems with... Description: IGERT Nonlinear Seminars Title: Noise-induced dynamics in bistable systems with delay Speaker: Lev Tsimring, Institute for Nonlinear Science, University of California, San Diego Abstract: In non-equilibrium systems, noise is known to produce surprising effects such as noise-induced phase transitions, stochastic resonance, etc. In this talk, we will consider noise-induced dynamics of a prototypical bistable system with delayed feedback. For small noise and magnitude of the feedback, the problem can be reduced to analysis of the two-state model with certain transition rates which depend on the earlier state of the system. In this two-state approximation, we find analytical formulae for the autocorrelation function, the power spectrum, and the linear response to a periodic perturbation. They show very good agreement with direct numerical simulations of the original Langevin equation. The power spectrum has a pronounced peak at the frequency corresponding to the inverse delay time, whose amplitude has a maximum at a certain noise level, thus demonstrating coherence resonance. The linear response to the external periodic force also has maxima at the frequencies corresponding to the inverse delay time and its harmonics. Contact: Paul Umbanhowar 847-7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: March 08, 2002 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Nicol: Euclidean extensions of dynamical systems Description: Nonlinear Seminar Title: Euclidean extensions of dynamical systems Speaker: Matt Nicol, Surrey Abstract: Recently there has been substantial progress in understanding the ergodic and mixing properties of generic group extensions of dynamical systems. Such models occur frequently in applications, especially physical systems modelled by PDEs with Euclidean symmetry. A group (G) extension of a base dynamical system consists of a map or flow on a manifold X, a skewing function g: X -> G and a skew-product map or flow on the product space X x G . We investigate the behaviour of E(n) (the Euclidean group of rotations, reflections and translation in n-dimensional space) extensions of various types of base dynamics on X. In particular we consider the topological and statistical properties of S E(n) extensions of periodic, quasiperiodic and chaotic dynamical systems. Contact: Paul Umbanhowar 467-7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: March 01, 2002 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Bronski: Passive Scalar Intermittency, Eigenvalue Asymptotic Description: Nonlinear Seminar Title: Passive Scalar Intermittency, Eigenvalue Asymptotics, and Small Ball Estimates for Fractional Brownian Motions Speaker: Jared Bronski, University of Illinois Urbana Champaign Abstract: Intermittency is one of the important phenomena in turbulence. Simply put intermittency is the fact that the probability distribution functions (PDF's) for quantities transported by a turbulent flow are asymptotically broad - wider than a Gaussian distribution. We present some work (with R.M. McLaughlin (UNC)) on a model of passive scalar intermittency originally due to Majda: \[T_t = \gamma(t) x \frac{\partial T}{\partial y} + \Delta T \] where $\gamma(t)$ is a random process, and $T$ is a passive scalar (for instance a dye) which is advected by the random (shear) flow. Majda was able to explicitly calculate moments of the distribution of the scalar $T$. McLaughlin and B. were able to calculate the large $N$ asymptotics of the moments of the distribution and thus calculate the width of the distribution of the quantity $T$. I will also talk about some recent work on a generalization of this model. A similar calculation can be done for this generalized model, which involves calculating the asymptotics of a certain eigenvalue problem. As a by-product of this calculation one finds the (previously unknown) optimal constants in a certain probabilistic "small ball" estimate for the probability that a fractional Brownian motion stays in a small ball in $L_2$. Contact: Bill Kath 491-8784 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 22, 2002 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Lathrop: Observing Intense Turbulent Vorticity and Strain Description: Nonlinear Seminar Title: Observing Intense Turbulent Vorticity and Strain Speaker: Daniel Lathrop, University of Maryland Abstract: Turbulent flows involve intense bursts of vorticity and independently intense events of strain(hyperbolic motions). We present new three-dimensional, non-intrusive, time-resolved measurements of a turbulent flow. Using a new optical method, we explore and characterize the interrelationship between dissipation (a measure of strain) and enstrophy (the square vorticity). Statistics of these quantities reveal the complex instabilities and saturation mechanisms in play in turbulent flows. An examination of individual events yields insight into the causes of intense events. Contact: Paul Umbanhowar 467-7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 15, 2002 Time: 2:00 PM Location: Technological Institute - Room M416 See the Evanston Campus Map Title: Ismagilov: Spatial Localization of Chemical Reactions... Description: IGERT Nonlinear Seminars Title: Spatial Localization of Chemical Reactions with Microfluidics Speaker: Rustem Ismagilov, University of Chicago Abstract: Complex chemical systems are composed of multiple chemical reactions that are maintained away from equilibrium and that interact non-linearly to give rise to new properties not present in isolated reactions. I would like to use microfluidics and microfabrication to control spatial localization of multiple chemical reactions in space and time, maintain them away from equilibrium, and control interactions among them. In this talk I will describe the first steps towards this goal: the use of multiple streams of laminar flow within a single microfluidic channel, and the use of crossings of two sets of parallel channels to localize chemical reactions. I will also describe two problems that we had to solve along the way -- lateral diffusion in laminar streams (with Abe Stroock and Howard Stone) and fluid flow through tangential channel crossings (with Wendy Zhang and Howard Stone). As a proof-of-principle experiment that shows that a pattern-forming system can be controlled with microstructures, I will describe control of Marangoni convective patterns by topographic perturbations of the heated surface over which the convection is occuring. Contact: Paul Umbanhowar 847-467-7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 08, 2002 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Greenside: The Existence and Properties of Spatiotemporal... Description: IGERT Nonlinear Seminar Title: The Existence and Properties of Spatiotemporal Chaos in a 2D Excitable Medium Speaker: Henry Greenside, Duke University Abstract: Excitable media such as heart tissue and chemical reaction-diffusion systems often have complex dynamical states involving many spirals or filaments that nucleate, interact, and annihilate over time. Such states raise challenging and interesting questions at the frontier of nonlinear dynamics such as how to characterize such states, are they chaotic, and are there universal features of the dynamics. I will discuss research by my group [*] that addresses some of these questions by numerical simulations of the Bar model, a model in which complex dynamics arises from repeated spiral instabilities caused by Doppler shifts. Our analysis shows that what seems to be chaotic behavior is actually long-lived transients whose average decay time grows exponentially with the system size. Although transient, the many-spiral states are well described as chaotic with a fractal dimension that grows extensively with the size of the system and whose value is related to the average density of spiral defects. Applications of these ideas to the analysis of surface voltage data on fibrillating hearts will be discussed. [*] "Size-dependent Transition to High-Dimensional Chaotic Dynamics in a Two-Dimensional Excitable Medium" by M. Strain and H. Greenside, Phys. Rev. Lett. 80(11):2307-2309 (1998). Contact: Hermann Riecke 847-491-8316 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 01, 2002 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Miller: Modeling a piece of the cerebral cortical circuit: Description: IGERT Nonlinear Seminar Title: Modeling the function and development of a piece of the cerebral cortical circuit: Cat V1 layer 4 Speaker: Ken Miller, University of California San Francisco Abstract: The primary visual cortex (V1) of the cat is perhaps the best-studied piece of the cerebral cortex. The cortical circuit is traditionally divided into six layers, with layer 4 specializing in receiving inputs from "lower" stations in the cortical hierarchy. In V1, layer 4 receives visual inputs from the lateral geniculate nucleus (LGN), which in turn receives the major output of the retina. Cells in LGN respond to light/dark edges but without distinction between edges of different orientations, whereas a given cell in layer 4 of V1 will respond only to a narrow range of orientations. The set of orientations to which a V1 cell will respond stays invariant under changes in stimulus magnitude or "contrast". Cells in LGN also respond well to fast-moving stimuli that do not activate V1 cells. We have developed a circuit model of layer 4 of V1 that accounts for a wide range of V1 response properties, including orientation preferences and their invariance under changes of stimulus contrast as well as speed preferences. Key to the model is a specific pattern of feedforward inhibition (LGN-driven, via cortical inhibitory interneurons) that counterbalances the direct LGN excitation. We show further that the model circuit will develop or "self-organize" from simple "Hebbian" rules of synaptic modification, under which the patterns of neural activity drive the modification of synapses.... CompleteAbstract Contact: Sara Solla 312-503-1408 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: January 25, 2002 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Zhang: Thread pinch-off: asymmetric cones in viscous flow. Description: IGERT nonlinear seminars Title: Thread pinch-off: asymmetric cones in viscous flow. Speaker: Wendy Zhang, University of Chicago Abstract: Dynamic singularities are ubiquitous. They arise in mathematical models of phenomena as grand as star formation or as familiar as the pinching of a thread of honey as it is being added to tea. Thread pinch-off allows one to study dynamics close to a singularity in a simple context which is also accessible to experiments. Recent works have revealed that a viscous thread close to pinch-off looks self-similar---the thread profile looks the same if the lengthscales are rescaled appropriately. Surprisingly, the presence of even small amounts of viscous dissipation in the surrounding can dramatically alter the self-similar profile. In particular, when no exterior viscous dissipation is present, the thread profile is symmetric about the point of pinch-off. When small amounts of exterior viscous dissipation are present, the thread profile becomes severely asymmetric. The precise origin of this asymmetry remains a puzzle. Contact: Paul Umbanhowar 847-467-7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: January 18, 2002 Time: 2:00 PM Location: Technological Institute - Room M416 See the Evanston Campus Map Title: Venkataramani: Crumpled sheets as a model system for... Description: IGERT Nonlinear Seminars Title: Crumpled sheets as a model system for studying multiple-scale behviors Speaker: Shankar Venkataramani, Universityof Chicago Abstract: There is much current interest in understanding multiple-scale behaviors in a variety of contexts ranging from composite materials to pattern formation to turbulence. In this talk, I will first give an overview of the mathematical issues that arise in the analysis of nonlinear systems displaying multiple scale behavior. I will then talk about elastic sheets, and describe a variety of problems as well as ideas that come up in the analysis of crumpling of elastic sheets. Finally, I will show how the analysis of the crumpling problem might generate techniques that are applicable to a range of other interesting problems. techniques that are applicable to a range of other interesting problems. Contact: Paul Umbanhowar 847-467-7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: January 11, 2002 Time: 2:00 PM Location: Technological Institute - Room M416 See the Evanston Campus Map Title: Kudrolli: Vortices and clustering in anisotropic and... Description: IGERT Nonlinear Seminar Title: Vortices and clustering in anisotropic and cohesive granular matter Speaker: Arshad Kudrolli, Clark University Abstract: Granular matter have been the topic of a considerable number of inter-disciplinary investigations because of the fascinating properties exhibited by them and their importance in industrial and geophysical situations. However most of the studies have focused on non-cohesive granular matter. We will discuss a series of experiments where attractive forces are also present between particles. If a small amount of liquid is introduced in a granular mixture, capillary bridges introduce attractive forces. We will discuss the effect of the cohesivity on the angle of repose and segregation of granular matter. In a second series of experiments, we will discuss the effect of magnetizing the granular particles. The experimental system consists of magnetized steel beads that are supplied energy by vertical vibrations. For high amplitudes of vibration, a gas of beads is observed, but as the amplitude is lowered, the magnetized particles are observed to form clusters. We use high speed imaging to measure the positions to understand the structure of the clusters. Time permiting we will also discuss the observation of vortex patterns in granular rods vertically in a container. Above a critical packing fraction, moving domains of nearly vertical rods are spontaneously observed to form which coexist with horizontal rods. Contact: Paul Umbanhowar 847-467-7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: December 07, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Glazier: Foams in Two and Three Dimensions Description: Nonlinear Seminar Title: Foams in Two and Three Dimensions Speaker: James A. Glazier, University of Notre Dame Abstract: We believe that we understand, much of the behavior of two dimensional foams. In particular, we have simple laws governing their kinetics, their topological and area distributions, and the existance of a scaling state. We can simulate foam evolution with high accuracy using the Potts model. In three dimensions the picture is much less clear. However, we are beginning to be able to use Magnetic Resonance Imaging to directly answer these questions. Besides the properties of so-called 'ideal' foams, we are now able to simulate complicating phenomena like irregular initial conditions, gravitational drainage and shear flow. Contact: Paul Umbanhowar 467-7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: November 30, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Abarbanel: Biological and Electronic Neurons: Expts. & Thry Description: Nonlinear Seminar Title: Biological and Electronic Neurons: Experiments and Theory Speaker: Henry Abarbanel, University of California, San Diego Abstract: The action potential dynamics of individual neurons underlies the way neurons communicate and biological nervous systems compute. The potnetial number of degrees of freedom expressed in these oscillations can be large, but experiments in our labs have shown that typically only three to five degrees of freedom are active. We discuss how we establish this from experimental data, the role of chaos in neural oscilaltions, the modeling of these neurons in numerical and electronic embodiments, and ways to test these models in a realistic fashion. Contact: Hermann Riecke 491-5396 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: November 16, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Stefanella Boatto : Dynamics of point vortices in the... Description: Nonlinear Seminar Title: Dynamics of point vortices in the plane and on the sphere: the non-linear stability of relative equilibria Speaker: Stefanella Boatto , Institute of Celestial Mechanics and Institute Henri Poincare, Paris Abstract: In 1883, while studying the atomic structure, J.J. Thomson investigated the linear stability of corotating point vortices in the plane. His interest was in configurations of identical vortices equally spaced along the circumference of a circle, i.e. located at the vertices of a regular polygon. He proved that for six or fewer vortices the polygonal configurations are stable, while for seven vortices - the Thomson heptagon - he erroneously concluded that the configuration is slightly unstable. It took more than a century to make some progress on this problem! In 1985, Dritschel proved that the Thomson heptagon is neutrally stable and that for eight or more vortices the corresponding polygonal configurations are linearly unstable. Recently (1999) Cabral & Schmidt proved that for seven or fewer vortices the polygonal configurations are non-linearly stable in the plane. For the spherical case, results are much more recent! In 1993 Dritschel & Polvani determined the ranges of linear stability - in terms of the latitude- of polygonal configurations. By a method similar to that used by Dritschel in the planar case, Dritschel & Polvani showed that at the pole, for N<7 the configuration is stable, for N=7 it is neutrally stable and for N>7 it is unstable. In 1998 Marsden & Pekarsky proved that for N=3 the range of non linear stability is the whole sphere (including results for vortices with different vorticities k1, k2 and k3). Cabral and I (2001) determined the ranges of non-linear stability for all N. Contact: Paul Umbanhowar 467-7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: November 06, 2001 Time: 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Ruelle: Positivity of Entropy Production for Quantum Spin... Description: Nonlinear Seminar Title: Positivity of Entropy Production for Quantum Spin Systems Speaker: Prof. David Ruelle, IHES, France Special Note: Please note unusual date and time Abstract: We consider a quantum spin system consisting of a finite subsystem connected to infinite reservoirs at different temperatures. In this setup we define nonequilibrium steady states and prove that the rate of entropy production in such states is nonnegative. Then we discuss the definition of entropy production, and see that there are physical questions which remain to be clarified. Contact: Amie Wilkinson 847-491-5486 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 26, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Wittenberg: Characterizing spatiotemporal chaos in the... Description: Nonlinear Seminar Title: Characterizing spatiotemporal chaos in the Kuramoto-Sivashinsky and related equations Speaker: Ralf Wittenberg, University of Michigan Abstract: We discuss some aspects of spatiotemporal chaos (STC) in the one-dimensional Kuramoto-Sivashinsky (KS) equation. In particular, we will describe how a wavelet projection may be used to elucidate features of the dynamics, and we will discuss some numerical experiments to probe the localization of the dynamics in space and scale. These experiments have motivated the construction of relatively low-dimensional, spatially localized models for a minimal "chaotic box", in the form of externally excited, periodized wavelet projections of the KS evolution equation. We will particularly comment on the importance of the large scales in maintaining the spatiotemporal disorder, and illustrate this with two other examples: The KS equation in the presence of an additional destabilizing linear term displays a transition from STC to a stationary shock-like solution, due to excitation at the large scales. We will also briefly discuss a sixth-order analogue of the KS equation in which STC is maintained by the coupling to large scales. Contact: Hermann Riecke (847) 491-3345 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 19, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Spiegel: Macroscopic Equations for Rarified Gas Dynamics Description: Nonlinear Seminar Title: Macroscopic Equations for Rarified Gas Dynamics Speaker: Edward Spiegel, Columbia University Abstract: A modification of the usual approach to deriving the continuum equations from kinetic theory will be presented, that leads to a set of equations different from the Navier Stokes (NS) equations. At small Knudsen numbers the NS equations are recovered, but for Knudsen numbers of order unity and larger, the proposed equations provide better agreement with experimental data on shock thickness and ultrasonic wave speed. Contact: Sandip Ghosal 467-5990 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 12, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Huepe: Finite-time blowup dynamics Description: Nonlinear Seminar Title: Finite-time blowup dynamics: A study on a "simple" system Speaker: Cristian Huepe-Minoletti, University of Chicago Abstract: Finite-time blowup behaviors appear in several physical realizations. These range from the pinch-off occurring when a fluid neck breaks to the gravitational collapse of a star into a black hole. I will present a study on the dynamics of the finite-time blowup solutions of a parabolic-elliptic system of partial differential equations. The goal is to understand how the system converges to a self-similar profile. The analysis of this relatively simple system should help us in developing techniques and intuition to approach other problems presenting finite-time singularities. Contact: Mary Silber 847-491-8782 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: September 28, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Dubovski: New Mathematical Models in Coagulation Kinetics Description: Nonlinear Seminar Title: New Mathematical Models in Coagulation Kinetics Speaker: Pavel Dubovski, Moscow Abstract: We consider two different mechanisms of particle interaction. The first mechanism yields the celebrated M. Smoluchowski equation, the second one leads us to a new kinetic model. It turns out that the new model is a discrete version of the Oort-Hulst-Safronov coagulation equation. We compare two models (Smoluchowski and new) both analytically and numerically and establish their similarity. This explains why Smoluchowski model describes the droplet interaction in atmospheric clouds in the right way. For the coagulation-fragmentation model we derive the fluid dynamic limit. Contact: Sasha Golovin 491-5346 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: June 01, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Tobin Driscoll : Fast simulation of semilinear... Description: Nonlinear Seminar Title: Fast simulation of semilinear time-dependent PDEs Speaker: Tobin Driscoll , University of Delaware Abstract: Many PDEs arising from the mathematical modelling of nonlinear phenomena have a special semilinear structure in which the highest spatial derivative appears only linearly. Well-known examples come from optics (cubic Schroedinger), water waves (Korteweg-de Vries, KP), fluid mechanics (Navier-Stokes), chaos (Kuramoto-Sivashinsky), self-organization (Cahn-Hilliard, Gray-Scott) and other fields. Numerical simulation of these models is an indispensable tool for their study. However, "textbook" methods can be unacceptably slow due to the presence of the high-order derivatives. In recent years a number of strategies for circumventing this problem have emerged. Many of the most successful techniques use a time integration method that varies with the local Fourier wavenumber. Although an ideal, general-purpose method has not yet been created, we are getting pretty close. In this survey I will introduce the major features of the problem, describe the important wavenumber-dependent methods, and show comparisons and experimental results. Beyond a very basic understanding of generic multistep and Runge-Kutta methods for ODEs, no numerical expertise will be assumed. Contact: Joanne Benge (847) 491-3904 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 25, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Shariff: Non-linear Lessons from Axisymmetric Vortex Rings Description: Nonlinear Seminar Title: Non-linear Lessons from Axisymmetric Vortex Rings Speaker: Karim Shariff, NASA Ames Research Center Abstract: The talk will present two types of phenomena, both recognizable to students of nonlinearity, that are exhibited by axisymmetric vortex rings in numerical and laboratory experiments. (1) The first type of phenomenon is reminiscent of inelastic solitons and is illustrated by the following examples: (a) A perturbed Hill's spherical vortex sheds a tail and returns to Hill's vortex. (b) The core shapes of two thin rings colliding head-on evolve through a sequence of shapes that happens to be Pierrehumbert's family of _planar_ steadily translating vortex pairs. This continues until the limiting member of the family is reached. Thereafter, the core shapes continue to approximately maintain this shape while continually depositing vorticity into a tail. (c) Fat rings that collide head-on, flatten but then the vortex cores grow a head in the shape of Pierrehumbert's limiting vortex pair. (d) When one tries to create a vortex ring at the edge of a pipe by pushing a piston forward, there is a limiting piston stroke length beyond which the vortex ring refuses to grow any larger. It then propagates away leaving a train of smaller rings behind. It is shown (heuristically) that the limiting stroke occurs when the apparatus is no longer able to keep ejecting energy at a rate compatible with the requirement (due to Kelvin) that a steadily translating vortex have maximum energy with respect to impulse preserving isovortical perturbations. (2) The second phenomenon is the heteroclinic tangle. Some vortex ring motions are periodic in time and therefore create a time periodic velocity field. The motion of fluid particles are solutions to a 2 dof system with a time periodic Hamiltonian. Contact: Joanne Benge 847-491-3904 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 18, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Movellan: Information integration and models of perception Description: Nonlinear Seminar Title: The Morton-Massaro Law of Information Integration: Implications for Models of Perception Speaker: Javier Movellan, UCSD Abstract: Information integration may be studied by analyzing the effect of two or more sources (e.g., auditory and visual) on subjects' responses. Experiments show that ratios of response probabilities often factorize into components selectively influenced by only one source (e.g., one component affected by the acoustic source and another one affected by the visual source). We call this the Morton-Massaro law (MML). We find conditions where the law is optimal and note that it reflects an implicit assumption about the statistics of the environment. Adherence to the MML can be used to assess whether the assumption is being made, and analyses of natural stimuli can be used to determine whether the assumption is reasonable. An important issue raised by the Morton--Massaro law is its potential incompatibility with interactive models of perception, i.e, models in which all the processing units may be coupled via feed-back and lateral connections. We present a class of interactive models governed by stochastic differential equations, and show that the Morton--Massaro law does not rule out either feed-back or lateral connections. Instead it is related to an architectural constraint that we called channel separability. We show how this constraint can be used to analyze how neurons integrate the effects of stimulus (classical receptive field) and context (non-classical receptive field). Contact: Joanne Benge 847-491-3904 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 11, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Zador: Computation in auditory cortex:cocktail party problem Description: Nonlinear Seminar Title: Synaptic mechanisms underlying computation in the auditory cortex in vivo: The cocktail party problem Speaker: Tony Zador, Cold Spring Harbor Laboratory Abstract: The neurons that provide the substrate for cortical computation appear quite noisy (although just how noisy remains a subject of some controversy), suggesting that the cortex uses a strategy rather different from that of a digital computer. The cortex is nevertheless able to solve hard computational problems, such as the cocktail party problem (an auditory analog of the visual scene segmentation problem), that remain beyond the capacity of the fastest computers. We have been combining experimental and theoretical approaches to study the special characteristics of cortical "wetware" that endow it with the capacity to solve such hard problems. I will discuss some recent experiments where we have used in vivo whole cell patch clamp recordings to study how acoustic signals are represented in the rodent primary auditory cortex. Contact: Joanne Benge 847-491-3904 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: May 04, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Fetz: Neural computation in dynamic recurrent networks Description: Nonlinear Seminar Title: Neural computation in dynamic recurrent networks Speaker: Eberhard Fetz, University of Washington Abstract: Neural network modeling represents a significant tool for systems neurophysiologists to investigate the mechanisms underlying computation in complex networks. Recordings of neurons in behaving animals have provided useful insights, but never lead to complete network solutions because the connectivity of the recorded cells is inevitably missing. Dynamic recurrent neural networks can provide complete model solutions that simulate behaviors; all the connections and activities are known and available for analysis. Such networks can be derived simply from examples of the behavior by gradient descent methods and can incorporate many physiological and behavioral "constraints". This talk will review some basic applications of dynamic recurrent neural networks to model premotor circuits of the primate, oscillating networks, integrators and differentiators, instructed delay tasks and short-term memory. Issues include [1] simulation of lesions and stimulation, [2] pruning techniques to derive minimal functional networks, [3] generalization beyond the training set, and [4] converting networks of sigmoidal units with continuous activation functions (necessary for analytic derivation of connectivity) into networks of integrate-and-fire units performing the same functions. Contact: Sara Solla 847-467-5080 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 27, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Nagel: Jamming Description: Nonlinear Seminar (jointly sponsored by Chemical Engineering) Title: Jamming Speaker: Sidney Nagel, University of Chicago Abstract: Jamming occurs in a wide variety of situations. Normally one thinks of traffic jams on a highway or the jamming that occurs when solid particles become impacted on leaving an orifice. I will argue that the transition from a flowing to a jammed state may be similar in many respects to other situations as well. The case I have in mind is the glass transition where a liquid becomes progressively sluggish as the temperature is lowered until it eventually becomes a glass where it stops moving entirely. Contact: Joanne Benge 847-491-3904 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 20, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Shraiman: Model Genetic Networks and Principles of Robust... Description: Nonlinear Seminar Title: Model Genetic Networks and Principles of Robust Transcription Control Speaker: Boris Shraiman, Bell Labs Abstract:Normal function and development of cells and organisms depends on the timely expression of appropriate genes. This process is controlled by the interactions of transcription factor proteins with regulatory segments of DNA which form links in the genetic network governing cell function. One of the remarkable properties of this system is its stability with respect to mutations. What are the principles underlying robust transcription control and governing the organization of such networks? The talk will address this question on a basis of a model network, then discuss avenues for comparing theoretical predictions with experimental data for the relatively simple example of E.coli. Contact: Paul Umbanhowar 847-467-7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 13, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Calabrese: Modeling and Experiments on Intersegmental... Description: Nonlinear Seminar Title: Modeling and Experiments on Intersegmental Coordination in the Leech Heartbeat Central Pattern Generator Speaker: Ron Calabrese, Emory University Abstract: A network of interneurons paces rhythmic activity in motor neurons that control the two longitudinal heart tubes of the leech. Pairs of mutually inhibitory interneurons in the 3rd and 4th segmental ganglia produce alternating rhythmic activity that paces activity in the interneuronal network. These two segmental oscillators (G3 and G4) are coordinated by two pairs of coordinating interneurons that originate in the 1st and 2nd ganglia. We investigated the mechanisms through which phase relations of the oscillators are established by the coordinating interneurons using a combination of modeling and experiments. In isolated nerve cords, the two segmental oscillators maintain a constant phase relation. To assess the period of the individual oscillators, we uncoupled them by reversibly blocking axonal conduction in the coordinating neurons (using sucrose solution). Upon recoupling, we found that the faster of the two oscillators led in phase and that the phase difference was proportional to the period difference between the oscillators. The period of the coupled system was the period of the faster of the two oscillators. These relations persisted even when the period differences was artificially enhanced by bathing one of the ganglia in an acceleratory modulator (myomodulin) or a decceleratory ion (Cs+ - probably acts by blocking h-current). Contact: Joanne Benge 847-491-3904 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: April 06, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Abbott: The effects of noisy synaptic input on neuronal... Description: Nonlinear Seminar Title: The effects of noisy synaptic input on neuronal responses Speaker: Larry Abbott, Brandeis University Abstract: Neurons in vivo typically receive a continuous barrage of both excitatory and inhibitory input. How does this affect their responses to stimuli? In a combination of modeling studies and in vitro recordings, we have found that background input, consisting of balanced excitatory and inhibitory components, can change the gain of neuronal responses. Furthermore, it allows neurons to accurately reflect the temporal dynamics of their inputs, even up to very high frequencies. These results suggest that neurons receive two classes of inputs; one class in which excitation and inhibition act in opposition to generate selective responses to various aspects of a stimulus, and another class in which excitation and inhibition work together to control the gain or amplitude of the selective response. Contact: Joanne Benge 847-491-3904 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: March 30, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Lee: Independent Component Analysis for Encoding Description: Nonlinear Seminar Title: Independent Component Analysis For Encoding Sensory Information Speaker: Te-Won Lee, University of California at San Diego Abstract: Independent Component Analysis (ICA) is a method for extracting independent sources given only mixtures of the unknown sources. This method has a wide range of applications in signal- and image processing and it has been suggested that ICA may reveal strategies for encoding sensory information in the brain. The goal of my talk is first to give an introduction to ICA with applications in signal and image processing and second to present recent results in using ICA for encoding sensory information such as speech signals and color images of natural scenes. I will briefly summarize the basic ICA assumptions and algorithms. This method is then applied to biomedical signals and images including EEG and fMRI data. The results show that ICA can separate weak brain signals in a noisy recording. Since ICA can be seen as a density estimator, it can be used to learn statistical features or codes for sensory data such as speech signals or images of natural scenes. The learned basis functions for speech signals can be used as features to improve the encoding process for a speech recognition system. In analyzing hyperspectral data of natural scenes, the learned basis functions resemble human cone sensitivities. The learned basis functions for color images show strong color opponency and result in significant redundancy reduction (data compression) that may reveal a strategy for efficiently encoding spatial and spectral information in the brain. A newly developed algorithm called the generalized Gaussian mixture model uses ICA and allows unsupervised classification as well as feature extraction. A few examples are presented for image classification and segmentation. Contact: Joanne Benge 847-491-3904 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: March 09, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Ermentrout: The excited cortex: phosphenes and other ... Description: Nonlinear Seminar Title: The excited cortex: phosphenes and other visual confections Speaker: Bard Ermentrout, University of Pittsburgh Abstract: We suggest that because the visual cortex sits in a nearly balanced state it is sensitive to a variety of external stimuli. We first discuss "LSD trails" which appear to arise due to a diminished amount of inhibition. We show that a spatially distributed neural network can produce sustained transient responses reminiscent of the responsed of disinhibited cortex. Next we discuss deep-pressure phosphenes which arise when subject press deeply on their eyeballs. We suggest that this phenomena arises due to asymmetric feedforward inputs to the excitatory and inhibitory neurons. Finally, we describe a model for flicker phosphenes - colorful spatial patterns that arise through parametric excitation of a cortical network. Contact: Joanne Benge 847-491-3904 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: March 07, 2001 Time: 4:00 PM Location: Tech M416 See the Evanston Campus Map Title: Ermentrout: Coupled oscillator arrays Description: Nonlinear Seminar Title: Coupled oscillator arrays Speaker: Bard Ermentrout, University of Pittsburgh Special Note: Introductory Lectures for the Nonlinear Seminar on March 9 aimed specifically at Graduate Students. The lecture will be video-taped. Abstract: This will be in some sense a continuation of the first talk in that I will focus specifically on a class of reduced models which arise with weakly coupled oscillators. I will use the reduction to first explain a psychophysical experiment in finger tapping. I will then briefly discuss 1-dimensional oscillator chains and applications to the olfactory lobe of the mollusk. I then describe general results in two- and three dimensions. The results in two-dimensions may apply to certain classes of waves seen in imaging of cortical networks. Contact: Joanne Benge 847-491-3904 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: March 06, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Bodenschatz: Why would a mosquito... CANCELLED Description: Nonlinear Seminar Title: Why would a mosquito care about turbulence? Speaker: E. Bodenschatz, Cornell University Special Note: DUE TO FLIGHT CANCELLATION THIS TALK HAS TO BE CANCELLED, UNFORTUNATELY. Abstract: We are reporting on Lagrangian particle tracking in a high Reynolds number turbulent flow. We are using silicon strip detectors developed for the CLEO3 detector at Cornell's e+e- collider. This detector allows us to follow particles in the flow at submicron resolution at a rate of 70,000 positions per second. We report results on particle accelerations and show that the acceleration is a very intermittent quantity and that variance scales according with the predictions of Kolomogrov. Particle size effects are also discussed. See http://www.nature.com/nsu/010301/010301-4.html Contact: Joanne Benge 847-491-3904 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: March 05, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Ermentrout : From Bowers House to our House: ... Description: Nonlinear Seminar Title: From Bowers House to our House: Simplifying neural networks. Speaker: Bard Ermentrout , University of Pittsburgh Special Note: Introductory Lectures for the Nonlinear Seminar on March 9 aimed specifically at Graduate Students. The lecture will be video-taped. Abstract: In this talk, I will describe a number of useful strategies for the reduction of biophysically based neurons to simple units that are more suitable for inclusion into large networks and which also have the benefit of allowing some analytic insights. I will first describe the form of biophysical models and synapses. Next I will use techniques from applied mathematics including average and bifurcation to reduce the dimensions of these models. I will compare solutions of the reduced models to the full models. Contact: Hermann Riecke 847-491-8316 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: March 01, 2001 Time: 4:00 PM Location: Tech M416 See the Evanston Campus Map Title: Socolar: Stabilization (or not) of periodic orbits via ... Description: Nonlinear Seminar Title: Stabilization (or not) of periodic orbits via time-delay feedback Speaker: Joshua Socolar, Duke University Special Note: Note unusual day Abstract: The idea of controlling chaos depends upon the feasibility of stabilizing periodic orbits embedded in an attractor. In certain situations, conventional control methods for stabilization are difficult to implement and time-delay techniques may be useful. I will discuss studies of the domains in parameter space over which time-delay techniques can work, in principle, for systems with just a few degrees of freedom, for the complex Ginzburg-Landau equation in 1D and 2D, and for related equations relevant for optical and electronic devices. The presence of zero-torsion instabilities prevents the extension of the 1D success to 2D. Contact: Joanne Benge 847-491-3904 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 23, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Pouget: Efficient computation and cue integration with... Description: Nonlinear Seminar Title: Efficient computation and cue integration with noisy population codes Speaker: Alexandre Pouget, University of Rochester Abstract: The brain represents sensory and motor variables through the activity of large populations of neurons. An important problem in neuroscience is to understand how the nervous system computes with these population codes, given that individual neurons are noisy and thus unreliable. I will focus on two general types of computation: function approximation and cue integration. I will show in particular that a class of neural networks, known as basis function networks with multidimensional attractors, can perform both types of computation optimally with noisy neurons. The architecture of these networks follows closely the one observed in the cortex, in that the networks use population codes and rely on feedforward, feedback and lateral connections to integrate information across layers. Moreover, neurons in the intermediate layers show response properties similar to those observed in several multimodal cortical areas. Thus, basis function networks with multidimensional attractors may be used by the brain to compute efficiently with population codes. Contact: Sara Solla 847-467-5080 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 16, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Chow: Collective dynamics of coupled neurons Description: Nonlinear Seminar Title: Collective dynamics of coupled neurons Speaker: Carson C. Chow, University of Pittsburgh Abstract: It is known that certain modes in the firing activity of neurons in the brain are correlated with behavioral and cognitive states. These various modes can include global synchronous oscillations over a range of frequencies, asynchronous stochastic firing, propagating waves, and localized self-sustained firing. Two goals (of many) in neuroscience are to understand the functional significance of these states and the neural mechanisms responsible for goals (of many) in neuroscience are to understand the functional significance of these states and the neural mechanisms responsible for their generation. While the first goal remains mostly within the realm of speculation, the second is starting to become unraveled. I will summarize some recent numerical and analytical results on the mechanisms of synchronization and self-sustained firing in a network of spiking neurons and discuss some possible implications to function. Contact: Paul Umbanhowar 847-467-7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 09, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Wayne: Counter-propagating waves on fluid surfaces and... Description: Nonlinear Seminar Title: Counter-propagating waves on fluid surfaces and the continuum limit of the Fermi-Pasta-Ulam model Speaker: Gene Wayne, Boston University Abstract: Recently methods have been developed with permit one to rigorously justify the use of ``amplitude'' or ``modulation'' equations that arise in a wide variety of physical contexts. In particular, Guido Schneider and I have recently shown that over the time and length scales commonly used to derive long-wave equations for fluid surfaces, the irrotational motion of an incompressible, inviscid fluid of finite depth can be described by a pair of uncoupled Korteweg-de Vries equations. In this talk I will review this result, describe the general method of proof, and apply this method in another context to show that in an appropriate scaling, the motion of the Fermi-Pasta-Ulam model of coupled, nonlinear oscillators can also be approximated by a pair of uncoupled KdV equations. Contact: Mary Silber 847-491-8782 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 07, 2001 Time: 4:00 PM Location: Tech M416 See the Evanston Campus Map Title: Egolf: Far-from-equilibrium, Extended, Chaotic Systems Description: Nonlinear Seminar Title: Toward a Predictive Theory of Far-from-equilibrium, Spatially Extended, Chaotic Systems Speaker: Prof. D. Egolf, Georgetown University Special Note: Note unusual time. Abstract: Systems maintained far-from-equilibrium often exhibit an intriguing persistent time dynamics. Whereas the chaotic dynamics of systems with only a few degrees of freedom is largely understood (or at least is well-characterized), more complex systems with large numbers of interacting degrees of freedom (heart tissue, fluid turbulence, planetary atmospheres and oceans, ecosystems, etc.) have proven quite difficult to understand despite abundant experimental and numerical data. Over the past 20 years, many researchers have focused on nonequilibrium, pattern-forming systems as test-beds for understanding these complicated systems. I will discuss two recent, exciting discoveries about these systems: a space-time localized mechanism for the generation of chaotic disorder and an equilibrium statistical mechanical description of a far-from-equilibrium system, and I will speculate about the broader applicability of the techniques behind the discoveries and the hope for a more general understanding of far-from-equilibrium systems. Contact: Hermann Riecke 847-491-8316 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: February 02, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Becker: A neural model of hippocampal-parietal... Description: Nonlinear Seminar Title: A neural model of hippocampal-parietal interactions in spatial memory Speaker: Suzanna Becker, McMaster University Abstract: In this talk, I will describe a computational model of the neural mechanisms in the parietal and temporal lobes that support spatial navigation, imagery, and episodic recall. Long-term allocentric representations of scenes are stored in the hippocampus and associated with object and texture information in the surrounding medial temporal lobe. Viewpoint-dependent representations are generated in the parietal part of the model to enable construction of an imagined retinotopic scene and generation of appropriate body movements. Bidirectional interactions between the hippocampal and parietal modules allows translation between allocentric and egocentric representations, with dynamical interactions between recalled episodic memories and viewer-centered mental images. Damage to the parietal part of the model produces symptoms of hemispatial neglect, including neglect in mental imagery that rotates with the imagined perspective of the observer, as in the famous Milan Square experiment with parietally lesioned patients reported by Bisiach and Luzatti (1978). This is joint work with Neil Burgess at UCL. Contact: Sara Solla 847-467-5080 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: January 26, 2001 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Quake: Single Molecule Biophysics: FemtoNewton Force... Description: Nonlinear Seminar Title: Single Molecule Biophysics: FemtoNewton Force Spectroscopy, Microfluidics, and Nanophotonics Speaker: Stephen Quake, Caltech Abstract: Thanks to recent advances in physics and chemistry it is now possible to image, manipulate and study single molecules. These new techniques are being used to investigate a diverse set of scientific problems, ranging from fundamental questions about the behavior of polymers to the development of sensitive new tools for nanofabrication and biological analysis. I will survey some projects in my group which demonstrate the power of single molecule science, ranging from single molecule force spectroscopy with femtoNewton resolution to the fabrication of nanometer scale optical devices. Contact: Annelise Barron 847-491-2778 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: December 08, 2000 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Nonlinear Models of Dynamics in Machining Processes Description: Title: Nonlinear Models of Dynamics in Machining Processes Speaker: Emily Stone, Utah State University Abstract: In this talk I will discuss research on the chatter instability in high-speed machining. Chatter is a self-excited oscillation between the machine tool and the workpiece that limits productivity of machining operations, reduces the quality of the product and shortens machine tool life. Up until recently all models of chatter have been linear, with delay effects in the case of regenerative chatter. These models only partially explain the instabilities observed in the machining process. In aircraft manufacture drilling is a critical machining process: over a million holes may be drilled in one wing of a commercial passenger jet. To address the problem of chatter in drilling, we are developing a suite of nonlinear models of metal cutting that can be merged with finite element studies of drill vibration modes and informed by large scale simulations of metal cutting operations. Typically, engineering studies of chatter have restricted themselves to the question of linear stability of a steady cutting solution; in addition to that we are studying the effects of the nonlinear terms in the model on the resulting dynamics. Ultimately contact will be made with laboratory results from experiments conducted in Seattle and St. Louis, with the ultimate goal of directing tool design and allowing machine operators to avoid chatter regimes in drilling. Contact: Mary Silber 847-491-8782 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: December 01, 2000 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Shaken Rayleigh-Benard Convection Description: Title: Shaken Rayleigh-Benard Convection Speaker: Jeffrey Rogers, Georgia Tech Abstract: Results from the first experimental study of thermal convection in the presence of modulated acceleration are reported. Predictions of enhanced conduction stability as well as flows in harmonic and subharmonic resonance to the modulation frequency are confirmed. Each temporal response displays a distinct characteristic spatial scale and examples of typical states are presented. In addition, the first observations of superlattice patterns in thermal convection are made over a parameter range where the two scales compete and coexist. A 4-wave resonance mechanism that is qualitatively different from the resonant triads responsible for complex-ordered patterns previously reported in other nonequilibrium systems selects the superlattice structure. Simulations are found to be in quantitative agreement with laboratory experiments. Additionally, numerical results show that the superlattices bifurcate directly from conduction at a codimension-two point. Contact: Hermann Riecke 847-491-8316 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: November 17, 2000 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Stress Structure and Fluctuation in Granular Matter Description: Title: Stress Structure and Fluctuation in Granular Matter Speaker: Dr. Dan Howell, Argonne National Laboratory Abstract: Granular systems have captured much recent interest because of their rich phenomenology and important applications. They exhibit dynamical states that resemble conventional thermodynamic phases (solid,liquid,gas), but lie outside the realm of equilibrium statistical mechanics. The challenge is to create new kinds of statistical physics and new analytical descriptions for the mean and fluctuating behavior of these materials. I will focus on two experiments used to examine force propagation and fluctuations in static and slowly driven granular systems. Both involve the stress structure; the first explores the stress imposed during construction of static piles, the second probes the fluctuations in this structure, and focuses specifically on a transition with density found in a slowly sheared material. Contact: Paul Umbanhowar 847-467-7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: November 10, 2000 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Interfacial fluid mechanics in microgeometries Description: Title: Interfacial fluid mechanics in microgeometries Speaker: Professor G.M. Homsy, Stanford University Abstract: The flow of fluids in microgeometries is important in many emerging technologies, including high speed spin coating in IC fabrication, coating over topographical features and imperfections for ultrathin lithographic applications, and motion and manipulation of dispersed bubbles in MEMS devices. These in turn lead to a class of fundamental fluid mechanics problems in which interfaces and interfacial forces play a major role. In this talk, I will discuss some of recent theoretical and experimental work in these areas, including instability of dynamic contact lines, flow over topography, optimal levelling of films by Marangoni stresses, and thermocapillary migration of bubbles in microchannels. Contact: B.J. Matkowsky 847-491-5396 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: November 03, 2000 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Comparing the Low Energy Landscape of a Quantum and... Description: Nonlinear Seminar Title: Comparing the Low Energy Landscape of a Quantum and a Classical Spin Glass Speaker: Prof. Susan Coppersmith, University of Chicago Abstract: The number of ground states of a classical Edwards-Anderson $\pm J$ spin glass grows exponentially with the system size. We calculate how a quantum tunneling term breaks this huge degeneracy in a two-dimensional system. The lowest energy states differ qualitatively from those of a classical system in which the magnetic degrees of freedom are coupled to lattice distortions. We relate our results to recent experiments by Aeppli, Rosenbaum, and collaborators probing the behavior of the disordered magnet $LiY_xHo_{1-x}F_4$ in a transverse magnetic field. Contact: Paul Umbanhowar 847-467-7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 31, 2000 Time: 1:30 PM Location: Tech M416 See the Evanston Campus Map Title: Acoustic Chaos Description: Title: Acoustic Chaos Speaker: Clive Ellegaard, Niels Bohr Institute Special Note: Note special date and time. Abstract: We use high resolution acoustic spectroscopy as an analog system to quantum systems. We study transitions between regularity and chaos and specific symmetry breaking. We compare with the predictions of Random Matrix Theory that was developed for the study of the complicated compound nuclear states. The acoustic system is interesting in its own right. It is governed by a vectorial wave-equation in contrast to the scalar Schroedinger equation, and we study specific features arising from this fact. Contact: Predrag Cvitanovic 847-491-3235 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 27, 2000 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Shock waves in supersonic sand Description: Title: Shock waves in supersonic sand Speaker: Dr. Chris Bizon, Colorado Research Associates Abstract: Granular media, such as sand, will flow like a fluid when sufficiently driven by an external force. Because collisions between grains are inelastic, granular temperature (defined in analogy with temperature as the width of the velocity distribution) decreases with time. Since the speed of sound in a fluid goes as the square root of its temperature, the speed of sound in a granular flow will decrease. Eventually, then, most granular flows enter a supersonic regime; shocks will be endemic. We report on the formation of shock waves in granular flows. Grains are poured into a quasi-two-dimensional vertical channel and flow downward, accelerated by gravity. Obstacles placed in the channel produce oblique shock waves analogous to those that form near obstacles in supersonic gas flows. Because these shocks are stationary and have a simple geometry, they provide ideal test beds for theories of rapid granular flow. Contact: Paul Umbanhowar 847-467-7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 13, 2000 Time: 2:00 PM Location: Tech M416 Title: Renormalization for stability analysis of Hamiltonian flows Description: Title: Renormalization for the analysis of stability in Hamiltonian flows Speaker: Professor C. Chandre, CEA Saclay Abstract: One of the main mechanism for the loss of stability in Hamiltonian flows is the break-up of invariant tori which plays an essential role in the large-scale dynamics. The aim of our approach is to determine the threshold of the break-up of invariant tori and its mechanism, by renormalization which is analogous to what has been done in the study of phase transition in statistical mechanics. The idea is to construct a renormalization transformation by a series of canonical changes of coordinates (performed on the Hamiltonian). This transformation combines an elimination of some non-resonant modes of the Hamiltonian (by perturbative techniques), with a rescaling of phase space that treats specifically the resonant modes of the Hamiltonian. This transformation acts as a microscope in phase space, i.e. it looks at the system at a smaller scale in phase space and at a longer time scale. From this renormalization transformation, one can deduce whether or not a torus exists, and the properties of critical tori (at the threshold of the break-up). For Hamiltonian systems with two degrees of freedom, the break-up of invariant tori is a universal mechanism and all the critical tori have statistically the same structure at all scales. The properties at criticality are deduced from a critical strange attractor of the renormalization transformation. Contact: P. Umbanhowar 467-7291 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: October 06, 2000 Time: 2:00 PM Location: Tech M416 See the Evanston Campus Map Title: Global secular dynamics in the planar three-body problem Description: Title: Global secular dynamics in the planar three-body problem Speaker: Professor Jacques Fejoz, Northwestern Abstract: The dynamics of the three-body problem in classical mechanics can be split into two parts: a fast, Keplerian dynamics, which describes the motion of the bodies along three ellipses as if each body underwent the attraction of only one fictitious center of attraction; and a slow, secular dynamics, which describes the deformations of these Keplerian ellipses, due to the fact that each body actually undergoes the attraction of the other two. I will show how to study this secular dynamics by building the so-called secular systems and applying some sophisticated version of KAM theorem. Contact: Hermann Riecke 847-491-8316 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

Date: September 29, 2000 Time: 2:00 PM - 3:00 PM Location: Tech M416 See the Evanston Campus Map Title: Interdisciplinary Seminar in Nonlinear Science Description: Title: Simulations of Oscillatory Binary Fluid Convection in Large Aspect Ratio Containers Speaker: Professor E. Knobloch, UC Berkeley Abstract: Direct numerical simulations of chevrons, blinking states and repeated transients in binary fluid mixtures with a negative separation ratio heated from below are described. The calculations are performed in two-dimensional containers using realistic boundary conditions and the parameter values used in the experiments of Kolodner (Phys. Rev.~E {\bf 47}, 1038, 1993). Particular attention is paid to the multiplicity of states, and their dependence on the applied Rayleigh number and the aspect ratio of the container. Quantitative agreement with the experiments is obtained, and a mechanism explaining the origin and properties of the repeated transients observed in the experiments is proposed. Contact: Hermann Riecke 847-491-8316 Audience: Public Group: Interdisciplinary Seminar in Nonlinear Science

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