Computations in Science Seminars
 Sep 3 Wed 12:15 Sayantan Majumdar, University of Chicago e-mail: Host: Leo Kadanoff () History dependent shear response in cross-linked actin networks

Is it possible to control the shear modulus of a material mechanically? We reconstitute an assembly of cross-linked actin filaments, a major component of cell cytoskeleton, to show that the system has remarkable property to respond under shear in a deformation history dependent manner. When a large shear stress pulse is applied to the system, the system remembers the direction of deformation long after the stress pulse is removed. For next loading cycle, shear response of the system becomes anisotropic; if the applied pulse direction is same as the previous one, the system behaves like a viscoelastic solid but a transient liquefaction is observed if the pulse direction is reversed with respect to the previous one. Further experiments suggest that this anisotropic response comes from stretching dominated and bending dominated deformation directions induced by the large shear deformation giving rise to a direction dependent mechano-memory. The long time scale over which the memory effect persists has relevance in various deformations in cellular and multicellular systems.

 Sep 10 Wed 12:15 Shashi Thutupalli, Princeton e-mail: Host: Leo Kadanoff () Collective dynamics in non-equilibrium systems: chimera clocks and flocking drops

The collective actions of heterogeneous individuals determine the course of some of the most intriguing phenomena in nature such as synchronization, flocking, multicellularity and inter-species ecology. In this talk, I will present collective dynamics in two different experimental systems: (i) a surprising symmetry broken synchronization state in coupled mechanical oscillators and (ii) hydrodynamically mediated flocking in a population of self propelled microdroplets.

In the first part, I will talk about an intriguing dynamic referred to as a chimera state. In the world of coupled oscillators, a chimera state is the co-existence of synchrony and asynchrony in a population of identical oscillators, which are coupled nonlocally. Following nearly 10 years of theoretical research, it has been an imminent question whether these chimera states exist in real systems. Recently, we built an experiment with springs, swings and metronomes and realised these symmetry breaking states in a purely physical system. Our mathematical model shows that the self-organization observed in the experiments is controlled by elementary dynamical equations from mechanics that are ubiquitous in many natural and technological systems such as power grids, optomechanical crystals, or cells communicating via quorum sensing in microbial populations.

In the second part, I will talk about microswimmers made from liquid crystalline emulsion droplets. Following a brief description of the swimming mechanism, I will discuss the effects of confinement on the collective effects that emerge in ensembles of millions of swimming droplets. Specifically, I dwell on hydrodynamic and volume exclusion interactions only through which these droplets can couple their motions.

 Oct 1 Wed 12:15 Stas Nagy, University of Chicago Host: Leo Kadanoff () snagy@uchicago.edu
 Oct 8 Wed 12:15 Bob Batterman, University of Pittsburgh e-mail: Host: Leo Kadanoff () Relative Autonomy and Minimal Modeling: Explaining the Robustness of Theories at Continuum Scales.

Bridging or connecting the descriptions and models of systems across widely separated scales is a deep problem that permeates many areas of scientific investigation. Unfortunately, philosophical discussion of this problem is often contextualized as an all or nothing'' dichotomy between reductionism and emergentism. This is much too crude.

This paper will discuss a set of mathematical techniques including the renormalization group and homogenization theory designed to upscale from models of systems that exhibit heterogeneities at small/micro scales to models that are homogeneous at continuum/everyday scales. The focus will be on two aspects of the use of such techniques. On the one hand, they appear to be essential to explain the existence of certain kinds of patterns in nature and the \emph{relative} autonomy of the continuum scale models from the lower scale details. Why, for example, do the equations that govern the scaling behavior of different fluids at criticality work so well when they completely ignore molecular scale details? Why, do the Navier-Cauchy equations for bending elastic beams work so well when they, too, essentially fail to reference any atomic or lower scale details?

On the other hand, we can also sometimes use models (toy models or \emph{minimal models}) to investigate and understand the behavior of real systems. For example, we can employ the Ising model and lattice gas automata to study the behavior of real systems---actual fluids that look absolutely nothing like these models at lower scales. The mathematics of the renormalization group and other techniques provide an account of how such \emph{non-representative} minimal models can be explanatory and can provide understanding. The paper discusses the importance of these mathematical techniques for answering the questions of autonomy, and the role and effectiveness of minimal models.

 Oct 15 Wed 12:15 Jane Wang, Cornell e-mail: Host: Leo Kadanoff ()
 Oct 21 Tue 4:00 PM Andy Ruina, Cornell Host: Leo Kadanoff () Gliders, bicycles, toys and walking robots
JFI Seminar - Room W301 - 4:00 PM

Many airplanes can, or nearly can, glide stably without control. So it seems natural that the first successful powered flight followed from mastery of gliding. Many bicycles can, or nearly can, balance themselves when in motion. Bicycle design seems to have evolved to gain this feature. Also, we can make toys and 'robots' that, like a stable glider or coasting bicycle, stably walk without motors or control in a remarkably human-like way. So it makes sense to use `passive-dynamics' as a core for developing the control of walking robots and to gain understanding of the control of walking people. That's what I used to think. But, so far, this has not led to robust walking robots. What about human evolution? We didn't evolve dynamic bodies and then learn to control them. Rather, people had elaborate control systems way back when we were fish and even worms. But if control is paramount, why is it that uncontrolled passive-dynamic walkers can walk so much like humans? It seems that energy optimal control, perhaps a proxy for evolutionary development, arrives at solutions that have features in common with passive-dynamics. Rather than thinking of good powered walking as passive walking with a small amount of control added, I now think of powered walking as highly controlled, but with much of the motor action titrated out.

 Oct 22 Wed 12:15 Andy Ruina, Cornell e-mail: Host: Leo Kadanoff () Non-holonomic stability and rotation with zero angular momentum: Demonstrations of stability and of the falling cat phenomenon go sour.

This talk is about two classes of (interesting, at least to me) physical behavior that follow from them impossibility of integrating some formulas that involve derivatives. First, systems with wheels or ice skates can be conservative yet have asymptotic stability. This is relevant to braking cars, flying arrows and the balance of skateboards and bicycles. Second, is the well known possibility that a system with zero angular momentum can, by appropriate deformations, rotate without any external torque. This effect explains how a cat that is dropped while upside down can turn over and of how various gymnastic maneuvers are performed. Both rolling contact and constancy of angular momentum are examples of the "non-integrability" of a "non-holonomic" equation. There are various simple demonstrations of these phenomena that can go bad. Cars can crash, bikes fall over and, in terrestrial experiments, various effects can swamp that which one wants to demonstrate. The talk describes the basic theory and then a collection of simple experiments that fail various ways for various reasons.

 Oct 23 Thu 5:30 PM Francesca Casadio, Art Institute of Chicago e-mail:
Special seminar at the Department of Art History, 166 Cochrane Woods Art Center
 Oct 29 Wed 12:15 Kerry Emanuel, MIT e-mail: Host: Leo Kadanoff ()
 Nov 5 Wed 12:15 Guenter Ahlers, UC Santa Barbara e-mail: Host: Leo Kadanoff ()
 Nov 12 Wed 12:15 Luis Bettencourt, Santa Fe Institute e-mail: Host: Leo Kadanoff ()
 Nov 19 Wed 12:15 Emil Martinec, University of Chicago e-mail:
 Dec 3 Wed 12:15 Susan Coppersmith, University of Wisconsin e-mail: Host: Leo Kadanoff ()
 Dec 10 Wed 12:15 Igor Aronson, Argonne e-mail: Host: Leo Kadanoff ()
 Jan 2015 7 Wed 12:15 Henry Cohn, Microsoft e-mail: Host: Leo Kadanoff ()
 Jan 2015 14 Wed 12:15 OPEN
 Jan 2015 21 Wed 12:15 Heinrich Jaeger, University of Chicago e-mail: Host: Leo Kadanoff ()
 Jan 2015 28 Wed 12:15 Seth Lloyd, MIT e-mail: Host: Leo Kadanoff ()
 Feb 2015 4 Wed 12:15 Zheng-Tian Lu, Argonne Host: Daniel Holz ()
 Feb 2015 11 Wed 12:15 OPEN
 Feb 2015 18 Wed 12:15 OPEN
 Feb 2015 25 Wed 12:15 OPEN
 Mar 2015 4 Wed 12:15 OPEN
 Mar 2015 11 Wed 12:15 OPEN
 Mar 2015 18 Wed 12:15 OPEN
 Mar 2015 25 Wed 12:15 OPEN
 Apr 2015 1 Wed 12:15 Andrea Bertozzi, UCLA e-mail: Host: Leo Kadanoff ()
 Apr 2015 8 Wed 12:15 OPEN
 Apr 2015 15 Wed 12:15 OPEN
 Apr 2015 22 Wed 12:15 OPEN
 Apr 2015 29 Wed 12:15 OPEN
 May 2015 6 Wed 12:15 OPEN
 May 2015 13 Wed 12:15 OPEN
 May 2015 20 Wed 12:15 OPEN
 May 2015 27 Wed 12:15 OPEN
 Jun 2015 3 Wed 12:15 OPEN
 Jun 2015 10 Wed 12:15 OPEN
 Jun 2015 17 Wed 12:15 OPEN
 Jun 2015 24 Wed 12:15 OPEN