Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session X5: Active Soft Matter: From Granular Rods to Flocks to Living Cells |
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Sponsoring Units: GSNP Chair: Cristina Marchetti, Syracuse University Room: 401/402 |
Thursday, March 19, 2009 2:30PM - 3:06PM |
X5.00001: Collective dynamics of rigid and deformable self-propelled particles Invited Speaker: We discuss a series of experiments with granular matter with novel shapes which self-organize upon excitation. Previously, we reported experiments with rigid rod shaped particles with asymmetric mass distributions which show directed motion on a vibrated plate [1]. Recognizing that such a system is a simple physical model of self-propelled particles, we discuss the observed collective behavior such as aggregation at the boundaries and swirling motion in the context of various minimal leaderless models of active living systems such as bacterial colonies and hoofed animal herds which show self-organization. We will introduce and discuss the dynamics of deformable shapes consisting of a head and a tail composed of a bead chain which is shown to undergo directed motion because of differential friction associated with the head and the body. [1]: ``Swarming and swirling in self-propelled granular rods," A. Kudrolli, G. Lumay, D. Volfson, and L. Tsimring, Phys. Rev. Lett. 100, 058001 (2008). [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:42PM |
X5.00002: Three dimensional reconstruction of starling flocks: an empirical investigation of collective animal behavior Invited Speaker: Bird flocking is a striking example of animal collective behaviour: thousands of birds gather above the roosting site, forming sharp-bordered flocks, which wheel and turn with remarkable coherence and synchronization. Despite an increasing theoretical interest, empirical investigations of collective motion have been limited so far by the difficulties of getting data on large systems. By means of stereoscopic photography and using statistical mechanics, optimization theory and computer vision techniques, we have measured for the first time the three-dimensional positions and trajectories of individual birds in groups of up to three thousands elements. This allowed us to analyze global morphological properties of the flocks, as well as structural and dynamical properties. Most notably, we investigated the nature of the inter-individual interaction. We found that the interaction between birds does not depend on their mutual metric distance, as most current models and theories assume, but rather on the topological distance (number of intermediate neighbors). In fact, we discovered that each individual interacts on average with a fixed number of neighbors (six-seven), rather than with all neighbors within a fixed metric distance. We argue that a topological interaction of this kind is indispensable to maintain flock's cohesion against the large density changes caused by external perturbations, typically predation. More recently, we characterized the velocity field, and computed dynamical observables. We showed that flocks exhibit long range correlations, which are a signature of their remarkable collective behavior. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 4:18PM |
X5.00003: Self Propelled Particles: from microdynamics to hydrodynamics Invited Speaker: In this talk I will illustrate the derivation of a unified continuum description of the large scale collective behavior of active matter from two specific physical microscopic dynamical models: stroke-averaged swimmers moving through a viscous fluid and self-propelled hard rods moving on a substrate. New results at large scales include a lowering of the density of the isotropic-nematic transition, an enhancement of longitudinal diffusion of the self-propelled orientable units, and a strong enhancement of boundary effects in confined self-propelled systems. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:54PM |
X5.00004: Active biopolymer gels: from cells to tissues Invited Speaker: Living cells are active soft materials that are far out of thermodynamic equilibrium. They continuously use up chemical energy in order to generate forces that drive processes such as cell migration and division. Moreover, cells actively remodel their surrounding extracellular matrix (primarily collagen), so whole tissues can also be regarded as active soft materials. The aim of our research is to understand the physical mechanisms underlying the self-organization and mechanics of cells and tissues. To this end we use an experimental approach and study simplified model systems for the cytoskeleton (purified actin, tubulin, and accessory proteins) and for tissues (fibroblast-populated collagen and fibrin gels). We use microscopy and rheology to investigate the structure and mechanics on different length scales, from the single protein up to macroscopic level. I will discuss two examples of active mechanical behavior, namely in purified actin-myosin networks, and in purified fibrin matrices with embedded contractile fibroblasts. In both cases we observe active contraction and active stiffening. We quantify the active forces and examine how the structure and mechanics of the active gels depend on motor/cell density. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:30PM |
X5.00005: Beller Lectureship Talk: Active response of biological cells to mechanical stress Invited Speaker: Forces exerted by and on adherent cells are important for many physiological processes such as wound healing and tissue formation. In addition, recent experiments have shown that stem cell differentiation is controlled, at least in part, by the elasticity of the surrounding matrix. We present a simple and generic theoretical model for the active response of biological cells to mechanical stress. The theory includes cell activity and mechanical forces as well as random forces as factors that determine the polarizability that relates cell orientation to stress. This allows us to explain the puzzling observation of parallel (or sometimes random) alignment of cells for static and quasi-static stresses and of nearly perpendicular alignment for dynamically varying stresses. In addition, we predict the response of the cellular orientation to a sinusoidally varying applied stress as a function of frequency and compare the theory with recent experiments. The dependence of the cell orientation angle on the Poisson ratio of the surrounding material distinguishes cells whose activity is controlled by stress from those controlled by strain. We have extended the theory to generalize the treatment of elastic inclusions in solids to ''living'' inclusions (cells) whose active polarizability, analogous to the polarizability of non-living matter, results in the feedback of cellular forces that develop in response to matrix stresses. We use this to explain recent observations of the non-monotonic dependence of stress-fiber polarization in stem cells on matrix rigidity. These findings provide a mechanical correlate for the existence of an optimal substrate elasticity for cell differentiation and function. \\[3pt] *In collaboration with R. De (Brown University), Y. Biton (Weizmann Institute), and A. Zemel (Hebrew University) and the experimental groups: Max Planck Institute, Stuttgart: S. Jungbauer, R. Kemkemer, J. Spatz; University of Pennsylvania: A. Brown, D. Discher, F. Rehfeldt. [Preview Abstract] |
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