Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session B39: Focus Session: Collective Dynamics of Self-Driven Particles |
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Sponsoring Units: GSNP DFD Chair: Sriram Ramaswamy, Indian Institute of Science, Bangalore Room: Morial Convention Center 231 |
Monday, March 10, 2008 11:15AM - 11:51AM |
B39.00001: Polar and apolar active matter Invited Speaker: Assemblies of interacting self-driven units form a new type of \textit{active} soft matter with collective behavior qualitatively different from that of its individual constituents, nonequilibrium phase transitions, and unusual mechanical and rheological properties. Examples include cytoskeletal filaments crosslinked by motor proteins, bacterial colonies, migrating cells, and vibrated layers of granular rods. In this talk I will review our work on using nonequilibrium statistical physics to derive a continuum description of these systems from specific models of single particle dynamics. This approach aims at understanding the interplay between physical mechanisms (such as formation or loss of physical connections, excluded volume effects, directional forces) and biochemical or other processes in regulating the large-scale organization and function of active matter. I will contrast the behavior of units with a head and a tail that can exhibit a macroscopic polar state, where all organisms move coherently in a preferred direction, with that of units with head-tail symmetry, that can order in a nematic state, with no net motion on macroscopic scale. Finally, I will use a simple model of active rods on a substrate to discuss the interplay between equilibrium steric effects and self-propulsion in controlling order and fluctuations in active fluids. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B39.00002: Swarming and swirling in self-propelled polar granular rods Arshad Kudrolli, Geoffroy Lumay, Dmitri Volfson, Lev Tsimring We discuss the dynamics of ``self-propelled" polar rods experimentally and numerically. In the experiment, the polar motion was achieved by vibrating rods with asymmetric mass distribution. In the numerics, we postulate a driving force acting along the axis of the rod. We observe aggregation of rods at the boundaries because of the inability of rods to turn around and escape for high enough density under low noise conditions. As vibration strength and thus noise is increased, the aggregation reduces and a uniformly distributed state displaying local orientation order and swirls are observed. We observe greater than $\sqrt{n}$ density fluctuations which are in a qualitative agreement with the Toner-Tu model, but this agreement should not be over-emphasized since the model is directly applicable to a nematic regime. Our findings elucidate an important and interesting interplay between the shape and the directed motion in {\em realistic} self-propelled rods which affects the phenomenology of their collective dynamics. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B39.00003: Rectification of Swimming Bacteria and Self Driven Particle Systems by Arrays of Asymmetric Barriers Charles Reichhardt, Mew Bing Wan, Cynthia Olson Reichhardt, Zohar Nussinov We show that the recent experimental observation of the rectification of swimming bacteria in a system with an array of asymmetric barriers occurs due to the ballistic component of the bacteria trajectories introduced by the bacterial ``motor.'' Each bacteria selects a random direction for motion and then moves in this direction for a fixed period of time before randomly changing its orientation and moving in a new direction. In the limit where the bacteria undergo only Brownian motion, rectification by the barriers does not occur. We also examine the effects of steric interactions between the bacteria and observe a clogging effect upon increasing the bacteria density. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B39.00004: Delay induced instabilities in self-propelling swarming particles Eric Forgoston, Ira Schwartz We consider a general model of self-propelling biological or artificial individuals interacting through a pairwise attractive force in a two-dimensional system in the presence of noise and communication time delay. Previous work has shown that a large enough noise intensity will cause a translating swarm of individuals to transition to a rotating swarm with a stationary center of mass. In this work, we use numerical simulations to show that with the addition of a time delay, the model possesses a transition that depends on the size of the coupling parameter. This transition is independent of the swarm state (traveling or rotating) and is characterized by the alignment of all of the individuals along with a swarm oscillation. By considering the mean field equations without noise, we show that the time delay induced transition is associated with a Hopf bifurcation. The analytical result yields good agreement with numerical computations of the value of the coupling parameter at the Hopf point. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B39.00005: ABSTRACT WITHDRAWN |
Monday, March 10, 2008 12:39PM - 12:51PM |
B39.00006: From Cannibalism to Active Motion of Groups Pawel Romanczuk, Lutz Schimansky-Geier The detailed mechanisms leading to collective dynamics in groups of animals and insect are still poorly understood. A recent study by Simpson et. al. suggests cannibalism as a driving mechanism for coordinated migration of mormon crickets [1]. Based on this result we propose a simple generic model of brownian particles interacting by asymmetric, non-conservative collisions accounting for cannibalistic behavior and the corresponding avoidance strategy. We discuss our model in one and two dimensions and show that a certain type of collisions drives the system out of equilibrium and leads to coordinated active motion of groups.\newline [1] Stephen J. Simpson, Gregory A. Sword, Patrick D. Lorch and Iain D. Couzin: \emph{Cannibal crickets on a forced march for protein and salt}, PNAS, 103:4152-4156, 2006 [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B39.00007: Spatial instability and bioturbulence in highly concentrated bacterial suspensions Andrey Sokolov, Igor Aranson We present an experimental study of spatio-temporal organization and transition to complex collective swimming regimes in highly concentrated suspensions of Bacillus subtilis. Experiments are performed in a free-standing thin-film sample with controlled thickness. Novel non-invasive high-resolution optical coherence tomography technique is used to probe the density distributions in the film in real time. Increasing the film thickness beyond certain threshold triggered a transition from quasi-to-dimensional collective swimming to three-dimensional turbulent state which is attributed to Oxygentaxis. We have studied effect of the controlled oxygen concentration on the bacterial collective behavior and transition to turbulent bioconvection. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B39.00008: Non-Coalescent, Self-Assembling Water Drops: Phase transitions, flows and hydrodynamics Mohan Srinivasarao, Vivek Sharma We study the collective nucleation, growth and self-assembly of non-coalescent water drops. These form and organize over evaporating polymer solutions exposed to a draft of moist air. The creation and evolution of a population of drops towards a closed packed array occurs in response to heat and mass fluxes involved in droplet condensation and solvent evaporation. We elucidate the kinetics and dynamics of droplet growth and assembly, by accounting for various transport and thermodynamic processes. These water drops template hexagonally ordered arrays of holes in polymer films. We thus have a useful and economical method for manufacturing porous films requiring only a drop of polymer solution (dilute) and a whiff of breath! [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B39.00009: Active nematics: fluctuations and coarsening Sriram Ramaswamy, Shradha Mishra, Francesco Ginelli, Hugues Chate, Sanjay Puri Nonequilibrium steady states with spontaneous nematic order are known to arise in collections of amoeboid cells as well as granular-rod monolayers. Recent studies [EPL 62 (2003) 196-202; PRL 96, 180602 (2006); PRL 97 (2006) 090602; Science 317 (2007) 105] have established that these states differ radically from thermal equilibrium systems of the same spatial symmetry. This talk will present results from our studies of microscopic as well as coarse-grained models of active nematics, highlighting the unique, fluctuation-dominated character of coarsening in these systems. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B39.00010: Long-range correlations in simulations of suspensions of swimming microorganisms Patrick Underhill, Juan Hernandez-Ortiz, Michael Graham Simulations of large populations of hydrodynamically interacting swimming particles have been performed at low Reynolds number in periodic and confined geometries. Our simulations show that the interactions of the particles lead to long-range spatial correlations in the fluid at scales larger than the size of a single organism. These long-range correlations lead to a large enhancement in the fluid transport properties. The diffusivity of passive, non-Brownian tracer particles diverges in the periodic geometry with increasing the simulation box size. This collective motion depends on the method the organism uses for propulsion. Simple scaling arguments have also been developed that can capture much of the physics of both the swimmer and tracer motions. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B39.00011: Simulated Flocking Dynamics of 2D Self-propelled Hard Particles Donald Blair Following a recent demonstration of long-lived giant number fluctuations in a swarming, granular nematic (Narayan et. al, Science {\bf 317}, 105 (2007)), we perform 2D simulations of hard, self-propelled particles which communicate only through contact. We vary particle end-shape, polarity, and aspect ratio and explore the effects on order, on the development of density fluctuations, and on the evolution of the swarm boundary. Connections to various forms of active matter (swimming bacteria, crawling cells) will be discussed. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B39.00012: Cell swarming leads to vortex flow in early embryo formation Ariel Balter, James A. Glazier A forming embryo can be though of as a confined region of incompressible medium. Vortex flow is observed in early embryo formation from \em drosophila \em fruit flies to mammals. The Navier-Stokes equation for fluid flow in a cavity is known to have stable vortex solutions. A model for cell motion in which cells move independently of their neighbors corresponds to high Reynolds number (\em Re\em) incompressible flow. An alternative cell-swarming model in which cells do influence their neighbors motion (through a mechanism known as \em contact following\em) corresponds to a flow model that is similar to low \em Re \em incompressible flow. Both models can potentially lead to stable vortex formation in a confined cavity. We investigate the applicability of both models to real biological systems [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B39.00013: Active elastic dimers: self-propulsion and current reversal on a featureless track Vijay Kumar Krishna Murthy, Sriram Ramaswamy, Madan Rao Directed motion without an imposed external gradient is seen not only in living systems but also in agitated granular matter. The essential ingredients are an external energy input and an inherent asymmetry. Unlike traditional ``Brownian ratchet models'', the asymmetry of interest in the above systems is \emph{internal} to the motile objects, and does not lie in an external periodic potential. In this work, we present a Brownian inchworm model of a self-propelled elastic dimer in the absence of an external potential. Nonequilibrium noise together with a stretch-dependent damping form the propulsion mechanism. Our model connects three key nonequilibrium features -- position-velocity correlations, a nonzero mean internal force, and a drift velocity. Our analytical results, including striking current reversals, compare very well with numerical simulations. The model unifies the propulsion mechanisms of DNA helicases, polar rods on a vibrated surface, crawling keratocytes and Myosin VI. We suggest experimental realizations and tests of the model. [Preview Abstract] |
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