Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session C14: Statistical Mechanics of Active MatterFocus
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Sponsoring Units: GSNP DBIO Chair: Michael Sinhuber, Stanford University Room: 273 |
Monday, March 13, 2017 2:30PM - 3:06PM |
C14.00001: A Materials Approach to Collective Behavior Invited Speaker: Nicholas Ouellette Aggregations of social animals, such as flocks of birds, schools of fish, or swarms of insects, are beautiful, natural examples of self-organized behavior far from equilibrium. Understanding these systems, however, has proved to be quite challenging. Determining the rules of interaction from empirical measurements of animals is a difficult inverse problem. Thus, researchers tend to focus on the macroscopic behavior of the group instead. Because so many of these systems display large-scale ordered patterns, it has become the norm in modeling animal aggregations to focus on this order. Large-scale patterns alone, however, are not sufficient information to characterize all the dynamics of animal aggregations, and do not provide stringent enough conditions to benchmark models. Instead, I will argue that we should borrow ideas from materials characterization to describe the macroscopic state of an animal group in terms of its response to external stimuli. I will illustrate these ideas with recent experiments on mating swarms of the non-biting midge Chironomus riparius, where we have developed methods to apply controlled perturbations and measure the detailed swarm response. Our results allow us to begin to describe swarms in terms of state variables and response functions, bringing them into the purview of theories of active matter. These results also point towards new, more detailed ways of characterizing and hopefully comparing collective behavior in animal groups. [Preview Abstract] |
Monday, March 13, 2017 3:06PM - 3:18PM |
C14.00002: Kinetics of motility-induced phase separation and swim pressure Adam Patch, David Yllanes, M. Cristina Marchetti Active Brownian particles (ABPs) represent a minimal model of active matter consisting of self-propelled spheres with purely repulsive interactions and rotational noise. We correlate the time evolution of the mean pressure towards its steady state value with the kinetics of motility-induced phase separation. For parameter values corresponding to phase separated steady states, we identify two dynamical regimes. The pressure grows monotonically in time during the initial regime of rapid cluster formation, overshooting its steady state value and then quickly relaxing to it, and remains constant during the subsequent slower period of cluster coalescence and coarsening. The overshoot is a distinctive feature of active systems. [Preview Abstract] |
Monday, March 13, 2017 3:18PM - 3:30PM |
C14.00003: Active Matter Chaos David A. Egolf, Edward J. Banigan, Charles Dawson Recently, researchers demonstrated that a model of soft, polydisperse, non-aligning, self-propelled particles in two dimensions exhibits a transition from a liquid-like state to a ``frozen" glassy state as the density is increased or the propulsion speed is decreased. Here we analyze the two states and the transition between them using nonlinear dynamical techniques. We find that the largest Lyapunov exponent indicates that the transition is a dynamical transition from a chaotic liquid state to a non-chaotic glassy state and that this transition is characterized by dynamical time and length scales that diverge as power laws. Near the transition, we also find that cooperative rearrangements of particles are anticipated by an increase in the finite-time exponent and a localization of the Lyapunov vector to the particles that will be involved in the rearrangement. Our results, in conjunction with similar previous results for granular matter, suggest the broad applicability of nonlinear dynamical techniques for exploring glassy and jamming transitions in a variety of media. [Preview Abstract] |
(Author Not Attending)
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C14.00004: Swift-Hohenberg-type model Anand Oza, Joern Dunkel Recent experiments from the Zvonimir Dogic Lab (Brandeis University) demonstrated that ATP-driven microtubule-kinesin bundles can self-assemble into two-dimensional active liquid crystals that exhibit a rich creation and annihilation dynamics of topological defects, reminiscent of particle-pair production processes in quantum systems. This remarkable discovery has sparked considerable theoretical and experimental interest. Here, we present and validate a minimal continuum theory for this new class of active matter systems by modifying the classical Landau-de Gennes theory for liquid crystals, obtaining a tensorial Swift-Hohenberg-type PDE. We simulate the resulting model numerically and develop an algorithm for tracking topological defects. We find that the resulting model agrees quantitatively with recently published data and predicts a regime of antipolar defect ordering. Ordered states go unstable as the activity parameter is increased, yet the chaotic defect dynamics still exhibit local antipolar ordering. Generally, our results suggest that complex nonequilibrium pattern-formation phenomena might be predictable from a few fundamental symmetry-breaking and scale-selection principles. [Preview Abstract] |
Monday, March 13, 2017 3:42PM - 4:18PM |
C14.00005: Snakes on a plane: modeling flexible active nematics Invited Speaker: Robin Selinger Active soft matter systems of self-propelled rod-shaped particles exhibit ordered phases and collective behavior that are remarkably different from their passive analogs. In nature, many self-propelled rod-shaped particles, such as gliding bacteria and kinesin-driven microtubules, are flexible and can bend. We model these ``living liquid crystals'' to explore their phase behavior, dynamics, and pattern formation. We model particles as short polymers via molecular dynamics with a Langevin thermostat and various types of activity, substrate, and environments. For self-propelled polar particles gliding on a solid substrate, we map out the phase diagram as a function of particle density and flexibility. We compare simulated defect structures to those observed in colonies of gliding myxobacteria; compare spooling behavior to that observed in microtubule gliding assays; and analyze emergence of nematic and polar order. Next we explore pattern formation of self-propelled polar particles under flexible encapsulation, and on substrates with non-uniform Gaussian curvature. Lastly, we impose an activity mechanism that mimics extensile shear, study flexible particles both on solid substrates and coupled to a lipid membrane, and discuss comparisons to relevant experiments. Work performed in collaboration with Michael Varga (Kent State) and Luca Giomi (Universiteit Leiden.) [Preview Abstract] |
Monday, March 13, 2017 4:18PM - 4:30PM |
C14.00006: Dynamic phases of active matter systems with quenched disorder Cynthia J. Olson Reichhardt, Csanad S\'andor, Andras Lib\'al, Charles Reichhardt Depinning and nonequilibrium transitions within sliding states in systems driven over quenched disorder arise across many size scales, ranging from nanoscale atomic friction, mesoscale flux motion in type-II superconductors, microscale colloidal motion in disordered substrates, and geological scale plate tectonics. We show that active matter or self-propelled particles interacting with quenched disorder under an external drive represent a new class of system that exhibits pinning-depinning phenomena, plastic flow phases, and nonequilibrium sliding transitions correlated with distinct velocity-force curve signatures. For strong particle-substrate interactions, a homogeneous pinned liquid phase forms that depins plastically into a uniform disordered phase and then dynamically transitions into a moving stripe coexisting with a pinned liquid and then into a moving phase separated state at higher drives. We numerically map the dynamical phase diagrams as a function of external drive, substrate interaction strength, and self-propulsion correlation length. These phases can be observed for active matter moving through random disorder. Our results indicate that intrinsically nonequilibrium systems can exhibit additional nonequilibrium transitions when subjected to an external drive. [Preview Abstract] |
Monday, March 13, 2017 4:30PM - 4:42PM |
C14.00007: Emergent Vortex Patterns in Systems of Self-Propelled, Chiral Particles Lorenz Huber, Jonas Denk, Emanuel Reithmann, Erwin Frey Self-organization of FtsZ polymers is vital for Z-ring assembly during bacterial cell division, and has been studied using reconstituted in vitro model systems. Employing Brownian dynamics simulations and a Boltzmann approach, we model FtsZ polymers as active particles moving along chiral circular paths. With both theoretical approaches we find self-organization into vortex structures and characterize different states in parameter states. Our work demonstrates that these patterns are robust and are generic for active chiral matter. Moreover, we show that the dynamics at the onset of pattern formation is described by a generalized complex Ginzburg-Landau equation. [Preview Abstract] |
Monday, March 13, 2017 4:42PM - 4:54PM |
C14.00008: Odd viscosity in chiral active liquids Vincenzo Vitelli, Debarghya Banerjee, Anton Souslov, Alexander Abanov Chiral active liquids, composed of self-rotating interacting units, are fluids that break both time reversal symmetry and parity. As a consequence, their viscous stress acquires an additional contribution called odd-viscosity (originally discovered in quantum Hall fluids) that is proportional to the angular momentum density. We construct a non-linear hydrodynamic theory of chiral active fluids, which captures previously neglected odd viscosity contributions. In the incompressible limit, the effect of odd viscosity is to modify the boundary pressure by an additional term proportional to the local vorticity. In the bulk, the odd viscosity affects the response of compressible chiral active fluids by generating transverse currents (with respects to applied pressure) in Burgers shocks. Finally we explore, the chiral vortex formation induced by the active rotation and its implication for the transition to turbulence. [Preview Abstract] |
Monday, March 13, 2017 4:54PM - 5:06PM |
C14.00009: Emergence of Chiral Phases in Active Torque Dipole Systems Ana Fialho, Elsen Tjhung, Michael Cates, Davide Marenduzzo The common description of active particles as active force dipoles fails to take into account that active processes in biological systems often exhibit chiral asymmetries, generating active chiral processes and torque dipoles. Examples of such systems include cytoskeleton filaments which interact with motor proteins and beating cilia and flagella. In particular, the generation of active torques by the actomyosin cytoskeleton has been linked to the break of chiral symmetry at a cellular level. This phenomenon could constitute the primary determinant for the break of left-right symmetry in many living organisms, e.g. the position of the human heart within the human body. In order to account for the effects of chirality, we consider active torque dipoles which generate a chiral active stress. We characterize quasi-1D and 2D systems of torque dipoles, using a combination of linear stability analysis and numerical simulations (Lattice Boltzmann). Our results show that activity drives a spontaneous breaking of chiral symmetry, leading to the self-assembly of a chiral phase, in the absence of any thermodynamic interactions favoring cholesteric ordering. At high values of activity, we also observe labyrinthine patterns where the activity-induced chiral ordering is highly frustrated. [Preview Abstract] |
Monday, March 13, 2017 5:06PM - 5:18PM |
C14.00010: Running and rotating: modelling the dynamics of migrating cell clusters Katherine Copenhagen, Nir Gov, Ajay Gopinathan Collective motion of cells is a common occurrence in many biological systems, including tissue development and repair, and tumor formation. Recent experiments have shown cells form clusters in a chemical gradient, which display three different phases of motion: translational, rotational, and random. We present a model for cell clusters based loosely on other models seen in the literature that involves a Vicsek-like alignment as well as physical collisions and adhesions between cells. With this model we show that a mechanism for driving rotational motion in this kind of system is an increased motility of rim cells. Further, we examine the details of the relationship between rim and core cells, and find that the phases of the cluster as a whole are correlated with the creation and annihilation of topological defects in the tangential component of the velocity field. [Preview Abstract] |
Monday, March 13, 2017 5:18PM - 5:30PM |
C14.00011: Pressure and tension in momentum-conserving active fluids Sriram Ramaswamy, Madan Rao We consider a fluid governed by the Navier-Stokes equation, driven by stresses carried by a suspension of rotationally diffusing self-propelled objects. We present results on the scale-dependence of the steady-state pressure and the dynamics of fluid interfaces in such a system. [Preview Abstract] |
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