Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session K53: Nonequilibrium Statistical Mechanics and Hydrodynamics of Active Matter IFocus

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Sponsoring Units: GSOFT GSNP DFD Chair: Dibyendu Mandal, Univ of California  Berkeley Room: LACC 513 
Wednesday, March 7, 2018 8:00AM  8:36AM 
K53.00001: Simulations of bulk and topologically constrained active matter Invited Speaker: Michael Hagan , Abhijeet Joshi , Gabriel Redner , Michael Norton , Matthew Peterson , Seth Fraden , Aparna Baskaran Active nematics are liquid crystals whose constituent particles transduce energy into motion. The ordered nematic state is unstable to the proliferation of topological defects, which undergo birth, streaming dynamics, and annihilation to yield a seemingly chaotic dynamical steadystate. I will describe several types of computational models for active nematics motivated by experiments at Brandeis in which microtubule bundles driven by ATPpowered motor proteins, and a few surprising things we have learned about the behaviors of active nematics in bulk and under confinement. These include heretofore unknown brokensymmetry phases in which the topological defects themselves undergo orientational ordering, renormalization of elastic moduli by activity, and a remarkable insensitivity to topological constraints even under high confinement. 
Wednesday, March 7, 2018 8:36AM  8:48AM 
K53.00002: Imaging the emergence of collective swarming using lightcontrolled bacteria Yi Peng , Xiang Cheng Collective motions of active fluids emerge at high densities and low noises, as illustrated vividly by bird flocks, fish schools and bacterial swarming. Although the disorderorder transition of active fluids has been extensively studied, the detailed kinetics of this transition have not been systematically explored in experiments. Here, using an engineered E. coli strain, whose locomotion can be reversibly controlled by light, we experimentally study the swarming transition in bacterial suspensions and explore its kinetic pathway. Particularly, we trigger bacterial swarming by tuning light intensity and image the dynamics of the emergence of collective motions in concentrated bacterial suspensions. We map the phase diagram of bacterial swarming as a function of total bacterial concentrations and the fractions of active swimmers. We find that, when the fraction of active swimmers is low, the transition shows an incubation period, indicating a finite energy barrier for the formation of swarming clusters. As the fraction of active swimmers increases, the incubation disappears. The kinetics of the transition are similar to that of spinodal decomposition. Our study provides new insights on nonequilibrium phase transition in active matter. 
Wednesday, March 7, 2018 8:48AM  9:00AM 
K53.00003: Active Brownian particles in a inhomogeneous activity or magnetic field Hidde Vuijk , Joseph Brader , Abhinav Sharma We study spherically symmetric active particles in a spatially inhomogeneous activity or magnetic field. In both cases the particles spontaneously orient themselves, which results in an inhomogeneous distribution of particles. Using GreenKubo approach we obtain analytical expression for the orientation of the particles. We find that the relevant equilibrium correlation function is the selfpart of the Van Hove function, which can be approximated accurately. Density does not have a linear response to the activity; however, an expression for the density is derived by using the orientation as input to dynamic density functional theory. All theoretical predictions are validated using Brownian dynamics simulations. 
Wednesday, March 7, 2018 9:00AM  9:12AM 
K53.00004: Flocking on Curved Surfaces and Topological Sound Suraj Shankar , Mark Bowick , M Cristina Marchetti Flocking, the spontaneous collective motion of a large collection of selfpropelled entities, is seen on many scales in nature, and can occur on a curved substrate. Generalizing the continuum TonerTu equations of an active polar fluid to a curved surface, we analytically obtain inhomogeneous ordered flocking states that also include the topological defects required by the underlying curved geometry. In addition to frustrating order, the curvature also generates a finite frequency threshold to excite the longwavelength sound modes that are present in an ordered polar flock. The breaking of time reversal symmetry due to spontaneous flow and the presence of a gap in the sound spectrum together gives rise to topologically protected sound modes that propagate unidirectionally and are localized to special geodesics on the surface (like the equator on a sphere). These excitations are analogous to edge states in electronic quantum Hall systems or wellknown equatorial waves in ocean and atmospheric flows, and provide unidirectional channels for information transport in the flock, robust against disorder and backscattering. 
Wednesday, March 7, 2018 9:12AM  9:24AM 
K53.00005: Transport of active spinning particles in porous media Shahrzad Yazdi , Juan Aragones , Alfredo AlexanderKatz Transport of active particles in confinement is of interest in multiple fields from biology to synthetic systems. Here, we explore how spinning particles behave in porous media and in particular how their transport is affected by the local structure of the medium. We demonstrate that spinning particles within a liquid in a 2D ordered array of posts display nonequilibrium states of ballistic transport. We further show that such states occur in the limit of small Reynolds number and high Peclet number. The origin of these states is found in the opening of a “directionality gap” in the density of states due to the breaking of rotational symmetry. Furthermore, we also show that these states are not affected by a large quantity of defects or disorder on the lattice, which resemble edge currents in topological insulators. Our results may be important in understanding transport of active mater in confined or (un)structured environments. 
(Author Not Attending)

K53.00006: Microscopic simulations of flexible active nematics on curved surfaces. Abhijeet Joshi , Aparna Baskaran , Michael Hagan Active nematics are liquid crystals which are driven out of equilibrium by energydissipating active stresses. Experiments and theory have demonstrated that in such systems the ordered nematic state is unstable to the proliferation of topological defects, which undergo birth, streaming dynamics, and annihilation to yield a seemingly chaotic dynamical steady state. Many biological systems consist of active agents confined on surfaces with inhomogeneous Gaussian curvature; yet we have limited theoretical understanding of such systems. Gaussian curvature couples to the elastic energy and topology of a nematic, thus altering the defect statistics in the system. This talk will describe computational results and comparison with theory for active nematics on curved manifolds, using a recently developed microscopic model that accounts for the inherent flexibility of the active filaments. 
Wednesday, March 7, 2018 9:36AM  9:48AM 
K53.00007: Transitioning from Active Nematic Films to Mesoscale Turbulence in 3D Channels Tyler Shendruk , Amin Doostmohammadi , Kristian Thijssen , Julia Yeomans Active fluids comprise a wide range of biological and synthetic materials that are driven out of equilibrium by energy injection from their microscopic elements, such as swimming bacteria or motor proteinmicrotubule bundles. The elongated nature of these constituents favours nematic alignment, while their activity continuously disturbs the orienational ordering leading to topological defects. Defects play an important role in the disorderly flows of 2D mesoscale turbulence and have been observed active films of microtubules, bacteria and monolayers of fibroblast and epithelial cells. However, it is less clear what role topological disclination lines play in 3D active flows. By numerically simulating a confining active nematic fluid between parallel plates, we characterize the transition from quasi2D to confined 3D chaotic flows. At small heights, the active nematic exhibits effectively 2D mesoscale turbulence, with straight disclination lines that span the channel gap and interact as 2D defects. Upon increasing channel height, we find that a twist contortion of the disclinations leads to a transition to fully 3D chaotic flows. By defining an order parameter based on the conformation of the disclinations, we characterize the transition to 3D mesoscale turbulence. 
Wednesday, March 7, 2018 9:48AM  10:00AM 
K53.00008: Structure and Dynamics of Active Nematics under Circular Confinement  A Microscopic Simulation Study Matthew Peterson , Abhijeet Joshi , Michael Hagan , Aparna Baskaran Experimental work on circularly confined active nematics has shown interesting phenomena such as buckling, phaseseparation and circulation, behaviors which are not readily captured in continuum models. Here, we use a particlebased computational model to study the dynamics of a confined active nematic, as a function of confinement size, activity, and the flexibility of the nematogens. We find that the behavior exhibited by this model qualitatively agrees with those of experiments. In particular, we find phaseseparated circulation for sufficiently large activities, formation of defect dipoles for moderate activities and stiffnesses, and buckling at high stiffnesses and strong confinement. 
Wednesday, March 7, 2018 10:00AM  10:12AM 
K53.00009: Surface Dynamics of Chiral Colloidal Gyrofluids Vishal Soni , Sofia Magkiriadou , Stefano Sacanna , Denis Bartolo , Michael Shelley , William Irvine Colloidal magnets, when spun by a rotating external field, coalesce into a fluidlike state. This fluid exhibits rich interfacial dynamics, which we explore in this talk. In particular, we observe experimentally and explain theoretically the propagation and dissipation of chiral surface waves. Unlike capillary waves in water, their propagation relies neither on inertia nor on surface tension. In contrast viscosity and spinning motion conspire against surface tension to sustain their unidirectional propagation. 
Wednesday, March 7, 2018 10:12AM  10:24AM 
K53.00010: NonEqilibrium Dynamics of Anisotropic Particles Open Up Novel Actuation Strategies Gabi Steinbach , Michael Schreiber , Sibylle Gemming , Artur Erbe The persistent fascination in locomotion at the mesoscale arises from the time reversibility of motion patterns in overdamped systems. To study the principles of locomotion via cyclic patterns, dispersed magnetic particles and subsequent selfassembled clusters are versatile systems. They rotate via alignment in a timedependent field, which can be used for controlled actuation. The challenge is to transform fielddriven rotation into an effective translation. To date, most examples of magnetic actuation (and other cyclic movers) exploit hydrodynamic coupling between rotation and translation. Here, we present a novel locomotion strategy that exploits internal interactions in multicomponent objects. We study selfassembled clusters of magnetic particles that exhibit an offcenter dipole moment. By theoretical modeling and in experiments with magnetic Janus particles, we demonstrate that the interaction between such anisotropic particles in the cluster breaks time reversibility. Using the same experimental particle system, we realize various modes of motion  ranging from stirrers to steerable movers with helical or directed path  depending on the magnetic configuration of a cluster. 
Wednesday, March 7, 2018 10:24AM  10:36AM 
K53.00011: Statistical mechanics of transport processes in active fluids Katherine Klymko , Dibyendu Mandal , Kranthi Mandadapu Active matter systems provide an opportunity to revisit the notions of statistical mechanics and condensed matter physics from a fresh nonequilibrium perspective. The concept of pressure has recently received particular emphasis. In order to explain the anomalous mechanical and transport characteristics in these systems, the balance equations of mass, momentum, and energy, and the associated microscopic expressions for the stress tensor and body forces need to be derived. We utilize the IrvingKirkwood procedure to derive the balance laws governing the macroscopic behavior of rotating active dumbbells starting from their microscopic dynamics. In deriving the balance of linear momentum, we find that the symmetry of the stress tensor is broken due to the presence of nonzero active torques on individual particles. The broken symmetry of the stress tensor induces internal spin in the fluid. In deriving the form for the balance of total angular momentum, we find microscopic expressions for the couple stress tensor that drives the spin field. We also show how the expressions for the stress tensor and body forces change in the case of purely convective dumbbells with no internal torques and comment on the implications for understanding pressure in convective and rotary active systems. 
Wednesday, March 7, 2018 10:36AM  10:48AM 
K53.00012: On Applying Novel Techniques to Control Active Matter Systems at the Microscale Etude O'NeelJudy , John Gibbs , Dylan Nicholls In this talk, I will discuss how photocatalysts can be exploited in artificial active matter at the microscale. Over the past decade, catalyzed chemical reactions have been used extensively for this purpose, particularly in the case of platinum, but photocatalysts have several advantages over traditional materials. The major benefit from using photocatalysts is that the activity can be switched onandoff by a light stimulus. Through codeposition of multiple matterials we can alter the bandgap and can actuate complex particles in different ways by switching the light source. Using these and other techniques, we investigate novel ways to control the motion of such active matter systems at the microscale. 
Wednesday, March 7, 2018 10:48AM  11:00AM 
K53.00013: The Rheocrystal : a flowing crystal of selfpropelled particles Olivier Dauchot , Guillaume Briand , Michael Schindler We experimentally and numerically study the dynamics of a monodisperse packing of self propelled hard disks inside a hexagonal arena [1]. The packings are dense, with area fractions close to the maximal geometrically allowed value φhcp in two dimension. Frustration of the hexagonal order, obtained by removing a few particles, leads to the formation of a rapidly diffusing “droplet”. Removing more particles, the whole system spontaneously forms a rotating flow while conserving an overall crystalline structure. We shall call this state a “rheocrystal”. Numerical simulations very well reproduce the experimental observations. They further demonstrate that when packing fraction is simply lowered without introducing frustration, the rheocrystal phase subsists, while the droplet regime disappears. Finally, the larger the system is, the faster the ”rheocrystal” rotates and the more ordered its structure. 
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