Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session A49: Focus Session: Self-Phoretic Colloids and Active Emulsions I |
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Sponsoring Units: GSOFT Chair: Cristina Marchetti, Syracuse University Room: 217D |
Monday, March 2, 2015 8:00AM - 8:12AM |
A49.00001: Swarming of active colloidal Janus particles: Polar waves and vortices Cong Xu, Jing Yan, Ming Han, Erik Luijten, Steve Granick The synthesis of artificial ``swarming" particles with tunable interaction represents a strong interest of the soft active matter community. Here, we demonstrate a straightforward design of swarming Janus colloids that exhibit transient mutual alignment within a certain frequency range of an applied AC electric field. In a dense two-dimensional suspension of these Janus colloids, we observe that coherent polar waves emerge at first, which then collide and merge into stable discrete vortices. Based upon a careful analysis of the pair interaction, we propose a simple mechanism that explains the formation of the polar waves, with agreement between experiment and simulation. A rich spectrum of phenomena, including dimer swarming, chain formation, and particle clustering, can be further achieved by changing the frequency of the AC electric field. [Preview Abstract] |
Monday, March 2, 2015 8:12AM - 8:24AM |
A49.00002: Confining collective motion Denis Bartolo, Antoine Bricard, Jean-Baptiste Caussin, Debasish Das, Charles Savoie, Vijayakumar Chikkadi, Kyohei Shitara, Oleskar Chepizhko, Fernando Peruani, David Saintillan Confined active materials are often found to display self-organization in the form of a macroscopic steadily rotating vortex, yet a unified description of the formation and structure of this pattern based on microscopic interaction rules remains lacking. We use a combination of experiments, numerical simulations and theory to address this question in the case of a confined population of colloidal rollers. We first demonstrate experimentally that upon increasing density this system undergoes a continuous phase transition from a dilute isotropic-gas phase to a heterogeneous polar-liquid phase. In the ordered phase, the entire population self-organizes into a single vortex lying at the onset of a phase separation.Numerical simulations confirm the existence of this non-equilibrium phase transition, and make it possible to single out the very ingredient responsible for the emergent-vortex structures: the competition between alignment and repulsive interactions. Building on this observation, we also establish a continuum theory and lay out a strong foundation for the description of emergent collective behavior in a broad class of motile populations constrained by geometrical boundaries. [Preview Abstract] |
Monday, March 2, 2015 8:24AM - 8:36AM |
A49.00003: Flocking at a distance in granular matter Harsh Soni, Sriram Ramaswamy A mixture of polar granular rods and spherical beads on a vibrated plate undergoes a phase transition to an orientationally ordered state above a critical bead concentration. We study this system using large scale numerical simulations with periodic boundary conditions. We find an intermediate state with banded structures between the disordered and the globally ordered state. We observe a single band whose width increases with rod concentration. We find that at high densities the rods and the beads phase separate. We also test the various theoretical predictions of the hydrodynamic theory in the ordered state. Our results, which are in good agreement with the theory, are following: We see a highly anisotropic dispersion relation are exhibited with two sound modes in all directions except along the flock. Further the rods are super diffusive in the transverse direction and exhibit large number fluctuations. [Preview Abstract] |
Monday, March 2, 2015 8:36AM - 8:48AM |
A49.00004: Exploring flocking via quantum many-body physics techniques Anton Souslov, Benjamin Loewe, Paul M. Goldbart Flocking refers to the spontaneous breaking of spatial isotropy and time-reversal symmetries in collections of bodies such as birds, fish, locusts, bacteria, and artificial active systems. The transport of matter along biopolymers using molecular motors also involves the breaking of these symmetries, which in some cases are known to be broken explicitly. We study these classical nonequilibrium symmetry-breaking phenomena by means of models of many strongly interacting particles that hop on a periodic lattice. We employ a mapping between the classical and quantum dynamics of many-body systems, combined with tools from many-body theory. In particular, we examine the formation and properties of nematic and polar order in low-dimensional, strongly-interacting active systems using techniques familiar from fermionic systems, such as self-consistent field theory and bosonization. Thus, we find that classical active systems can exhibit analogs of quantum phenomena such as spin-orbit coupling, magnetism, and superconductivity. The models we study connect the physics of asymmetric exclusion processes to the spontaneous emergence of transport and flow, and also provide a soluble cousin of Vicsek's model system of self-propelled particles. [Preview Abstract] |
Monday, March 2, 2015 8:48AM - 9:00AM |
A49.00005: Dynamics of cluster growth in a system of self-propelled particles Michelle Driscoll, Melissa Ferrari, Jeremie Palacci, Paul Chaikin Self-propelled particles are a simple realization of an active-matter system; particles are continuously driven and thus inherently out of equilibrium. It has recently been shown that this leads to a rich variety of behaviors, including self-organization into dynamic clusters. Particles aggregate into clusters which are not static, but are constantly growing and shrinking by exchanging particles, coalescing, and breaking apart. Here we study in detail the dynamics of this process in a system of photo-activated colloidal swimmers. Soon after the self-propulsion mechanism is activated, a large fraction of the swimmers quickly incorporate into clusters. We measure the distributions of cluster size, and show that a substantial population of small clusters can persist even at late times. Additionally, we examine how geometric confinement of the system can alter cluster growth dynamics. [Preview Abstract] |
Monday, March 2, 2015 9:00AM - 9:12AM |
A49.00006: Transitions between homogeneous phases of polar active liquids Olivier Dauchot, Khanh Dang Nguyen Thu Lam, Michael Schindler Polar active liquids, composed of aligning self-propelled particle exhibit large scale collective motion. Simulations of Vicsek-like models of constant-speed point particles, aligning with their neighbors in the presence of noise, have revealed the existence of a transition towards a true long range order polar-motion phase. Generically, the homogenous polar state is unstable; non-linear propagative structures develop; and the transition is discontinuous. The long range dynamics of these systems has been successfully captured using various scheme of kinetic theories. However the complexity of the dynamics close to the transition has somewhat hindered more basics questions. Is there a simple way to predict the existence and the order of a transition to collective motion for a given microscopic dynamics? What would be the physically meaningful and relevant quantity to answer this question? Here, we tackle these questions, restricting ourselves to the study of the homogeneous phases of polar active liquids in the low density limit and obtain a very intuitive understanding of the conditions which particle interaction must satisfy to induce a transition towards collective motion. [Preview Abstract] |
Monday, March 2, 2015 9:12AM - 9:48AM |
A49.00007: Active self-assembly and collective chemotaxis of catalytic colloids Invited Speaker: Ramin Golestanian The non-equilibrium dynamics of phoretically active colloids could lead to spontaneous formation of interesting structures and patterns due to the presence of long-range Coulomb-like interactions. We examine theoretically the consequences of this interaction, and present some results that exemplify the type of emergent properties that could result from them. In particular, we discuss the following: (1) spontaneous formation of small stable clusters or ``molecules'' that can exhibit functionality that depends on geometry (2) collective chemotaxis in a solution of catalytically active colloids that could lead to cluster formation, aster condensation, and spontaneous oscillations, and (3) swarming - in the form of a comet - of light-induced thermally active colloids with negative Soret coefficient due to a shadowing interaction. [Preview Abstract] |
Monday, March 2, 2015 9:48AM - 10:00AM |
A49.00008: Hydrodynamics of Turning Flocks Xingbo Yang, M. Cristina Marchetti We present a hydrodynamic model of flocking that generalizes the familiar Toner-Tu equations to incorporate turning inertia of well polarized flocks. The continuum equations are derived by coarse graining the inertial spin model recently proposed by Cavagna et al. The interplay between orientational inertia and bend elasticity of the flock yields spin waves that mediate the propagation of turning information throughout the flock. When the inertia is large, we find a novel instability that signals the transition to complex spatio-temporal patterns of continuously turning and swirling flocks. [Preview Abstract] |
Monday, March 2, 2015 10:00AM - 10:12AM |
A49.00009: Interactions and Collective Behaviour of Chemotactic Active Colloids Suropriya Saha, Surbhi Hablani, Ramin Golestanian, Sriram Ramaswamy Artificial realizations of motility point in the direction of a new paradigm in engineering, through the design of emergent behavior by manipulating properties at the scale of the individual components. Catalytic colloidal swimmers are a particularly promising example of such systems. Here we present a comprehensive theoretical description of gradient-sensing of an individual swimmer, leading controllably to chemotactic or anti-chemotactic behavior. We use it to study the scattering of such a swimmer off a reactant source and construct a framework for studying their two body interactions and finally their collective behavior. We find that both the positional and the orientational degrees of freedom of the active colloids can exhibit condensation, signalling formation of clusters and asters. We present prescriptions for how to tune the surface properties of the colloids during fabrication to achieve each type of behavior. We further study the athermal fluctuations of a pointed tracer particle in a bath of such swimmers. [Preview Abstract] |
Monday, March 2, 2015 10:12AM - 10:24AM |
A49.00010: The Swim Pressure of Active Matter John Brady, Sho Takatori, Wen Yan Through their self-motion, active matter systems generate a unique ``swim pressure'' that is entirely athermal in origin. This new source for the active stress exists at all scales in both living and nonliving active systems, and also applies to larger organisms where inertia is important. Here we explain the origin of the swim stress and develop a simple thermodynamic model to study the self-assembly and phase separation in active soft matter. Our new swim stress perspective may help analyze and exploit a wide class of active soft matter, from swimming bacteria and catalytic nanobots, schools of fish and birds, and molecular motors that activate the cellular cytoskeleton. [Preview Abstract] |
Monday, March 2, 2015 10:24AM - 10:36AM |
A49.00011: Diffusive properties of Brownian agents interacting via metric-free aligning-forces Francisco J. Sevilla, Luis Alberto G\'omez Nava, Jos\'e Luis Mirada Olvera, Miguel Alejandro P\'erez Contreras The diffusive properties of active particles moving at constant speed in two dimensions andinteracting through metric-free aligning-forces are studied. Exponential and scale free networks are used as the backbone for the interactions among agents. Averages over the trajectories of hundred thousand agents are performed to compute the single particle mean-square displacement, and the kurtosis of the spatial distribution, as a function of time. These quantities provide a mean to evaluate the effects of interaction. In contrast to what one would expect, the diffusion constant, increases with the intensity of the alignment. [Preview Abstract] |
Monday, March 2, 2015 10:36AM - 10:48AM |
A49.00012: Self-consistent nonlocal feedback theory for electrocatalytic swimmers Amir Nourhani, Vincent H. Crespi, Paul E. Lammert The phoretic propulsion mechanisms of electrocatalytic micro/nano-motors has received considerably more theoretical attention than the heterogeneous electrochemical processes underlying their operation. We present a flexible approach to such heterogeneous electrochemistry with nonlocal feedback using a surface bias potential field as a control parameter field with a locally open-circuit reference state. The framing in operational terms permits both convenient contact to experiment and potential for implementation in Frumkin-Butler-Volmer kinetics. Previous results are recovered in a simple approximation, and an approximate scaling form is deduced for motor speed as function of fuel concentration and swimmer size which is more consistent with data from the literature than the original linear fits. [Preview Abstract] |
Monday, March 2, 2015 10:48AM - 11:00AM |
A49.00013: ABSTRACT MOVED TO Z49.00009 |
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