Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session H34: Active Matter VFocus
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Sponsoring Units: GSOFT DBIO GSNP/DFD Chair: Erik Luijten, Northwestern University Room: 337 |
Tuesday, March 15, 2016 2:30PM - 2:42PM |
H34.00001: Active colloids propelled by induced-charge electrophoresis Ming Han, Erik Luijten Populations of motile organisms exhibit a variety of collective behaviors, ranging from bacterial colony formation to the flocking of birds. Current understanding of these active motions, which are typically far from equilibrium and based on the collective behavior of self-propelled entities, is far from complete. One approach is to reproduce these observations in systems of synthetic active colloids. However, one of the standard self-propulsion mechanisms, induced-charge electrophoresis (ICEP) of a dielectric Janus colloid remains not fully understood by itself, especially the strong dependence of the resultant particle motion on the frequency of the external field. Resolution of this outstanding problem requires detailed study of the time-resolved dielectric response of the colloid and the dynamics of the electric double layer. Through molecular dynamics simulations coupled with an efficient dielectric solver, we elucidate the underlying mechanism of the frequency dependence of ICEP and the polarization of a metallodielectric Janus colloid. [Preview Abstract] |
Tuesday, March 15, 2016 2:42PM - 2:54PM |
H34.00002: Driving magnetic colloidal polymers Joshua Dempster, Monica Olvera de la Cruz Magnetic colloids are of growing interest for applications such as drug delivery and in vitro tissue growth. Recent experiments have synthesized 1D chains of magnetic colloids into permanent colloidal polymers. We study magnetic colloidal polymers theoretically and computationally under the influence of time-varying external fields and find a rich set of controllable, dynamic conformations. By iterating through a sequence of conformations, these polymers can perform mechanical functions. We discuss possible roles for these polymers beyond those considered for single colloids. [Preview Abstract] |
Tuesday, March 15, 2016 2:54PM - 3:06PM |
H34.00003: Broken detailed balance observed for hydrodynamically coupled colloidal particles in two optical traps Christoph F. Schmidt, Jannes Gladrow, Nikta Fakhri, Chase P. Broedersz Optical traps can be approximated as harmonic potentials for refractile colloidal particles. If two particles are trapped by two optical traps at close distance, particles remain hydrodynamically coupled. With two traps at different laser powers, one can create a non-equilibrium situation, where one particle can feed energy to the other particle. We show that this situation leads to the breaking of detailed balance that can be visualized as finite flux in a coarse-grained phase space spanned by the displacements of the two particles. We test if the slight temperature difference caused by different laser powers drives the flux or if it is due to energy dissipation associated with scattered light, a second-order effect for non-absorbing particles. [Preview Abstract] |
Tuesday, March 15, 2016 3:06PM - 3:42PM |
H34.00004: Collective motion in populations of colloidal bots Invited Speaker: Denis Bartolo One of the origins of active matter physics was the idea that flocks, herds, swarms and shoals could be quantitatively described as emergent ordered phases in self-driven materials. From a somehow dual perspective, I will show how to engineer active materials our of colloidal flocks. I will show how to motorize colloidal particles capable of sensing the orientation of their neighbors and how to handle them in microfluidic chips. These populations of colloidal bots display a non-equilibrium transition toward collective motion. A special attention will be paid to the robustness of the resulting colloidal flocks with respect to geometrical frustration and to quenched disorder. [Preview Abstract] |
Tuesday, March 15, 2016 3:42PM - 3:54PM |
H34.00005: Self-similarity in active colloid motion Colin Constant, Sergey Sukhov, Aristide Dogariu The self-similarity of displacements among randomly evolving systems has been used to describe the foraging patterns of animals and predict the growth of financial systems. At micron scales, the motion of colloidal particles can be analyzed by sampling their spatial displacement in time. For self-similar systems in equilibrium, the mean squared displacement increases linearly in time. However, external forces can take the system out of equilibrium, creating active colloidal systems, and making this evolution more complex. A moment scaling spectrum of the distribution of particle displacements quantifies the degree of self-similarity in the colloid motion. We will demonstrate that, by varying the temporal and spatial characteristics of the external forces, one can control the degree of self-similarity in active colloid motion. [Preview Abstract] |
Tuesday, March 15, 2016 3:54PM - 4:06PM |
H34.00006: Paramagnetic colloids in rotating fields: from chains through chaos to clusters. Hamed Abdi, Rasam Soheilian, Randall Erb, Craig Maloney We present computer simulations and experiments on dilute suspensions of superparamagnetic particles subject to rotating magnetic fields. ~We focus on short chains of particles and their decay routes to stable structures. ~At low rate, the chains track the external field. ~At intermediate rate, the short chains break up but perform a periodic (albeit complex) motion. ~At sufficiently high rates, the chains generally undergo chaotic motion at short times and decay to either close-packed clusters or more dispersed colloidal~``molecules'' at long times. ~We show that the transition out of the chaotic states follows a first order reaction kinetics. [Preview Abstract] |
Tuesday, March 15, 2016 4:06PM - 4:18PM |
H34.00007: Hydrodynamic self-organization and mixing in suspensions of micro-rotors Kyongmin Yeo, Enkeleida Lushi, Petia Vlahovska Self-organization of active objects has attracted considerable attention recently, especially in the context of living systems. Hydrodynamic interactions can play a crucial role in the emerging behavior when the objects are immersed in fluid, especially in the low Reynolds number regime. While self-propelled active objects have been extensively investigated, the collective behavior of rotating active particle has received limited attention. To elucidate the transition to collective behavior and especially the role of multi-body hydrodynamic interactions, we numerically study systems of co- and counter-rotating spheres by varying the mixture ratio as well as the total volume fraction. With increasing volume fraction, we observe the emergence of intriguing patterns such as lanes, vortices of same-spin rotors as well as dynamic crystals composed of both types of rotors. We consider how the motion of the rotating particles and fluid they collectively generate affects the dispersion or clustering of passive sphere particles or mixing of passive scalar fields in the system. [Preview Abstract] |
Tuesday, March 15, 2016 4:18PM - 4:30PM |
H34.00008: Collective dynamics of rotating colloidal particles Sofia Magkiriadou, Vishal Soni, Benny van Zuiden, Denis Bartolo, Vincenzo Vitelli, William T.M. Irvine We study magnetic colloidal particles in suspension under the influence of a rotating magnetic field. When in aggregates, these particles show rich dynamics that are governed by magnetic and hydrodynamic interactions. By tuning these interactions, we probe the phase diagram of this system and study the emergent collective dynamics. Finally, we begin to investigate whether we can control this phase diagram with geometry. [Preview Abstract] |
Tuesday, March 15, 2016 4:30PM - 4:42PM |
H34.00009: Helical motion of chiral liquid crystal droplets Takaki Yamamoto, Masaki Sano Artificial swimmers have been intensively studied to understand the mechanism of the locomotion and collective behaviors of cells and microorganisms. Among them, most of the artificial swimmers are designed to move along the straight path. However, in biological systems, chiral dynamics such as circular and helical motion are quite common because of the chirality of their bodies, which are made of chiral biomolecules. To understand the role of the chirality in the physics of microswimmers, we designed chiral artificial swimmers and the theoretical model for the chiral motion. We found that chiral liquid crystal droplets, when dispersed in surfactant solutions, swim in the helical path induced by the Marangoni effect. We will discuss the mechanism of the helical motion with our phenomenological model. [Preview Abstract] |
Tuesday, March 15, 2016 4:42PM - 4:54PM |
H34.00010: Rolling and spinning swimmers Michelle Driscoll, Melissa Ferrari, Mena Youssef, Paul Chaikin, Stefano Sacanna We study the dynamics and collective interactions that occur in a system of rotating active matter: an oscillating, externally applied magnetic field is used to drive motion in a system of confined, magnetic colloids. By adjusting the orientation, frequency, and amplitude of the applied field we can drive a wide range of particle motions, from rolling to spinning. These rotations lead to a large variety of collective behaviors, which are driven both by particle-particle magnetic interactions as well as long-range hydrodynamic flows. We observe that the clustering which results from in-plane spinning can be strongly modulated by changing inter-particle magnetic interactions. We explore the strength of this clustering as a function of particle interaction, and can isolate the effect of magnetic and hydrodynamic interactions. We also observe that particle rotation can lead to complex and large-scale flows for both the case of rolling and spinning particles. [Preview Abstract] |
Tuesday, March 15, 2016 4:54PM - 5:06PM |
H34.00011: Emergent order in ensembles of active spinners Benjamin C. van Zuiden, Jayson Paulose, William T. M. Irvine, Denis Bartolo, Vincenzo Vitelli Interacting self-propelled particles is proxy to model many living systems from cytoskeletal motors to bird flocks, while also providing a framework to investigate fundamental questions in non equilibrium statistical mechanics. A surge of recent studies have shown that self-propulsion significantly modifies the phase behavior of particles interacting via potential interactions. A prototypical example is the so-called Motility Induced Phase Separation occurring in ensembles of self-propelled hard spheres. In stark contrast, our understanding of active spinning, as opposed to self-propulsion, remains very scarce. Here, we study a system of self-spinning dimers, interacting via soft repulsive forces. Upon varying the density and activity, we observe a range of emergent phases characterized by different degrees of spatiotemporal order in the position and orientation of the dimers. Changes in bulk properties, including crystallization, melting, and freezing, are reflected in the collective motion of the particles. We rationalize our numerical findings theoretically and demonstrate some of these concepts in a active granular experiment. [Preview Abstract] |
Tuesday, March 15, 2016 5:06PM - 5:18PM |
H34.00012: Emergent collective dynamics in ensembles of magnetic colloidal rollers Alexey Snezhko Strongly interacting colloids driven out-of-equilibrium by an external periodic forcing often develop nontrivial collective dynamics. Ferromagnetic micro-particles immersed in water and sediment on the bottom surface of the flat cell are energized by a single-axis homogeneous alternating magnetic field applied perpendicular to the surface supporting the particles. Upon application of the alternating magnetic field the magnetic torque on each particle is transferred to the mechanical torque giving rise to a rolling motion of the particle. Experiments reveal a rich collective dynamics of magnetic rollers in a certain range of excitation parameters. Flocking and spontaneous formation of steady vortex motion have been observed. The effects are fine-tuned and controlled by the parameters of the driving magnetic field. Formation of the self-organized collective states spontaneously breaking the symmetry of the underlying interactions has been attributed to the interplay of inelastic inter-particle collisions and self-induced hydrodynamic flows in the system. The research was supported by the U.S. DOE, Office of Basic Energy Sciences, Division of Materials Science and Engineering. [Preview Abstract] |
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