Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session S15: Focus Session: Active Soft Matter III - Soft, Self-Propelled Particles |
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Sponsoring Units: DPOLY DBIO GSNP Chair: Juan Aragones, Massachusetts Institute of Technology Room: 304 |
Thursday, March 6, 2014 8:00AM - 8:12AM |
S15.00001: Surprises in the nonequilibrium self-organization of active Janus particles Jie Zhang, Jing Yan, Steve Granick As a minimal model of natural active matter and nonequilibrium system, self-propelled colloidal particles are excellent for the study of collective behavior that is common in biological systems spanning from mesoscopic to macroscopic scales, such as swarming, pattern formation and anomalous fluctuations. Here we use induced-charge electrophoretic (ICEP) Janus particles to show experimentally that self-propelled active colloidal particles can display macroscopic phase aggregation despite only particle-particle repulsion in this system. The evolution of the active dense phase grows with time following a power law of 1/2. Strikingly, this agrees with the attachment-detachment coarsening mechanism of phase separation systems at thermodynamic equilibrium. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S15.00002: Self-propelled Janus particles in an asymmetric channel: Effects of rectification and autonomous pumping Vyacheslav R. Misko, Pulak K. Ghosh, Fabio Marchesoni, Franco Nori Using numerical simulations, Brownian transport of self-propelled overdamped microswimmers (i.e., Janus particles) in a two-dimensional periodically compartmentalized asymmetric channel has been investigated for different compartment geometries, boundary collisional dynamics, and particle rotational diffusion [1]. The resulting time-correlated active Brownian motion is subject to rectification in the presence of spatial asymmetry. We demonstrate that ratcheting of Janus particles is much stronger than for ordinary thermal potential ratchets and thus experimentally accessible. In particular, we show that autonomous pumping of a large mixture of passive particles can be induced by just adding a small fraction of self-propelled Janus particles. \\[4pt] [1] Pulak K. Ghosh, Vyacheslav R. Misko, Fabio Marchesoni, and Franco Nori, Phys. Rev. Lett. {\bf 110}, 268301 (2013). [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S15.00003: Territory Covered by $N$ Self-Propelled Brownian Agents in 2 dimensions Francisco J. Sevilla, Luis Alberto G\'omez Nava We consider the problem of the territory covered by $N$ non-interacting self-propelled Brownian agents where self-propulsion is modeled by a non-linear friction term in the Langevin-like equations of motion for each agent. Our study generalizes, to a continuous time and space description, the well known problem of the territory explored by $N$ Random Walkers [1]. Numerical and analytical approaches are presented to exhibit the effects of self-propulsion on the many independent agents exploring two dimensional homogenous regions. Our results may have a wide range of applications in a variaty of non-equilibrium systems. \\[4pt] [1] L Hernan et al. Territory covered by $N$ diffusing particles. Nature 355, 423-426 (1992). [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S15.00004: Self-propelling microswimmer made of a bi-faced hydrogel Alexander Alexeev, Svetoslav V. Nikolov, Peter Yeh We use dissipative particle dynamics to design a new self-propelling microswimmer. Our microswimmer features a simple design and represents an X-shaped gel layer. The gel has two distinct sides or facets: one facet is responsive and swells when an external stimulus is applied, whereas the other facet is passive. We show that when an external stimulus is applied periodically, the swimmer propels itself unidirectionally in a highly viscous fluid. The propulsion is associated with periodic shape changes. When the stimulus is applied, the responsive facet swells, inducing internal stresses in the gel causing both lateral expansion and bending of the microswimmer. When the external stimulus is removed, the responsive layer contracts, and elastic forces cause the microswimmer to straighten and to recover the initial shape. The combination of the sequential expansion, bending, contraction, and straightening produces a time irreversible motion pattern leading to net propulsion at low Reynolds number. We examine how the swimming speed can be enhanced by selecting material properties and geometry of the swimmer. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S15.00005: Phase separation of biphasic mixture of active Janus colloids Jing Yan, Ming Han, Erik Luijten, Steve Granick Recently there is a surge of interest in the phase behavior of active matter in which building blocks display self-propelling motion. Although much has been known from theory and simulation, experimental examples are very rare. Specifically, the epitomic problem of a binary mixture of active matter defies any experiment or theory so far. Here we present an experimental realization of binary mixture of particles, which only acquires activity when they collisionally interact with the opposite kind. We used a system in which the only difference in the two particles is the phase in their cyclic motion, precluding any artifact due to difference in interparticle potential. We observe phenomena strikingly similar to spinodal decomposition of molecular system, in addition to new features due to the nonequilibrium nature of the system. We derived a general, effective Flory-Huggins theory for spinodal decomposition of bicomponent active system, and rationalized the 1/3 power law growth of the domain size in regions where thermodynamic analogy is valid. The system also presents a plethora of nonequilibrium phenomena such as critical fluctuation, lane formation, and dynamic absorbing state in different parameter space. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S15.00006: Optimal hydrodynamic synchronization of colloidal rotors Pietro Cicuta, Jurij Kotar, Nicolas Bruot, Luke Debono, Stuart Box, Stephen Simpson, David Phillips, Simon Hanna Synchronization of driven oscillators is a key in flow generation in artificial and biological systems at the micro-scale. Flow can be driven efficiently by filaments undergoing periodic motion. These filaments are typically colloidal-scale and semi-flexible, and thus have many conformational degrees of freedom and are subject to thermal noise; in the case of biological cilia, they are driven in a complex fashion by internal molecular motors that induce bending. The question of synchronization is thus best addressed by simpler systems, such as individual driven spheres, in which the multiple degrees of freedom are coarse-grained into a few control parameters which can be tuned and understood theoretically, and in which the hydrodynamic interaction is readily described. The system of `rotors' is considered here: spheres are driven along predefined trajectories, with a given force law. In this model it is possible to address quantitatively the conditions for hydrodynamic synchronization. Previous theoretical work pointed to the importance of two factors: modulation of the driving force around the orbit, or the deformability of the trajectory. We show via experiments, numerical simulations and theory that both factors are to be considered, and at play in biological systems. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S15.00007: Mode-coupling theory for the glassy dynamics of self-propelled particles Grzegorz Szamel, Elijah Flenner, Ludovic Berthier We examine glassy dynamics of self-propelled particles. The self-propulsion is modeled as a random force that evolves according to the Ornstein-Uhlenbeck process. Starting from the microscopic description of the dynamics, we derive an effective many-particle diffusion equation describing the time evolution of the probability density of the particles' positions. Next, we assume pair-wise additivity of the effective many-particle interaction and use the standard procedure to derive mode-coupling equations for the time-dependence of density fluctuations. The most important consequence of the self-propulsion is the replacement of the equilibrium structure factor by the self-propulsion-dependent steady state structure factor. To test the theory, we use steady state structure factors obtained from computer simulations of self-propelled particles. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S15.00008: Accelerated Self-Replication under Non-Equilibrium, Periodic Energy Delivery Rui Zhang, Monica Olvera de la Cruz Self-replication is a remarkable phenomenon in nature that has fascinated scientists for decades. In a self-replicating system, the original units are attracted to a template, which induce their binding. In equilibrium, the energy required to disassemble the newly assembled copy from the mother template is supplied by thermal energy. The possibility of optimizing self-replication is explored by controlling the frequency at which energy is supplied to the system. A model system inspired by a class of light switchable colloids is considered where light is used to control the interactions. Conditions under which self-replication can be significantly more effective under non-equilibrium, cyclic energy delivery than under equilibrium constant energy conditions are identified. Optimal self-replication does not require constant energy expenditure. Instead, the proper timing at which energy is delivered to the system is an essential controllable parameter to induce high replication rates. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S15.00009: Dynamics and Jamming for Run-and-Tumble Swimmers in the Presence of Quenched Disorder Cynthia Olson Reichhardt, Charles Reichhardt We consider run-and-tumble swimmers in two dimensions that interact with each other and with a random array of obstacles. In the absence of obstacles, there is a well-defined transition to a cluster or living crystal state for increasing density and run length. We apply a directional drift force to the particles such that they would move at an average drift velocity in the absence of obstacles. In the presence of quenched disorder, the average drift velocity initially increases with increasing run time before reaching a maximum and then decreasing for increasing run time. We correlate the regime where the drift velocity decreases with the onset of the clustering phase. We also show that for increasing density, the obstacles induce an active matter jamming phase, and that the density at which the jamming occurs decreases for increasing run length. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S15.00010: Microscopic walkers in concentrated colloidal monolayers: Oscillators, rollers and spinners Juan Luis Aragones, Joshua Steimel, Alfredo Alexander-Katz We have studied the dynamical behavior of paramagnetic particles under a rotational magnetic field in concentrated passive colloidal monolayers, and their effects on the dynamics and structure of the monolayer. Depending on the direction of the applied rotating magnetic field, paramagnetic particles will rotate parallel or perpendicular to the substrate plane, generating two types of active particles: \textit{rollers}, whose angular momentum is converted to translational motion through the force of friction between the particles and substrate, and \textit{spinners}, which rotate parallel to the substrate. Additionally, \textit{oscillators} can be created from \textit{rollers} by applying oscillating magnetic fields. We have carried out experiments and simulations to analyze the dynamics of these active particles in dense colloidal monolayers. The non-equilibrium nature of these systems confers quite interesting behavior; we observed activity-induced phase separation and local vortex formation around \textit{rollers} and \textit{spinners} due to the fluid media. These vortices interact between them creating patterns and cooperative movements. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S15.00011: Microscopic Tribotactic Walkers Joshua Steimel, Juan Aragones, Alfredo Alexander-Katz The translational motion of a rotating object near a surface is strongly dependent on the friction between the object and the surface. The process of friction is inherently directional and the friction coefficient can be anisotropic even in the absence of a net friction coefficient gradient. This is macroscopically observed in the ordering motif of some animal hair or scales and a microscopic analog can be imagined where the friction coefficient is determined by the strength and density of reversible bonds between a rotating object and the substrate. For high friction coefficients most of the rotational motion is converted into translational motion; conversely for low friction coefficients the object primarily rotates in place. We exploited this property to design and test a new class of motile system that displays tribotaxis, which is the process by which an object detects differences in the local friction coefficient and moves accordingly either to regions of higher or lower friction. These synthetic tribotactic microscopic walkers, composed of a pair of functionalized superparamagnetic beads, detect gradients in the spatial friction coefficient and migrate towards high friction areas when actuated in a random fashion. The effective friction between the walkers and the substrate is controlled by the local density of active receptors in the substrate. The tribotactic walkers also displayed trapping in high friction areas where the density of free receptors is higher. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S15.00012: Swimming bacteria in liquid crystal Andrey Sokolov, Shuang Zhou, Igor Aranson, Oleg Lavrentovich Dynamics of swimming bacteria can be very complex due to the interaction between the bacteria and the fluid, especially when the suspending fluid is non-Newtonian. Placement of swimming bacteria in lyotropic liquid crystal produces a new class of active materials by combining features of two seemingly incompatible constituents: self-propelled live bacteria and ordered liquid crystals. Here we present fundamentally new phenomena caused by the coupling between direction of bacterial swimming, bacteria-triggered flows and director orientations. Locomotion of bacteria may locally reduce the degree of order in liquid crystal or even trigger nematic-isotropic phase transition. Microscopic flows generated by bacterial flagella disturb director orientation. Emerged birefringence patterns allow direct optical observation and quantitative characterization of flagella dynamics. At high concentration of bacteria we observed the emergence of self-organized periodic texture caused by bacteria swimming. Our work sheds new light on self-organization in hybrid bio-mechanical systems and can lead to valuable biomedical applications. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S15.00013: ``Casimir effect'' with active swimmers Dipanjan Ray, Lena Lopatina, Cynthia Olson Reichhardt, Charles Reichhardt In recent years, active matter has increasingly found applications in nanoengineering.\footnote{R. DiLeonardo et al, Proc. Natl. Acad. Sci. U.S.A. 107, 9541 (2010); B. Kaehr and J. B. Shear, Lab on a Chip, 9, 2632 (2009).} Here we show using molecular dynamics simulations that the natural motion of ``run-and-tumble'' bacteria will push together two parallel walls arranged in a Casimir geometry. This effect is robust as long as the wall separation is comparable to or smaller than the bacterial run-length, so that the bacterial motion is not Brownian on the length scale of the walls. The magnitude of the attractive force between the walls exhibits an unusual exponential dependence on the wall separation. The attraction arises from a depleted concentration of bacteria in the region between the plates; this is caused by the tendency of the bacteria to slide along the walls, which breaks time-reversal symmetry and allows a density difference to develop. The same mechanism was used recently to explain bacterial rectification.\footnote{M. B. Wan et al, Phys. Rev. Lett., 101, 018102 (2008); J. Tailleur and M. E. Cates, Europhys. Lett. 86, 60002 (2009).} The inclusion of steric interactions between the bacteria reduces the attraction between the plates but does not eliminate it. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S15.00014: Confined dynamics of non-aligning self-propelled particles in the small box limit Yaouen Fily, Aparna Baskaran, Michael Hagan Recent years have brought the realization that even the simplest of active particle models can exhibit rich behavior. Here we study the confined dynamics of non-aligning self-propelled particles when the size of the confining box is small compared with the distance traveled by a particle before its orientation decorrelates. Using a combination of analytical and computational tools, we characterize the inhomogeneities of the density for a class of box shapes. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S15.00015: Swimming against the flow - An orientational disorder to order transition Chih-kuan Tung, Florencia Ardon, Alyssa G. Fiore, Lian Hu, Susan S. Suarez, Mingming Wu Micro-organisms often need to swim against fluid flows for their survival. In native state, mammalian sperm swim against a flow to reach the egg. Using bull sperm as a model system, we studied the impact of fluid flows on sperm swimming behavior. Interestingly, we find that a directional swimming pattern emerges as the fluid flow rate exceeds a critical value. Using the average directional vector, $<$$S_{x}$$>$ or $<$$S_{y}$$>$ of the sperm head, as an order parameter, and fluid flow rate (along $x$-axis) as a control parameter, we find that $<$$S_{x}$$>$ or $<$$S_{y}$$>$ increases continuously with the increase of flow rate above the onset point, following a power law with an exponent close to 0.5. We will discuss the sources of this transition, and implications in both physics and biology. [Preview Abstract] |
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