Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session A34: Active Matter IFocus
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Sponsoring Units: GSOFT DBIO GSNP/DFD Chair: Joern Dunkel, Massachusetts Institute of Technology Room: 337 |
Monday, March 14, 2016 8:00AM - 8:12AM |
A34.00001: And yet it moves - propulsion of colloidal clusters under reciprocal actuation Gabi Steinbach, Sibylle Gemming, Artur Erbe In the regime of low Reynolds numbers, the challenge of torque-based magnetic actuation lies in the conversion of torque into an effective force via symmetry breaking without inertial effects. Most reported systems rely on the hydrodynamic coupling between rotation and translation by an asymmetry in the environment (surfaces/interfaces) or the object shape. There, net translation can be realized only under non-reciprocal actuation given by precessing and rotating fields. In contrast, under oscillating fields, which are easier to realize, hydrodynamic coupling intrinsically leads to cyclic, reciprocal translation (Scallop theorem) unless the object has a certain flexible shape such as a flagellum. We present an alternative approach where symmetry breaking can be realized by magnetically interacting colloids which have been effectively modeled by spheres with shifted dipoles. If such colloids self-assemble, they form rigid clusters. We show how the collective, non-equilibrium dynamics of the colloids under oscillating fields propel the cluster. Depending on the configuration of the cluster it can rotate, translate and perform screw-like motion. [Preview Abstract] |
Monday, March 14, 2016 8:12AM - 8:24AM |
A34.00002: Hyperuniformity in periodically sheared dilute suspensions Sam Wilken, Rodrigo Guerra, David J Pine, Paul M Chaikin Periodically sheared dilute, non-Brownian suspensions explore new configurations through collisions in an otherwise reversible flow. Below a critical strain, the particles remain active until they find a configuration with no collisions and reach an absorbing state. Recent simulations by Hexner and Levine have shown that the configuration of particles in the critically absorbing state is hyperuniform. The particle number fluctuations of hyperuniform systems decrease with counting box size more rapidly than random systems (like the same suspension that is not in a critically absorbing state). We built a compact, lightweight uni-axial shear cell where particle coordinates can be measured while shearing with a confocal microscope. We have identified hyperuniform structures with density fluctuation measurements in colloidal suspensions of up to 40\% volume fraction in the critically absorbing state with a strain ramp down protocol and find hyperuniform scaling of the density fluctuations. [Preview Abstract] |
Monday, March 14, 2016 8:24AM - 8:36AM |
A34.00003: Convection-driven aggregation of micron sized capsules Oleg Shklyaev, Henry Shum, Anna Balazs Collective dynamics of microcapsules often serve as a model for understanding behavior observed in colonies of biological cells. Using computer simulations, we explore the capability of chemically generated convection to assemble microcapsules into a colony with neighbors close enough to facilitate chemical communication. The microcapsules are assumed to carry a supply of chemical fuel. When this fuel, leaking out of the capsules, reacts at enzyme-covered sites of the chamber, the reaction generates fluid density variations driving flows. These flows carry the microcapsules, which tend to aggregate into colonies on and near the enzyme-covered sites. This aggregation continues until the reagent has been depleted and convection stops. We show that capsule colonies of predesigned shapes can be assembled by patterning the enzyme-covered surface. [Preview Abstract] |
Monday, March 14, 2016 8:36AM - 8:48AM |
A34.00004: Electric-field mediated propulsion in binary colloidal suspensions Laura Colon-Melendez, Matthew Spellings, Sharon C. Glotzer, Michael J. Solomon We observe propulsion of pairs of unequally sized dielectric colloidal spheres in a plane perpendicular to the applied AC electric field. The fully reversible and reconfigurable effect is observed at different applied voltages and frequencies. Using confocal microscopy and particle tracking methods, we study the degree of active motion as a function of the number of particles in the dynamic clusters. The observed phenomenon is consistent with previous observations of asymmetric dumbbell propulsion in electric fields attributed to asymmetric electrohydrodynamic flow (Ma et al, PNAS 2015 112 (20) 6307-6312). [Preview Abstract] |
(Author Not Attending)
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A34.00005: Self-assembly of active colloidal molecules with dynamic function Rodrigo Soto, Ramin Golestanian Catalytically active colloids maintain non-equilibrium conditions in which they produce and deplete chemicals at their surface. While individual colloids that are symmetrically coated do not exhibit dynamical activity, the concentration fields resulting from their chemical activity decay as 1/r and produce gradients that attract or repel other colloids depending on their surface chemistry and ambient variables. This results in a non-equilibrium analogue of ionic systems, but with the remarkable novel feature of action-reaction symmetry breaking. In dilute conditions these active colloids join up to form molecules via generalized ionic bonds. Colloids are found to join up to form self-assembled molecules that could be inert or have spontaneous activity in the form of net translational velocity and spin depending on their symmetry properties and their constituents. As the interactions do not satisfy detailed-balance, it is possible to achieve structures with time dependent functionality. We study a molecule that adopts spontaneous oscillations and another that exhibits a run-and-tumble dynamics similar to bacteria. Our study shows that catalytically active colloids could be used for designing self-assembled structures that posses dynamical functionalities. [Preview Abstract] |
Monday, March 14, 2016 9:00AM - 9:12AM |
A34.00006: Optimal Navigation of Self-Propelled Colloids in Microstructured Mazes Yuguang Yang, Michael Bevan Controlling navigation of self-propelled microscopic `robots' subject to random Brownian motion in complex microstructured environments (e.g., porous media, tumor vasculature) is important to many emerging applications (e.g., enhanced oil recovery, drug delivery). In this work, we design an optimal feedback policy to navigate an active self-propelled colloidal rod in complex mazes with various obstacle types. Actuation of the rods is modelled based on a light-controlled osmotic flow mechanism, which produces different propulsion velocities along the rod's long axis. Actuator-parameterized Langevin equations, with soft rod-obstacle repulsive interactions, are developed to describe the system dynamics. A Markov decision process (MDP) framework is used for optimal policy calculations with design goals of colloidal rods reaching target end points in minimum time. Simulations show that optimal MDP-based policies are able to control rod trajectories to reach target regions order-of-magnitudes faster than uncontrolled rods, which diverges as maze complexity increases. An efficient multi-graph based implementation for MDP is also presented, which scales linearly with the maze dimension. [Preview Abstract] |
Monday, March 14, 2016 9:12AM - 9:24AM |
A34.00007: Active colloids that slosh through passive matrices. Jie Zhang, Steve Granick Studies of natural and artificial active matter have focused on systems with a large mismatch of the time and length scales for active and passive elements, but in a variety of non-equilibrium condensed matter systems, including numerous biological processes, actively driven elements have a crowded environment of surrounding passive ``solvent'' elements of comparable size. Here we study self-propelled colloidal particles in a passive matrix of comparable size. Particles with high activity take straight lines and sharp turns through the soft 2-D crystal matrix to ensure rapid healing of the crystal structure. Effective attraction between active particles arises when the concentration of active particles or the hardness of the matrix increases; active particles tend to segregate in the grain boundaries of the crystal matrix. [Preview Abstract] |
Monday, March 14, 2016 9:24AM - 9:36AM |
A34.00008: Diffusion of torqued active Brownian particles Francisco J Sevilla An analytical approach is used to study the diffusion of active Brownian particles that move at constant speed in three-dimensional space, under the influence of passive (external) and active (internal) torques. The Smoluchowski equation for the position distribution of the particles is obtained from the Kramer-Fokker-Planck equation corresponding to Langevin equations for active Brownian particles subject to torques. In addition of giving explicit formulas for the mean square-displacement, the non-Gaussian behavior is analyzed through the kurtosis of the position distribution that exhibits an oscillatory behavior in the short-time limit. FJS acknowledges support from PAPIIT-UNAM through the grant IN113114 [Preview Abstract] |
Monday, March 14, 2016 9:36AM - 9:48AM |
A34.00009: ABSTRACT WITHDRAWN |
Monday, March 14, 2016 9:48AM - 10:00AM |
A34.00010: Motile Microbots from Dynamically Interacting and Self-Reconfiguring Assemblies of Metallo-Dielectric Janus Microcubes Koohee Han, C. Wyatt Shields IV, Bhuvnesh Bharti, Gabriel P. Lopez, Orlin D. Velev A new class of dynamically and reversibly reconfigurable active matter made by magnetic assembly and actuation of metallo-dielectric microcubes will be presented. We describe how magnetically responsive Janus microcubes can be assembled hierarchically into dynamically reconfiguring microclusters. Ferromagnetic cobalt patches of the cubes act as assembly directors. The residual magnetic polarization of the metal-coated facets leads to directional dipole-dipole and field-dipole interactions and reconfiguration of the neighboring cubic particles, which is directed by the conformational restrictions. Dynamic reconfiguration of assembled clusters can be achieved by on-demand switching between the dipole-field interaction and the residual dipole-dipole interaction when the field is turned on and off. We show how pre-assembled Janus microcube clusters can be directionally motile in non-Newtonian fluids by applying asymmetric magnetic fields. The modulation of the viscosity of non-Newtonian fluids upon varying the shear rate allowed demonstrating directional motion, resulting from time-asymmetric stroke patterns (e.g., rapid opening and slow closing). These motile clusters can serve as early prototypes of self-propelling microswimmers capable of \textit{in-situ} assembly. [Preview Abstract] |
Monday, March 14, 2016 10:00AM - 10:12AM |
A34.00011: Spontaneous Oscillations in an Active Matter System Robert Hayes, Boyce Tsang, Steve Granick Active matter (which consumes energy to move about) can organize into dynamic structures more interesting than those possible at steady-state. Here we show spontaneous periodic self-assembly in a simple three-component system of water, oil phase, and surfactant at constant room temperature, with emphasis on one model system. Benchtop experiments show that liquid crystal oil droplets spontaneously and collectively oscillate like a `beating heart' for several hours; contract, relax, and subsequently re-contract in a petri dish at a rate of a few `beats' per minute. These oscillations, emergent from the cooperative interaction of the three components, are driven by the competition between positive and negative feedback processes. This illustration of feedback in action reveals a new way to program self-assembled structures to vary with time. [Preview Abstract] |
Monday, March 14, 2016 10:12AM - 10:24AM |
A34.00012: Catalytic particles induced Marangoni flow: motion, pumping and self-assembly Paolo Malgaretti, Alvaro Dominguez, Mihail N Popescu, Siegfried Dietrich When catalytic particles, such as Janus particles, or enzymes are in the vicinity of a fluid-fluid interface, their behavior can be strongly modulated by the presence of the interface and/or by the inhomogeneity in the transport properties of the two fluid phases. Hence, the effective interaction with the interface can lead to novel dynamical regimes absent in homogeneous fluids. For example, if the by-products of the catalysis are surface active their spatial distribution will affect the local value of the surface tension. In such a scenario, when a catalytic particle approaches a fluid-fluid interface a Marangoni flow will set up as a response to the inhomogeneity in the surface tension induced by the byproducts of the catalysis. The onset of such a flow will attract the catalytic particle towards the interface. Interestingly the strength of such an effective attraction is strongly affected by the affinity of the byproduct to the interface as well as by the transport properties of the two fluid phases. In particular, for water-oil interfaces such an effect overwhelms other means of active transport such as self-diffusiophoresis and makes it suitable to enhance particle accumulation close to fluid-fluid interfaces. Finally I will discuss the onset of collective behavior. [Preview Abstract] |
Monday, March 14, 2016 10:24AM - 10:36AM |
A34.00013: Dynamics of fractal cluster colloidal gels with embedded active Janus particles Michael Solomon, Megan Szakasits, Wenxuan Zhang We find that fractal cluster gels of colloids in which platinum-coated Janus particles have been embedded exhibit enhanced mobility when the Janus particles are made active by the addition of hydrogen peroxide. Gelation is induced through addition of a divalent salt, magnesium chloride, to an initially stable suspension of Janus and polystyrene colloids, each of size about 1 micron. After the gels have been created, the embedded Janus colloids are activated by hydrogen peroxide, which is delivered to the system through a porous hydrogel membrane. We vary the ratio of active to passive colloids in the gels from about 1:20 to 1:8. Changes in structure and dynamics are visualized by two channel confocal laser scanning microscopy. By image analysis, we determine the particle positions and compute the mean squared displacement (MSD) of all particles in the gel. We measure the mobility enhancement in the fractal gels as a function of hydrogen peroxide concentration and Janus particle concentration and discuss the results in terms of the force provided by each active particle to the fractal gel network. [Preview Abstract] |
Monday, March 14, 2016 10:36AM - 10:48AM |
A34.00014: Rotation-translation hydrodynamic coupling of a particle in a gradient of viscosity Naomi Oppenheimer, Shahin Navardi, Howard Stone We study the translation-rotation hydrodynamic coupling of spherical particles in Stokes flow where there are gradients of viscosity. In particular, we examine cases in which symmetry is broken by temperature gradients in the fluid. Using the Lorentz reciprocal theorem, we derive analytical expressions for the coupling tensor when the viscosity variations caused by the temperature gradients are small. We examine two cases. In the first, the temperature gradient is external and the particle moves perpendicular to it. In the second the translating object is a Janus sphere that creates its own local temperature gradient. We find that in the first case, translation induces rotation, and in the second it suppresses it. Our results may illuminate recent experimental results of Janus particles activated by light. [Preview Abstract] |
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