Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session G15: Active Colloids I |
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Sponsoring Units: DFD DSOFT Chair: Arvind Gopinath, University of California, Merced Room: 210/212 |
Tuesday, March 3, 2020 11:15AM - 11:51AM |
G15.00001: Active Microrheology, Hall Effect, and Jamming in Active Chiral Fluids Cynthia Reichhardt, Charles Reichhardt We examine the motion of a probe particle driven through an active chiral fluid composed of circularly swimming disks. We find that the probe particle travels in both the longitudinal direction, parallel to the driving force, and in the transverse direction, perpendicular to the driving force, giving rise to a Hall angle. Under constant driving force, we show that the probe particle velocity in both the longitudinal and transverse directions exhibits nonmonotonic behavior as a function of the activity of the circle swimmers. The Hall angle is maximized when a resonance occurs between the frequency of the chiral disks and the motion of the probe particle. As the density of the chiral fluid increases, the Hall angle gradually decreases before reaching zero when the system enters a jammed state. We show that the onset of jamming depends on the chiral particle swimming frequency, with a fluid state appearing at low frequencies and a jammed solid occurring at high frequencies. |
Tuesday, March 3, 2020 11:51AM - 12:03PM |
G15.00002: Hydrodynamic Spin Lattices Pedro J Saenz, Giuseppe Pucci, Sam E Turton, Ruben R. Rosales, Jorn Dunkel, John W M Bush In this talk, we introduce hydrodynamic spin lattices (HSLs) of walking droplets as a new class of highly tunable active spin analog systems. Millimetric liquid droplets can walk across the surface of a vibrating fluid bath, self-propelled through a resonant interaction with their own guiding wave fields. A walking droplet, or walker, may be trapped by a submerged circular well at the bottom of the fluid bath, leading to a clockwise or counterclockwise angular motion centered at the well. When a collection of such wells is arranged in a 1D or 2D lattice geometry, a thin fluid layer between wells enables wave-mediated interactions between neighboring walkers. For sufficiently strong pair-coupling, wave interactions between neighboring droplets may induce local spin flips leading to ferromagnetic or antiferromagnetic order. Transitions between these two forms of magnetic order can be induced through variations in non-equilibrium driving, lattice geometry and Coriolis forces mimicking an external magnetic field. Our experimental results agree with theoretical predictions from a generalized Kuramoto model, establishing HSLs as a generic paradigm for active phase oscillator dynamics with complex particle-wave coupling. |
Tuesday, March 3, 2020 12:03PM - 12:15PM |
G15.00003: Emergence of multi-vortex states in swarms of active magnetic rollers Koohee Han, Gašper Kokot, Andreas Glatz, Alexey Snezhko Active matter represents an emergent class of out-of-equilibrium dissipative systems demonstrating complex self-organization, collective behavior, and tunable functionalities. In both artificial and biological active systems, the vortex phase is of great interest because of its spontaneous emergence from a translational motion of elementary units and its ability to maintain the emergent dynamic structures. However, the generation of long-lived multi-vortex states without geometrical confinement has been challenging. We report that flocking magnetic rollers driven by a vertical a.c. magnetic field [1,2] can form long-lived multi-vortex states in an unconfined environment. We reveal that the absence of confining boundaries allows the emergence of multiple vortices with a wide distribution of vortex size and angular speed. In addition, these vortices exhibit equal probability distribution of clockwise or counterclockwise rotation with characteristic opposite chiralities of nearest neighboring vortices. |
Tuesday, March 3, 2020 12:15PM - 12:27PM |
G15.00004: Controlled Self-Assembly of Vortex in Ensembles of Active Magnetic Rollers Andrey Sokolov, Gašper Kokot, Alexey Snezhko Magnetically driven colloids, an example of active matter, are able to demonstrate complex collective behavior and self-organize into coherent dynamic patterns via short- and long-range interactions. Here we present a method for guided self-assembly of ferromagnetic rolling particles energized by a uniform AC magnetic field into a stationary vortex via magnetic interaction with an additional strongly localized magnetic field. By tuning the parameters of the additional field we effectively control vortex dimensions, internal order, and a number of entrapped rollers. We find that vortex self-organization is assisted by field-induced magnetic steering and controlled by a phase shift between alternating magnetic fields. The presented method for assisted self-organization of rolling colloids into a vortex with on-demand characteristics suggests a new paradigm for active matter control and may lead to the development of new approaches for microscopic transport in active particles systems. |
Tuesday, March 3, 2020 12:27PM - 12:39PM |
G15.00005: Self-phoretic helical colloids William Uspal, Ruben Poehnl Chemically active colloids self-propel by catalyzing the decomposition of molecular "fuel" available in the surrounding solution. If the various molecular species involved in the reaction have distinct interactions with the colloid surface, and if the colloid has some intrinsic asymmetry in its surface chemistry or geometry, there will be phoretic flows in an interfacial layer surrounding the particle, leading to directed motion. Most studies of chemically active colloids have focused on spherical, axisymmetric “Janus” particles, which (in the bulk, and in absence of fluctuations) simply move in a straight line. For particles with complex (non-spherical and non-axisymmetric) geometry, the dynamics can be much richer. Here, we consider chemically active helices and helix-sphere dimers. Via numerical calculations and slender body theory, we study how the translational and rotational velocities of the particle depend on geometry and the distribution of catalytic activity over the particle surface. Significantly, we find that both tangential and circumferential concentration gradients contribute to the particle velocity, and that the relative importance of these effects, which can be tuned by varying the particle geometry, determines the topology of the surrounding flow field. |
Tuesday, March 3, 2020 12:39PM - 12:51PM |
G15.00006: 3D printed active starships and other nonuniaxial swimmers Samia Ouhajji, Daniela Jutta Kraft Microorganisms exhibit autonomous locomotion through complex media and can react to changes in their environment. Leptospira bacteria, for example, swim towards regions of high viscosity [1]. This ability of bacteria and living cells to adapt their motion in response to external stimuli is called taxis [2]. While different kinds of taxis have been studied, most notably chemotaxis, the mechanism behind viscotaxis was not investigated until recently. Liebchen et al. found that swimmers with nonuniaxial body shapes swim up viscosity gradients due to the generation of asymmetric viscous torques acting on different parts of the swimmer [3]. We are testing these theoretical predictions using artificial microswimmers created by 3D printing based on two-photon lithography that gives us access to virtually any shape. We have started with the fabrication of trimers on the micrometre scale, the simplest form of a nonuniaxial body shape, but we are also exploring the motion of other nonuniaxial swimmers such as starships. |
Tuesday, March 3, 2020 12:51PM - 1:03PM |
G15.00007: Light Programmable Locomotions of Semiconductor Nanomotors with Optically Tunable In-Phase Electric Polarization Zexi Liang, Daniel Teal, Donglei (Emma) Fan To develop active nanomaterials that can instantly respond to external stimuli with designed mechanical motions is an important step towards the realization of nanorobots. Herein, we present our finding of a versatile working mechanism that allows instantaneous change of alignment direction and speed of semiconductor nanowires in an external electric field with simple visible-light exposure. The light induced alignment switch can be cycled over hundreds of times and programmed to express words in Morse code. With theoretical analysis and simulation, the working principle can be attributed to the optically tuned real-part (in-phase) electrical polarization of a semiconductor nanowire in aqueous suspension. The manipulation principle is exploited to create a new type of microscale stepper motor that can readily switch between in-phase and out-phase modes, and agilely operate independent of neighboring motors with patterned light. This work could inspire the development of new types of micro/nanomachines with individual and reconfigurable maneuverability for many applications. |
Tuesday, March 3, 2020 1:03PM - 1:15PM |
G15.00008: Bulk Viscosity of Dilute Gases using Nonequilibrium Molecular Dynamics Simulations Bhanuday Sharma, Rakesh Kumar Recent studies have reported that bulk viscosity, μb, may play an important role in several fluid mechanical phenomena including fluid instabilities, turbulence, and hypersonic flows. However, accurate estimation of μb is a challenging task and relies on indirect techniques like acoustic spectroscopy, Rayleigh-Brillouin scattering, and Green-Kubo method. These techniques have several limitations. In the present work, a new method is proposed for the estimation of the bulk viscosity of dilute gases using nonequilibrium molecular dynamics simulations. In this method, the fluid is expanded or compressed with a known expansion/compression rate (▽.v), where v is the velocity of the fluid. During this volume change process, mechanical (pmech) and thermodynamic pressures (pthermo) are estimated using instantaneous translational and total kinetic energy, respectively. The μb is then obtained by the relation: μb = (pthermo-pmech) / ▽.v. The proposed method is applied to estimate the μb of dilute nitrogen gas. The results are compared with available experimental data and a good agreement is observed. Effects of temperature, pressure, rate and direction (i.e., expansion or compression) of volume change, and humidity on μb are reported. (Sharma, B., & Kumar, R., Phys. Rev. E 100, 013309) |
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