Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session P35: Microscale Flows: Active Particles |
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Chair: John Buchner, TU Delft Room: 617 |
Monday, November 25, 2019 5:16PM - 5:29PM |
P35.00001: MicroPIV measurements of flows induced by achiral microswimmers Jamel Ali, Liyuan Tan, Xiangcheng Shi, Dalhyung Kim, Min Jun Kim, U Kei Cheang We report on the low Reynolds number hydrodynamics induced by achiral microswimmers and analysis of their flow fields using particle image velocimetry. The flows produced by two types of rigid swimmers are examined. The first swimmer consists of three self-assembled magnetic beads bonded together with avidin-biotin complexes. The second swimmer consist of thin geometries in the shape of the letter `L' produced though photolithography and thin-film deposition. Both swimmers were wirelessly actuated in precessing magnetic fields using electromagnetic coils positioned in an approximate Helmholtz configuration. A high speed camera was used to capture the motion of fluorescent seeding particles as well as track the rotation of the microswimmers. Analysis of microPIV data revealed microvortices produced during swimming, while the magnitude of the local flows scaled linearly with increasing rotation rate. The attractive and repulsive flows observed may be useful for applications such as non-contact micro manipulation and assembly/disassembly of modular swimmers. [Preview Abstract] |
Monday, November 25, 2019 5:29PM - 5:42PM |
P35.00002: Templated self-assembly of colloidal microswimmers Antoine Aubret, Jeremie Palacci Biological living systems are prototypical examples of Active Matter. Cells, for instance, exhibit far-from-equilibrium behavior such as autonomous regulation or organization. Here, we show how we can carve non-equilibrium pathways for the controlled self-assembly of colloidal microswimmers using light as a tool. We use photocatalytic colloidal microparticles as primary building blocks for self-assembly. We specifically designed the particles to self-propel, and sense light gradients. Following sequential light-patterns, the particles autonomously assemble into robust self-spinning structures, or microgears. The gears interact with contactless `teeth', synchronizing their motion. We characterize the interaction potential, and show that the synchronization originates from the coupling between the chemical clouds generated by the catalytic activity of the gears and hydrodynamic interactions between their constituents. Following, the gears constitute the fundamental components of synchronized micro-machineries that auto-regulate and whose dynamics is tuned by the spins of their internal components. Our study demonstrate the potential of non-equilibrium interactions to program self-assembly of dynamical colloidal architectures. [Preview Abstract] |
Monday, November 25, 2019 5:42PM - 5:55PM |
P35.00003: Steady streaming in a simple reciprocal swimmer Nicholas Derr, Christopher Rycroft, Daphne Klotsa While biolocomotion at high and low Re has been well-studied, swimming at intermediate Re $\sim$ 1--1000---where both viscous and inertial forces are important---is less understood. Most previous investigations at intermediate Re have centered on individual species, implicitly focusing on a single example of the many ways such organisms self-propel. As a result, few underlying generic mechanisms that unify the many disparate intermediate-Re swimming methods have been identified. One possible such mechanism is steady streaming---the generation, due to inertial effects, of lower-order steady flow by periodic large-scale motion. In this talk, we examine the role of steady streaming in the locomotion of a simple reciprocal swimmer at intermediate Re. After asymptotically expanding the Navier--Stokes equations, we solve for time-periodic solutions to the resulting set of unsteady Stokes equations. We present the swimming speed and efficiency over a range of intermediate Re and comment on similarities to motility mechanisms at low Re. [Preview Abstract] |
Monday, November 25, 2019 5:55PM - 6:08PM |
P35.00004: Life in the fast layer Ernest B. Van Der Wee, Brennan Sprinkle, Isaiah Katz, Mena Youssef, Stefano Sacanna, Aleksandar Donev, Michelle M. Driscoll Microrollers are rotating particles that become active close to a wall due to an asymmetric flow of the fluid around the particles. They can be experimentally realized by driving magnetic colloidal particles hovering above a wall with a rotating magnetic field. Introducing a small fraction of fluorescently labeled microrollers, we can measure their velocities using microscopy and particle tracking. We compare our results to high resolution Brownian dynamics simulations which include lubrication effects. The velocity of a microroller is much slower than the velocity of the fluid pumped around it. Therefore, a particle put in the flow field around a microroller will have a velocity much higher than the microroller itself. As a consequence, the average velocity of a suspension of microrollers increases as a function of their density. In addition, at higher densities the particles form two layers: a slow one close to the wall and a much faster one above it. We find that the microrollers switching between the slow and the fast layer, and characterize the lifetime of the particles in the two layers. [Preview Abstract] |
Monday, November 25, 2019 6:08PM - 6:21PM |
P35.00005: Flow field on the interface induced by swimming and driven colloids Mehdi Molaei, Jiayi Deng, Tianyi Yao, Nicholas Chisholm, John Crocker, Kathleen Stebe An active colloid trapped on or near a fluid interface generates a complex flow field that differs from the bulk flow. The flow field depends on the modes of motion, the mechanics of the interface, and hydrodynamic coupling with the bulk fluids. To characterize the flow field, we introduce a method based on the correlated motion of active colloids and passive tracer particles. The challenge is to extract weak biased motions of probes via interaction with the active colloids given the noisy environment and significant Brownian displacement. We examine a gallery of active motions and their flow fields; we simultaneously measure the rheology of the interface. We first investigate a 2d bacterial suspension on an oil-water interface as a model active colloidal system. We analyze the flow field induced by pusher and puller bacteria (models of force dipoles) and ``pirouetting'' bacteria (stationary rotlet dipoles) and driven magnetic microbeads (Stokeslets). The measurements are performed for the interfaces with different viscoelasticity. Tracer particle displacement fields at various lag times are compared to calculated displacement fields for hydrodynamic modes permitted in interfacial layers as a function of rheology and compressibility. [Preview Abstract] |
Monday, November 25, 2019 6:21PM - 6:34PM |
P35.00006: Relating microswimmer synthesis to hydrodynamic actuation and rheotactic tunability Enkeleida Lushi, Quentin Brosseau, Florencio Balboa Usabiaga, Yang Wu, Leif Ristroph, Jun Zhang, Michael Ward, Michael J. Shelley We explore the behavior of micron-scale autophoretic Janus (Au/Pt) rods, having various Au/Pt length ratios, swimming near a wall in an imposed background flow. We find that their ability to robustly orient and move upstream, i.e. to rheotax, depends strongly on the Au/Pt ratio, which is easily tunable in synthesis. Numerical simulations of swimming rods actuated by a surface slip show a similar rheotactic tunability when varying the location of the surface slip versus surface drag. Slip location determines whether swimmers are Pushers (rear-actuated), Pullers (front-actuated), or in between. Our simulations and modeling show that Pullers rheotax most robustly due to their larger tilt angle to the wall, which makes them responsive to flow gradients. Thus, rheotactic response infers the nature of difficult to measure flow-fields of an active particle, establishes its dependence on swimmer type, and shows how Janus rods can be tuned for flow responsiveness. We demonstrate the effectiveness of a simple geometric sieve for rheotactic ability. [Preview Abstract] |
Monday, November 25, 2019 6:34PM - 6:47PM |
P35.00007: Mobility mechanisms condition the instabilities in active microdrops Matvey Morozov, Sebastien Michelin Chemically active droplets submerged in the bulk of surfactant solution self-propel with straight, helical, or random trajectories. Here we employ numerical simulations to establish the link between the behavior of an active drop and its interfacial properties. To this end, we consider a drop that converts the gradients of surfactant concentration into flow via two different mobility mechanisms: diffusiophoresis and the Marangoni effect. The resulting surfactant advection is the only nonlinear effect and, thus, the only source of dynamical complexity in the model. Our numerical simulations indicate that strong advection may destabilize the regime of straight and steady self-propulsion. For axisymmetric flow, this instability results in a regime of symmetric extensile flow around a stationary droplet. If advection is strengthened further, chaotic oscillations may develop. In 3D, a drop driven by diffusiophoresis alone does not exhibit extensile flow and the random behavior emerges right after the steady self-propulsion becomes unstable. Our results reveal that the thresholds of these instabilities depend heavily on the balance between diffusiophoresis and the Marangoni effect. [Preview Abstract] |
Monday, November 25, 2019 6:47PM - 7:00PM |
P35.00008: Direct Numerical Simulations of Electric Field Driven Hierarchical Self-assembly in Mixtures of Particles S. B. Pillapakkam, Suchandra Das, Edison Amah, Ian Fischerq, Pushpendra Singh We have numerically studied the process of self-assembly in particle mixtures when they are subjected to an externally applied electric field. The inter-particle electric forces cause mixtures of micron to nano sized particles to self-assemble into molecular-like hierarchical arrangements consisting of composite particles which are organized in a pattern. As in experiments for micron sized particles, the structure of a composite particle depends on factors such as the relative sizes and the number ratio of the particles, their polarizabilities, and the electric field intensity. The minimum electric field intensity required for manipulation is larger for nanoparticles for which the electric field induced lateral forces must also overcome Brownian forces. Also, for nanoparticles, the composition of composite particles was relatively more uniform because of the mixing induced by Brownian motion. Particles of mixtures containing only positively or negatively polarizable particles arrange in chains and columns which become aligned in the electric field direction, but when both type of particles are present they come together to form clusters. [Preview Abstract] |
Monday, November 25, 2019 7:00PM - 7:13PM |
P35.00009: Collisions and rebounds of active droplets Kevin Lippera, Matvey Morozov, Michael Benzaquen, Sebastien Michelin Active droplets undergoing gradual micellar dissolution and spontaneous self-propulsion have recently received much interest as prototypical experimental realisations of synthetic micro-swimmers. While the self-propulsion of a single droplet has been widely studied and is known to arise above a critical advection-to-diffusion ratio, interactions and motion in complex environments remain mostly unexplored due to the non-linearity of the transport equation and its coupling with the flow that prevent the use classical superposition methods.\\ Using a novel numerical framework relying on bi-spherical coordinate we solve the nonlinearly coupled hydrodynamics and solute dynamics exactly, enabling to characterise the rebound for various advection-to-diffusion ratios and to unravel the dominant interactions. [Preview Abstract] |
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