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 C35: Microscale Flows: Locomotion |
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Chair: Bhargav Rallabandi, University of California, Riverside Room: 617 |
Sunday, November 24, 2019 8:00AM - 8:13AM |
C35.00001: How to catch a microswimmer Ivan Tanasijevic, Eric Lauga An active particle swimming in a flow undergoing solid-body rotation always ends up diverging away from the centre of rotation. Is it possible to design a Stokes flow vortex that would trap an active particle? We address mathematically the fate of swimmers modelled as spheres, or prolate spheroids, swimming with prescribed velocities in a number of elementary vortical flows in 2D and 3D. We predict theoretically the existence of bounded orbits, which are confirmed by numerical simulations. We also address the role of fluctuations for the swimmer. These results open the possibility of designing microswimmer traps, thereby facilitating biophysical studies of cell motility. [Preview Abstract] |
Sunday, November 24, 2019 8:13AM - 8:26AM |
C35.00002: Self-propulsion of chemically active colloids Bhargav Rallabandi, Fan Yang, Howard Stone Phoretic particles are able to propel in the presence of externally applied or self-generated chemical gradients. Focusing on the non-Brownian limit, we discuss the autonomous motion of colloids that generate chemical gradients through surface reactions and consequently self-propel via diffusiophoresis. We develop a general framework by combining truncated multipole expansions for the hydrodynamic and chemical fields with the Lorentz reciprocal theorem. Applying the framework to particles with uniform surface chemical fluxes, we find that the particles translate primarily through chemical interactions, whereas hydrodynamic interactions are typically subdominant. We discuss the utility of truncated expansions in accurately describing particle trajectories, while also contrasting autophoretic motions in ionic and non-ionic solvents. We apply the model to a mixture of two particle populations and find behaviors that include pair-chasing, attraction and cluster formation. [Preview Abstract] |
Sunday, November 24, 2019 8:26AM - 8:39AM |
C35.00003: Propulsion of spherical microparticles through spontaneous symmetry breaking in mucus Henry Fu, Louis Rogowski, Jamel Ali, Xiao Zhang, MinJun Kim Symmetries have long been used to understand when propulsion is possible in microscale systems. For biological force- and torque-free swimmers the Scallop theorem applies kinematic reversibility of Stokes flow has been to understand which swimming strokes are asymmetric under time-reversal and capable of propulsion. Currently, artificially propelled magnetic micro- and nanoparticles are being utilized in a variety of techniques including hyperthermia, drug delivery, and magnetic resonance imaging. Rotation of rigid magnetic particles by an external magnetic field are a promising category of such artificial propulsion. So far it has been expected that spherical beads are too symmetric to be propelled in this fashion, but here we present experiments demonstrating that rotating spherical magnetic beads in a solution of mucin seem to display a spontaneous symmetry-breaking propulsion. We perform a perturbative analysis of a rotating sphere in a nonlinear polymeric fluid that elucidates the physical mechanism behind such a symmetry breaking. [Preview Abstract] |
Sunday, November 24, 2019 8:39AM - 8:52AM |
C35.00004: Locomotion of a rotating cylinder pair with periodic gaits at low Reynolds numbers Lingbo Ji, Wim M. van Rees We investigate the effect of periodic gaits on the self-propulsion of two side-by-side rotating cylinders at low Reynolds numbers. This cylinder-pair model serves as a prototype for engineered micro-swimmers due to its simple design and operation. To study periodic gaits, we impose a zero-net-rotation constraint on each cylinder for each swimming cycle. By numerically solving the Stokes flow around the cylinders, we can identify the optimal rotation patterns that enable the cylinder pair to travel the largest distance for a range of energy budgets. We extend this study to three dimensions by investigating the optimal swimming strategies for pairs of spheroids within a range of aspect ratios, and compare their performance to traditional Purcell swimmers and other locomotion strategies at low Reynolds numbers. [Preview Abstract] |
Sunday, November 24, 2019 8:52AM - 9:05AM |
C35.00005: Microswimmer and obstacle interactions mediated by pressure fields Florencio Balboa Usabiaga, Quentin Brosseau, Enkeleida Lushi, Yang Wu, Leif Ristroph, Jun Zhang, Michael Ward, Michael J. Shelley We revisit the problem of microswimmer orientation near boundaries. We will show how elongated swimmers create high and low pressure nodes around them and how these pressure nodes are enough to tilt the swimmers around obstacles. We address this problem with a combination of theory, simulations and experiments. We use the Rigid multiblob method to model phoretic swimmers near walls and compare the predictions with experiments of bimetallic micro-rods swimming in hydrogen peroxide solutions. Finally, we will use our mechanical explanation to suggest how phoretic swimmers can be designed to have preferred hydrodynamic interactions with walls. [Preview Abstract] |
Sunday, November 24, 2019 9:05AM - 9:18AM |
C35.00006: Hydrodynamics of Microswimmers Trapped at Fluid-Fluid Interfaces Nicholas Chisholm, Kathleen Stebe Microswimmers on or near fluid interfaces have pronounced changes in their motion, in their interactions with neighboring colloids, and in their collective behaviors. These motions and interactions can be harnessed to direct colloid assembly or alter interfacial transport conditions. However, the manner in which a fluid interface alters microswimmer motion is not yet understood. To address this gap, we develop a flow singularity model to identify leading order modes for active colloids on interfaces in the limit of small Reynolds and capillary numbers. We discuss a ``clean'' interface in which there is no jump in tangential stresses at the interface. We also consider an incompressible interface, as is typical for colloids on interfaces in the presence of surfactant. Thereafter, we examine interactions between a microswimmer and passive ``tracer'' particles for two microswimmer trajectory types: swimming in a straight line or in circular paths. These results will be useful in future work on the use of active colloids to direct and enhance transport at interfaces. [Preview Abstract] |
Sunday, November 24, 2019 9:18AM - 9:31AM |
C35.00007: Active Particles Powered by Quincke Rotation in a Bulk Fluid Debasish Das, Eric Lauga How are groups of living organisms such as flocks of birds, sheep, schools of fish and bacterial colonies able to self-organize and display collective motion? This question has fascinated scientists for decades and has given rise to the new field of 'active matter'. One of the key features of active matter is that it is composed of self-propelled units that move by consuming energy from their surrounding with a direction of self-propulsion typically set by their own anisotropy (shape or function) rather than by an external field. In this work, we show how an electrohydrodynamic instability called Quincke rotation can be exploited to develop an ideal self-propelled particle in a bulk fluid. Dielectric particles suspended in a weakly conducting fluid are known to spontaneously start rotating under the action of a sufficiently strong uniform DC electric field due to Quincke rotation. This rotation can be converted into translation when the particles are placed near a surface providing useful model systems for active matter. Using a combination of numerical simulations and theoretical models, we demonstrate that it is possible to convert the spontaneous Quincke rotation into spontaneous translation even in the absence of surfaces by relying on geometrical asymmetry instead. [Preview Abstract] |
Sunday, November 24, 2019 9:31AM - 9:44AM |
C35.00008: Harnessing low Reynolds number flow for net migration: Locomotion of a deformable microcapsule by random fluid forces Takuji Ishikawa, Takeru Morita, Toshihiro Omori, Yohei Nakayama, Shoichi Toyabe Random noise in low Reynolds number flow has rarely been used to obtain net migration of microscale objects. In this paper, we show that net migration of a microscale object can be extracted from random directional fluid forces, by introducing deformability and inhomogeneous density into the object. As a model system, we considered an elastic microcapsule containing fluid and a rigid sphere with different densities. The numerical results showed that the microcapsule could migrate vertically downward when random forces were applied using a migration mechanism based on non-reciprocal body deformation in Stokes flow. We also developed a mathematical framework to describe the deformation-induced migration caused by noise. The proposed theory showed good agreement with the simulation results, and illustrated that drag asymmetry acts like a rachet to generate net downward motion under noise. These results provide a basis for understanding the noise-induced migration of a micro-swimmer and are useful for harnessing energy from low Reynolds number flow. [Preview Abstract] |
Sunday, November 24, 2019 9:44AM - 9:57AM |
C35.00009: Generalised geometric swimming for Stokes flow Lyndon Koens, Eric Lauga Shapere and Wilczek first demonstrated that the displacement of a microscopic swimmer was related to path integrals over a gauge field. This field is a function of the swimmers configuration and laboratory frame position. For simple 1D swimmers, Stokes theorem can be used to relate the net displacement from a stroke to the area within a curve. This provides an effective method to determine the optimal strokes for displacement. However, a similar visualization is difficult for many swimmers because of complex configuration spaces or non-commuting variables. In this talk I will use a Purcell swimmer to demonstrate how to overcome these issues. These techniques reveal general properties about the displacement of microswimmers while offering a new method to optimise them. [Preview Abstract] |
Sunday, November 24, 2019 9:57AM - 10:10AM |
C35.00010: Magnetocapillary Swimmers: a Self-Assembled System to Study Locomotion, Transport Cargoes and Mix Fluids. Galien Grosjean, Ylona Collard, Maxime Hubert, Alexander Sukhov, Jens Harting, Ana-Suncana Smith, Nicolas Vandewalle Because of capillary forces, small objects floating on a liquid tend to aggregate. Combined with a magnetic induction field, this effect can be used to assemble soft-ferromagnetic spheres into tunable structures. When they are exposed to oscillating magnetic fields, these assemblies spontaneously move along the interface. This is due to a breaking of time-reversal symmetry in their adopted shapes. These structures are conceptually simple, chemically inert, and spontaneously form without direct intervention or complex microfabrication process. Therefore, they offer a very wide range of possibilities, from the experimental study of the basic physical principles of locomotion, to the development of complex tasks such as cargo transport and fluid mixing. [Preview Abstract] |
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