Session U09: Microscale Flows: Locomotion (8:45am - 9:30am CST)

Viscosity destabilizes the propulsion dynamics of active droplets.

Biological micro-organisms have developed sophisticated swimming behaviors such as run-and-tumble or switch-and-flick. These complex functions depend on their complicated biophysical machinery. In efforts to develop artificial micro-swimmers, the aim is to build a minimal system based on the principles of out-of-equilibrium physics that is able to mimic such complex behaviors. In this work, we show that an active droplet, undergoing micellar solubilization, experiences unsteady self-propulsion in response to an increase in the viscosity of the swimming medium. The origins of this seemingly counterintuitive behavior is explained using theory in conjunction with a novel experimental technique to simultaneously visualize the hydrodynamic and chemical fields around the droplet. By varying the viscosity we can tune the propulsion dynamics and observe behaviors reminiscent of natural micro-swimmers.   

A note on helical locomotion in a porous medium.

Microorganisms and artificial microswimmers often need to swim through environments that are more complex than purely viscous liquids in their natural habitats or operational environments, such as gel-like mucus, wet soil and aquifer. The question of how properties of these complex environments affect locomotion has attracted considerable recent attention. In this work, we focus on helical locomotion for its ubiquity as a propulsion mechanism adopted by many swimming bacteria. We present a theoretical model to examine how the additional resistance due to the network of stationary obstacles in a porous medium affects helical locomotion. Compared with previous theoretical and experimental results, we will elucidate the effects of the resistance on various types of helical locomotion. We also remark on the limitations as well as potential connections of our results with experimental measurements of bacterial swimming speeds in polymeric solutions.   

Self-Phoretic Helical Particles

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. 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 surrounding flow field.   

Rheotaxis of droplet swimmers in confinements

Biological microswimmers commonly navigate confinements having liquid flows, e.g. locomotions of spermatozoa through the reproductive tract and bacteria in the gut. The directed motion of the microorganisms in response to the gradients in external flow velocity is called `rheotaxis'. Recently, rigorous efforts have been made to understand the rheotaxis of microorganims, specifically bacteria. In contrast, there is little quantitative understanding of rheotaxis of artificial microswimmers. Here, we elucidate the swimming dynamics of a common type of artificial microswimmer, i.e. active droplets, in micro-confinements having Poiseuille flow. We experimentally quantify the rheotaxis of these droplet microswimmers, intrinsically undergoing Marangoni stress dominated `self-propulsion', in response to varying velocity gradients. The active droplets exhibit unique oscillatory rheotaxis in a confinement, which we try to understand using a hydrodynamic model. The latter takes into consideration the hydrodynamic interactions of a finite-sized swimmer with the confining walls. We strongly feel that detailed understanding of artificial active matter rheotaxis will make significant contributions towards better design optimization for practical applications.   

Metallic microswimmers driven up the wall by gravity

As a natural and functional behavior, various microorganisms exhibit gravitaxis by orienting and swimming upwards against gravity. We study the swimming of autophoretic nanomotors, which are bimetallic and rod-shaped particles, and find that when moving near inclined walls, these tail-heavy rods preferentially orient upwards and swim up along the wall. Through experiment and theory, we identify two mechanisms that contribute to their gravitactic behavior. First, a buoyancy or gravitational torque acts on these rods to align them upwards. Further, hydrodynamic interactions of the rod with the inclined wall induce a fore-aft drag asymmetry on the rods that reinforces their orientation bias and promotes their upward motion.   

Generating net rotational motion at low Reynolds number via reinforcement learning

Locomotion at the microscopic scale encounters stringent constraints due to the absence of inertia. Here we apply a recent framework based on reinforcement learning (Tsang et al., Phys. Rev. Fluids, \textbf{5}, 074101, 2020) to generate net rotational motion at low Reynolds numbers. Without prior knowledge of locomotion, the system develops effective policies based on its interactions with the surrounding environment. We compare the results with previously known strategies and remark on the possibility of more complex maneuvers.   

Propulsion via flexibility in shear-thinning fluids

In the low Reynolds number regime, flexibility can be exploited to enable propulsion via the interaction of an elastic body and its surrounding fluid. Biological fluids such as blood and mucus are typically shear-thinning. However, the impact of this ubiquitous non-Newtonian rheology on elastohydrodynamic propulsion remains largely unknown. Here we consider a minimal model to examine how shear-thinning viscosity could modify the fluid-structure interaction and hence the propulsion performance. The results may be useful for the design of artificial micro-propellers in biological fluids.   

Dynamic 3D velocity control with microfluidic device

Many innovative techniques for microscopic manipulation have been developed due to the growing need for precise experimental probes into the world of microorganisms. Here we present a technique rooted deeply in the symmetry granted by the low Reynolds number regime, coupled with a well-chosen device geometry which exploits that symmetry. We show that the use of this pairing allows for the finely-controlled application of pressure to generate microscale 3D flows with symmetry-induced qualities that are purely directional and strain-free, or stationary and purely strainful. Further, we demonstrate that this set of flow modes can be dynamically used to control the velocity for any prescribed motion of microobjects through a fluid. Thus, our apparatus fills a unique need to control microobjects that are sensitive to the mechanical properties of the medium in which they are swimming, and to stimulate them only as experimentally desired.   

Novel colloidal probes to quantify hydrodynamic and phoretic interactions

Janus microswimmers are considered model systems in the study of active matter. They are composite particles constituted of a chemically inert and catalytic component. They consume the surrounding fuel, converting available free energy into work by harnessing the interfacial phenomenon of diffusiophoresis, the migration in a gradient of chemicals. As they move the fluid, microswimmers interact hydrodynamically and via phoresis. Quantifying and understanding non-equilibrium interactions is of primary importance for the design of artificial micromachines made of active components and emergent properties in collections of active particles. Generic colloidal tracers are ill-defined for this task as they are sensitive to gradients (phoresis) and advected by flows (hydrodynamics). We develop and synthesize novel colloidal tracers with tunable phoretic mobility to quantify and disentangle phoretic and hydrodynamic interactions. We demonstrate that the particles allow us to better understand the behavior of individual microswimmers and the contributing interactions of more complex machines, e.g. self-spinning microgears. Our approach will constitute an adequate benchmark to guide the design of complex interactions in assemblies of active microparticles.