Passive swimming of a microcapsule in vertical fluid oscillation

Soft Microswimmer Powered by Fluid Oscillation

In this letter, we review the results of our recent studies on a soft microswimmer powered by fluid oscillations. The microswimmer consists of an elastic membrane with a prolate spheroidal reference shape containing a rigid sphere. The swimming direction can be controlled by appropriately applying fluid oscillations. The obtained knowledge will be useful for future artificial microswimmer designs.

Active droplet driven by a collective motion of enclosed microswimmers

Active fluids containing self-propelled particles are relevant for applications such as self-mixing, micropumping, and targeted drug delivery. With a confined boundary, active fluids can generate bulk flow inside the system, which has the potential to create self-propelled active matter. In this study, we propose that an active droplet is driven by a collective motion of enclosed microswimmers. We show that the droplet migrates via the flow field generated by the enclosed microswimmers; moreover, the locomotion direction depends on the swimming mode of these internal particles. The locomotion mechanism of the droplet can be well explained by interfacial velocity, and the locomotion velocity shows good agreement with the Lighthill-Blake theory. These findings are essential to understand the interplay between the motion of self-propelled particles and the bulk motion response of active matter.

Harnessing random low Reynolds number flow for net migration

Random noise in low Reynolds number flow has rarely been used to obtain net migration of microscale objects. In this study, we numerically show that net migration of a microscale object can be extracted from random directional fluid forces in Stokes flow, by introducing deformability and inhomogeneous density into the object. We also developed a mathematical framework to describe the deformation-induced migration caused by noise. These results provide a basis for understanding the noise-induced migration of a microswimmer and are useful for harnessing energy from low Reynolds number flow.

Stability of a Dumbbell Micro-Swimmer

A squirmer model achieves propulsion by generating surface squirming velocities. This model has been used to analyze the movement of micro-swimmers, such as microorganisms and Janus particles. Although squirmer motion has been widely investigated, motions of two connected squirmers, i.e., a dumbbell squirmer, remain to be clarified. The stable assembly of multiple micro-swimmers could be a key technology for future micromachine applications. Therefore, in this study, we investigated the swimming behavior and stability of a dumbbell squirmer. We first examined far-field stability through linear stability analysis, and found that stable forward swimming could not be achieved by a dumbbell squirmer in the far field without the addition of external torque. We then investigated the swimming speed of a dumbbell squirmer connected by a short rigid rod using a boundary element method. Finally, we investigated the swimming stability of a dumbbell squirmer connected by a spring. Our results demonstrated that stable side-by-side swimming can be achieved by pullers. When the aft squirmer was a strong pusher, fore and aft swimming were stable and swimming speed increased significantly. The findings of this study will be useful for the future design of assembled micro-swimmers.