Microstructure appearing in soft microswimmer suspensions

Microstructures in a suspension of microswimmers are of interest in various fields including bioengineering and robotics. Previous studies using rigid swimmers have reported that microstructures are heavily dependent on near-field hydrodynamic interactions. Although the near-field interactions are altered by the deformability of swimmers, its effect on the microstructures are largely unknown. In this study, we numerically investigated the effect of swimmer deformation on the suspension microstructures. The soft microswimmer was modeled as a capsule with a hyperelastic membrane. Thrust is generated by the torque distribution acting slightly above the membrane, mimicking ciliary activity of a ciliate. We solved the fluid mechanics of Stokes flow by a boundary element method and solid mechanics of membrane deformation by a finite element method. By increasing the membrane deformability, the polar order observed for pullers is weakened. On the other hand, the swimming velocity and the straightness in the path are gained as the deformability is increased. The maximum tension on the membrane and its position differ depending on the swimmer type. These results are useful for understanding suspension properties of biological swimmers and soft microrobots.