Phase Separation and Coexistence of Hydrodynamically Interacting Microswimmers

TL;DR

This study investigates how hydrodynamic interactions influence phase separation and coexistence in microswimmers, revealing the effects of propulsion strength, density, and swimmer type on their collective behavior and phase diagram.

Contribution

It extends previous models by incorporating hydrodynamics in squirmer simulations, analyzing phase behavior, and identifying the influence of swimmer type and propulsion on phase separation.

Findings

Hydrodynamics significantly affect phase separation boundaries.

Dense clusters exhibit diffusive coarsening followed by ballistic compactification.

Swimmer type shifts phase boundaries and alters phase states.

Abstract

A striking feature of the collective behavior of spherical microswimmers is that for sufficiently strong self-propulsion they phase-separate into a dense cluster coexisting with a low-density dis- ordered surrounding. Extending our previous work, we use the squirmer as a model swimmer and the particle-based simulation method of multi-particle collision dynamics to explore the influence of hydrodynamics on their phase behavior in a quasi-two-dimensional geometry. The coarsening dynamics towards the phase-separated state is diffusive in an intermediate time regime followed by a final ballistic compactification of the dense cluster. We determine the binodal lines in a phase diagram of P\'eclet number versus density. Interestingly, the gas binodals are shifted to smaller densities for increasing mean density or dense-cluster size, which we explain using a recently introduced pressure balance [S. C. Takatori et al., Phys. Rev. Lett. 113, 028103 (2014)] extended by a hydrodynamic contribution. Furthermore, we find that for pushers and pullers the binodal line is shifted to larger P\'eclet numbers compared to neutral squirmers. Finally, when lowering the P\'eclet number, the dense phase transforms from a hexagonal "solid" to a disordered "fluid" state.