Clustering of microswimmers: Interplay of shape and hydrodynamics

TL;DR

This study investigates how shape and hydrodynamic interactions influence clustering and phase separation in microswimmers, revealing opposing effects for spherical versus elongated particles through mesoscale simulations.

Contribution

It demonstrates that hydrodynamics suppress MIPS in spherical microswimmers but enhance it in elongated ones, highlighting the complex interplay of shape and hydrodynamics.

Findings

Hydrodynamics suppress MIPS in spherical squirmers.

Hydrodynamics enhance MIPS in elongated squirmers.

Shape determines whether hydrodynamics promote or inhibit clustering.

Abstract

The spatiotemporal dynamics in systems of active self-propelled particles is controlled by the propulsion mechanism in combination with various direct interactions, such as steric repulsion, hydrodynamics, and chemical fields. Yet, these direct interactions are typically anisotropic, and come in different 'flavors', such as spherical and elongated particle shapes for steric repulsion, pusher and puller flow fields for hydrodynamics, etc. The combination of the various aspects is expected to lead to new emergent behavior. However, it is a priori not evident whether shape and hydrodynamics act synergistically or antagonistically to generate motility-induced clustering (MIC) and phase separation (MIPS). We employ a model of prolate spheroidal microswimmers - called squirmers - in quasi-two-dimensional confinement to address this issue by mesoscale hydrodynamic simulations. For comparison, non-hydrodynamic active Brownian particles (ABPs) are considered to elucidate the contribution of hydrodynamic interactions on MIC and MIPS. For spherical particles, the comparison between ABP and hydrodynamic-squirmer ensembles reveals a suppression of MIPS due to hydrodynamic interactions. The fundamental difference between ABPs and squirmers is attributed to an increased reorientation of squirmers by hydrodynamic torques during their collisions. In contrast, for elongated squirmers, hydrodynamics interactions enhance MIPS. Thus, hydrodynamic interactions show opposing effects on MIPS for spherical and elongated microswimmers.