Collective behavior of strongly confined suspensions of squirmers

TL;DR

This study uses numerical simulations to explore how confinement and swimmer type influence the collective behavior, flow patterns, and fluctuations in suspensions of spherical squirmers.

Contribution

It reveals the dominance of source dipoles in confined flows and highlights the importance of near-field interactions and swimmer type on collective dynamics.

Findings

Flow is dominated by source dipoles due to confinement.

Polar states occur only in neutral squirmers.

Near-field interactions are crucial for particle alignment.

Abstract

We run numerical simulations of strongly confined suspensions of model spherical swimmers called "squirmers". Because of the confinement, the Stokeslet dipoles generated by the particles are quickly screened and the far-field flow is dominated by the source dipole for all the different kinds of squirmers. However, we show that the collective behavior of the suspension still depends on the self-propelling mechanism of the swimmers as polar states can only be observed for neutral squirmers. We demonstrate that the near-field hydrodynamic interactions play a crucial role in the alignment of the orientation vectors of spherical particles. Moreover, we point out that the enstrophy and the fluid fluctuations of an active suspension also depend on the nature of the squirmers.