Orientational order in concentrated suspensions of spherical microswimmers

TL;DR

This paper uses numerical simulations to study the orientational dynamics of concentrated spherical microswimmer suspensions, revealing persistent instabilities and long-term polar order influenced by swimmer type.

Contribution

It demonstrates that isotropic suspensions of spherical swimmers are inherently unstable and develop polar order, extending previous dilute-limit predictions to concentrated regimes.

Findings

Isotropic suspensions are always unstable.

Long-time polar order develops regardless of initial conditions.

Order depends on swimmer type, weakly on volume fraction.

Abstract

We use numerical simulations to probe the dynamics of concentrated suspensions of spherical microswimmers interacting hydrodynamically. Previous work in the dilute limit predicted orientational instabilities of aligned suspensions for both pusher and puller swimmers, which we confirm computationally. Unlike previous work, we show that isotropic suspensions of spherical swimmers are also always unstable. Both types of initial conditions develop long-time polar order, of a nature which depends on the hydrodynamic signature of the swimmer but very weakly on the volume fraction up to very high volume fractions.