Sedimentation and structure of squirmer suspensions under gravity

TL;DR

This study investigates how gravity influences the collective behavior and sedimentation of microswimmers, revealing transitions to ordered states and differences between puller and pusher types under varying gravitational forces.

Contribution

It introduces a DPD simulation approach to analyze microswimmer sedimentation and order formation under gravity, highlighting differences between swimmer types.

Findings

Sedimentation leads to ordered hexagonal layers at the bottom.

Pullers better maintain hexagonal order than pushers at high gravity.

Microswimmers' collective behavior depends on swimmer type and gravitational strength.

Abstract

The effect of gravity on the collective motion of living microswimmers, such as bacteria and micro-algae, is pivotal to unravel not only bio-convection patterns but also the settling of bacterial biofilms on solid surfaces. In this work, we investigate suspensions of microswimmers under the influence of a gravitational field and hydrodynamics, simulated via dissipative particle dynamics (DPD) coarse-grained model. We first study the collective sedimentation of passive colloids and microswimmers of the puller and pusher types upon increasing the imposed gravitational field and compare with previous results. Once sedimentation occurs, we observe that, as the gravitational field increases, the bottom layer undergoes a transition to an ordered state compatible with a hexagonal crystal. In comparison with passive colloids, both pullers and pushers easily rearrange at the bottom layer to anneal defects. Specifically, pullers are better than pushers in preserving the hexagonal order of the bottom mono-layer at high gravitational fields.