Micro-swimmer collective dynamics in Brinkman flows

TL;DR

This study investigates how resistance in Brinkman flows influences the collective behavior of micro-swimmers, showing that increased resistance delays or suppresses collective instabilities, with implications for understanding microorganism dynamics in complex habitats.

Contribution

It introduces a continuum model of micro-swimmers in Brinkman flows and demonstrates how resistance impacts collective dynamics and stability, extending previous homogeneous flow analyses.

Findings

Resistance delays the onset of collective instabilities.

High resistance can completely suppress collective swimming behavior.

Simulations confirm the stabilizing effect of resistance on swimmer dynamics.

Abstract

Suspensions of swimming micro-organisms are known to undergo intricate collective dynamics as a result of hydrodynamic and collision interactions. Micro-swimmers, such as bacteria and micro-algae, naturally live and have evolved in complex habitats that include impurities, obstacles and interfaces. To elucidate their dynamics in a heterogeneous environment, we consider a continuum theory where the the micro-swimmers are embedded in a Brinkman wet porous medium, which models viscous flow with an additional resistance or friction due to the presence of smaller stationary obstacles. The conservation equation for the swimmer configurations includes advection and rotation by the immersing fluid, and is coupled to the viscous Brinkman fluid flow with an active stress due to the swimmers' motion in it. Resistance alters individual swimmer locomotion and the way it disturbs the surrounding fluid, and thus it alters its hydrodynamic interactions with others and and such affects collective dynamics.The entropy analysis and the linear stability analysis of the system of equations both reveal that resistance delays and hinders the onset and development of the collective swimming instabilities, and can completely suppress it if sufficiently large. Simulations of the full nonlinear system confirm these. We contrast the results with previous theoretical studies on micro-swimmers in homogeneous viscous flow, and discuss relevant experimental realizations.