Collective Chemotactic Dynamics in the Presence of Self-Generated Fluid Flows

TL;DR

This paper investigates how self-generated fluid flows influence chemotactic behavior in micro-swimmer suspensions, revealing distinct instabilities and aggregation dynamics driven by hydrodynamic interactions.

Contribution

It introduces a kinetic theory coupling chemotaxis with collective fluid flows, identifying separate chemotactic and hydrodynamic instabilities in isotropic suspensions.

Findings

Hydrodynamic interactions can limit chemotactic aggregation.

Puller suspensions form mutually-repelling aggregates.

Pusher suspensions exhibit flow-induced fragmentation of aggregates.

Abstract

In micro-swimmer suspensions locomotion necessarily generates fluid motion, and it is known that such flows can lead to collective behavior from unbiased swimming. We examine the complementary problem of how chemotaxis is affected by self-generated flows. A kinetic theory coupling run-and-tumble chemotaxis to the flows of collective swimming shows separate branches of chemotactic and hydrodynamic instabilities for isotropic suspensions, the first driving aggregation, the second producing increased orientational order in suspensions of "pushers" and maximal disorder in suspensions of "pullers". Nonlinear simulations show that hydrodynamic interactions can limit and modify chemotactically-driven aggregation dynamics. In puller suspensions the dynamics form aggregates that are mutually-repelling due to the non-trivial flows. In pusher suspensions chemotactic aggregation can lead to destabilizing flows that fragment the regions of aggregation.