Activity-induced clustering in model dumbbell swimmers: The role of hydrodynamic interactions

TL;DR

This study uses fluid-particle dynamics simulations to show that hydrodynamic interactions significantly influence clustering and motility in active suspensions of model bacteria, altering their steady state behaviors.

Contribution

It demonstrates how activity-induced hydrodynamic forces promote clustering and modify motility, providing new insights into collective behaviors in active suspensions.

Findings

Hydrodynamic interactions enhance dynamic clustering.

Hydrodynamic trapping leads to clustering behavior.

Density dependence of motility differs with hydrodynamics.

Abstract

Using a fluid-particle dynamics approach, we numerically study the effects of hydrodynamic interactions on the collective dynamics of active suspensions within a simple model for bacterial motility: each microorganism is modeled as a stroke-averaged dumbbell swimmer with prescribed dipolar force pairs. Using both simulations and qualitative arguments, we show that, when the separation between swimmers is comparable to their size, the swimmers' motions are strongly affected by activity-induced hydrodynamic forces. To further understand these effects, we investigate semidilute suspensions of swimmers in the presence of thermal fluctuations. A direct comparison between simulations with and without hydrodynamic interactions shows these to enhance the dynamic clustering at a relatively small volume fraction; with our chosen model the key ingredient for this clustering behavior is hydrodynamic trapping of one swimmer by another, induced by the active forces. Furthermore, the density dependence of the motility (of both the translational and rotational motions) exhibits distinctly different behaviors with and without hydrodynamic interactions; we argue that this is linked to the clustering tendency. Our study illustrates the fact that hydrodynamic interactions not only affect kinetic pathways in active suspensions, but also cause major changes in their steady state properties.