Diffusion and spatial correlations in suspensions of swimming particles

TL;DR

This paper investigates how different swimming modes of microorganisms affect fluid motion and tracer diffusion, revealing that pushers enhance diffusion more than pullers, with effects depending on system size and explained by a mean field theory.

Contribution

It introduces a simulation study comparing pushers and pullers in suspensions, highlighting the qualitative differences in fluid dynamics and diffusion enhancement.

Findings

Pushers cause greater diffusivity enhancement than pullers.

Diffusivity and correlation length increase with system size for pushers.

A mean field theory explains the origin of these effects.

Abstract

Populations of swimming microorganisms produce fluid motions that lead to dramatically enhanced diffusion of tracer particles. Using simulations of suspensions of swimming particles in a periodic domain, we capture this effect and show that it depends qualitatively on the mode of swimming: swimmers ``pushed'' from behind by their flagella show greater enhancement than swimmers that are ``pulled'' from the front. The difference is manifested by an increase, that only occurs for pushers, of the diffusivity of passive tracers and the velocity correlation length with the size of the periodic domain. A physical argument supported by a mean field theory sheds light on the origin of these effects.