Simulations of Brownian tracer transport in squirmer suspensions

TL;DR

This paper uses detailed simulations to analyze how swimming microorganisms influence the transport and diffusion of tracer particles in suspensions, revealing the effects of hydrodynamic and steric interactions and their dependence on swimmer concentration.

Contribution

It introduces advanced simulation methods to study tracer transport in squirmer suspensions, providing new insights into the effects of swimmer interactions on diffusion.

Findings

Hydrodynamic interactions significantly enhance tracer diffusion.

Steric interactions influence tracer displacement distribution.

Tracer diffusivity scales non-linearly with swimmer volume fraction in semi-dilute regimes.

Abstract

In addition to enabling movement towards environments with favourable living conditions, swimming by microorganisms has also been linked to enhanced mixing and improved nutrient uptake by their populations. Experimental studies have shown that Brownian tracer particles exhibit enhanced diffusion due to the swimmers, while theoretical models have linked this increase in diffusion to the flows generated by the swimming microorganisms, as well as collisions with the swimmers. In this study, we perform detailed simulations based on the force-coupling method and its recent extensions to the swimming and Brownian particles to examine tracer displacements and effective tracer diffusivity in squirmer suspensions. By isolating effects such as hydrodynamic or steric interactions, we provide physical insight into experimental measurements of the tracer displacement distribution. In addition, we extend results to the semi-dilute regime where the swimmer-swimmer interactions affect tracer transport and the effective tracer diffusivity no longer scales linearly with the swimmer volume fraction.