Bacterial activity hinders particle sedimentation

TL;DR

This study demonstrates that swimming bacteria significantly slow down and disperse passive particle sedimentation in fluids, with effects depending on bacterial concentration and activity levels, modeled effectively by an advection-diffusion framework.

Contribution

It provides new insights into how bacterial activity influences particle sedimentation dynamics, introducing a predictive parameter based on flow speeds.

Findings

Bacteria reduce sedimentation speed even at low concentrations.

Bacterial activity increases particle dispersion during sedimentation.

A simple ratio predicts sedimentation front speed effectively.

Abstract

Sedimentation in active fluids has come into focus due to the ubiquity of swimming micro-organisms in natural and industrial processes. Here, we investigate sedimentation dynamics of passive particles in a fluid as a function of bacteria E. coli concentration. Results show that the presence of swimming bacteria significantly reduces the speed of the sedimentation front even in the dilute regime, in which the sedimentation speed is expected to be independent of particle concentration. Furthermore, bacteria increase the dispersion of the passive particles, which determines the width of the sedimentation front. For short times, particle sedimentation speed has a linear dependence on bacterial concentration. Mean square displacement data shows, however, that bacterial activity decays over long experimental (sedimentation) times. An advection-diffusion equation coupled to bacteria population dynamics seems to capture concentration profiles relatively well. A single parameter, the ratio of single particle speed to the bacteria flow speed can be used to predict front sedimentation speed.