Sedimentation and diffusion of passive particles in suspensions of swimming Escherichia coli

TL;DR

This study experimentally examines how swimming bacteria like E. coli influence the sedimentation and diffusion of passive particles in water, revealing bacteria slow sedimentation, increase diffusion, and cause complex front development.

Contribution

It introduces a coupled advection-diffusion and bacterial population model to explain sedimentation behavior in active bacterial suspensions.

Findings

Bacteria reduce sedimentation velocity even at low concentrations.

Bacteria increase the effective diffusion coefficient of passive particles.

High bacteria concentrations lead to two sedimentation fronts due to bacterial death.

Abstract

Sedimentation in active fluids has come into focus due to the ubiquity of swimming micro-organisms in natural and artificial environments. Here, we experimentally investigate sedimentation of passive particles in water containing various concentrations of the bacterium E. coli. Results show that the presence of living bacteria reduces the velocity of the sedimentation front even in the dilute regime, where the sedimentation velocity is expected to be independent of particle concentration. Bacteria increase the effective diffusion coefficient of the passive particles, which determines the width of the sedimentation front. For higher bacteria concentration, we find the development of two sedimentation fronts due to bacterial death. A model in which an advection-diffusion equation describing the settling of particles under gravity is coupled to the population dynamics of the bacteria seems to capture the experimental trends relatively well.