Colloids in a bacterial bath: simulations and experiments

TL;DR

This study combines experiments and simulations to investigate how bacterial motion influences the diffusion of micron-scale colloids in a two-dimensional Bacillus subtilis suspension, revealing a crossover from super-diffusive to diffusive behavior.

Contribution

It introduces a novel 2D bacterial suspension setup and compares experimental results with simulations, highlighting the importance of steric interactions in bacterial collective motion.

Findings

Super-diffusive to diffusive crossover observed in colloid motion

Simulations with self-propelled dumbbells match experimental data quantitatively

Steric collisions significantly influence bacterial collective behavior

Abstract

We present a joint experimental and computational study of the effect of bacterial motion on micron-scale colloids contained in a two-dimensional suspension of Bacillus subtilis. With respect to previous work using E. coli, here we introduce a novel experimental set-up that allows us to realise a two-dimensional bacterial suspension insensitive to either evaporation or fluid flow. By analysing the mean square displacements of both bacteria and colloids, we confirm the existence of a crossover from super-diffusive behaviour at short time scales to normal diffusion at longer times. We also study the same two-dimensional system by means of numerical simulations, using a suspension of self-propelled dumbbells or the Vicsek model, which has been previously used to study the dynamics of active particles. Our numerical results obtained with both models are in broad agreement with the experimental trends, but only the dumbbell simulations can match the experimental data quantitatively. The level of agreement we find suggest that steric interactions due to collisions are important players in determining collective motion of the bacterial bath, and should complement hydrodynamic interactions in experiments.