Fluid dynamics of bacterial turbulence

TL;DR

This paper investigates bacterial turbulence in concentrated Bacillus subtilis suspensions through experiments and a minimal theoretical model, revealing how activity influences fluid dynamics and providing quantitative agreement between theory and experiments.

Contribution

It introduces a minimal fourth-order vector-field theory that quantitatively explains experimental observations of bacterial turbulence in 3D suspensions.

Findings

Velocity statistics and correlations match experimental data

Fluid memory decreases with increased bacterial activity

Energy scales linearly with enstrophy

Abstract

Self-sustained turbulent structures have been observed in a wide range of living fluids, yet no quantitative theory exists to explain their properties. We report experiments on active turbulence in highly concentrated 3D suspensions of Bacillus subtilis and compare them with a minimal fourth-order vector-field theory for incompressible bacterial dynamics. Velocimetry of bacteria and surrounding fluid, determined by imaging cells and tracking colloidal tracers, yields consistent results for velocity statistics and correlations over two orders of magnitude in kinetic energy, revealing a decrease of fluid memory with increasing swimming activity and linear scaling between energy and enstrophy. The best-fit model parameters allow for quantitative agreement with experimental data.