Fluctuations and Rheology in Active Bacterial Suspensions

TL;DR

This paper investigates the non-equilibrium rheological properties of active bacterial suspensions by measuring correlations and responses of tracer particles, revealing fluctuation-dissipation violations and characteristic stress fluctuation spectra.

Contribution

It introduces a combined experimental and theoretical approach to characterize active stress fluctuations and their spectral properties in bacterial baths.

Findings

Violation of fluctuation-dissipation theorem observed.

Power spectrum of active stress fluctuations characterized.

Identification of 1/√ω scaling in noise spectrum.

Abstract

We probe non-equilibrium properties of an active bacterial bath through measurements of correlations of passive tracer particles and the response function of a driven, optically trapped tracer. These measurements demonstrate violation of the fluctuation-dissipation theorem and enable us to extract the power spectrum of the active stress fluctuations. In some cases, we observe $1/\sqrt{\omega}$ scaling in the noise spectrum which we show can be derived from a theoretical model incorporating coupled stress, orientation, and concentration fluctuations of the bacteria.