Diffusion of an ellipsoid in bacterial suspensions

TL;DR

This study investigates how elongated ellipsoids diffuse in bacterial suspensions, revealing nonlinear enhancement and novel coupling between translation and rotation caused by bacterial flows, advancing understanding of active matter transport.

Contribution

It introduces the first detailed analysis of anisotropic tracer diffusion in bacterial baths, uncovering a universal coupling mechanism driven by bacterial-induced flows.

Findings

Ellipsoids exhibit nonlinear enhancement in translational and rotational diffusion.

An anomalous translation-rotation coupling is observed, forbidden in classical Brownian motion.

Bacterial swimming induces stretching flows responsible for the observed anomalies.

Abstract

Active matter such as swarming bacteria and motile colloids exhibits exotic properties different from conventional equilibrium materials. Among these properties, the enhanced diffusion of tracer particles is generally deemed as a hallmark of active matter. Here, rather than spherical tracers, we investigate the diffusion of isolated ellipsoids in quasi-two-dimensional bacterial bath. Our study reveals a nonlinear enhancement of both translational and rotational diffusions. More importantly, we uncover an anomalous coupling between translation and rotation that is strictly prohibited in the classic Brownian diffusion. Combining experiments with theoretical modeling, we show that such an anomaly arises from generic stretching flows induced by swimming bacteria. Our work illustrates a universal organizing principle of active matter and sheds new light on fundamental transport processes in microbiological systems.