Motile bacteria in a critical fluid mixture

TL;DR

This study investigates how extit{E. coli} bacteria swim near a critical fluid mixture, revealing that their trails are caused by local phase re-organisation, which can be used to probe fluid dynamics.

Contribution

It demonstrates that bacterial trails in a critical fluid are due to phase re-organisation, contrasting with shear effects in liquid crystals, and uses this to analyze fluid fluctuations.

Findings

Trails are caused by local phase re-organisation, not shear effects.

Bacterial trails can be used to probe critical fluctuations.

Quantitative analysis matches simulations of phase re-organisation.

Abstract

We studied the swimming of \textit{Escherichia coli} bacteria in the vicinity of the critical point in a solution of the non-ionic surfactant \chem{C_{12}E_{5}} in buffer solution. In phase contrast microscopy, each swimming cell produces a transient trail behind itself lasting several seconds. Comparing quantitative image analysis with simulations show that these trails are due to local phase re-organisation triggered by differential adsorption. This contrasts with similar trails seen in bacteria swimming in liquid crystals, which are due to shear effects. We show how our trails are controlled, and use them to probe the structure and dynamics of critical fluctuations in the fluid medium.