Turning bacteria suspensions into a "superfluid"

TL;DR

This study investigates the rheological behavior of active Escherichia coli suspensions under shear, revealing a transition to a superfluid-like state where viscosity vanishes due to bacterial activity overcoming viscous dissipation.

Contribution

It provides experimental evidence of a superfluid-like transition in active bacterial suspensions, highlighting the role of activity and shear in organizing bacteria and reducing viscosity.

Findings

Effective viscosity decreases with concentration in dilute regime.

Suspensions exhibit a superfluid-like transition with vanishing viscosity.

Active bacteria can overcome viscous dissipation under shear.

Abstract

The rheological response under simple shear of an active suspension of Escherichia coli is determined in a large range of shear rates and concentrations. The effective viscosity and the time scales characterizing the bacterial organization under shear are obtained. In the dilute regime, we bring evidences for a low shear Newtonian plateau characterized by a shear viscosity decreasing with concentration. In the semi-dilute regime, for particularly active bacteria, the suspension display a "super-fluid" like transition where the viscous resistance to shear vanishes, thus showing that macroscopically, the activity of pusher swimmers organized by shear, is able to fully overcome the dissipative effects due to viscous loss.