Symmetric shear banding and swarming vortices in bacterial "superfluids"

TL;DR

This study investigates the unique shear responses of bacterial suspensions, revealing symmetric shear bands and swarming vortices in active fluids, supported by experiments and hydrodynamic modeling, advancing understanding of active matter physics.

Contribution

It provides the first experimental observation and theoretical explanation of symmetric shear bands and swarming vortices in bacterial superfluids under shear.

Findings

Bacterial suspensions form symmetric shear bands under shear stress.

Hydrodynamic model accurately predicts shear-banding and vortex formation.

Active superfluids exhibit unconventional flow behaviors.

Abstract

Bacterial suspensions--a premier example of active fluids--show an unusual response to shear stresses. Instead of increasing the viscosity of the suspending fluid, the emergent collective motions of swimming bacteria can turn a suspension into a "superfluid" with zero apparent viscosity. Although the existence of active "superfluids" has been demonstrated in bulk rheological measurements, the microscopic origin and dynamics of such an exotic phase have not been experimentally probed. Here, using high-speed confocal rheometry, we study the dynamics of concentrated bacterial suspensions under simple planar shear. We find that bacterial "superfluids" under shear exhibit unusual symmetric shear bands, defying the conventional wisdom on shear-banding of complex fluids, where the formation of steady shear bands necessarily breaks the symmetry of unsheared samples. We propose a simple hydrodynamic model based on the local stress balance and the ergodic sampling of nonequilibrium shear configurations, which quantitatively describes the observed symmetric shear-banding structure. The model also successfully predicts various interesting features of swarming vortices in stationary bacterial suspensions. Our study provides new insights into the physical properties of collective swarming in active fluids and illustrates their profound influences on transport processes.