Journal Article

Symmetric shear banding and swarming vortices in bacterial superfluids

Xu, Xinliang, Peng, Yi, Guo, Shuo, Cheng, Xiang, Samanta, Devranjan

2018

Overview

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 insights into the physical properties of collective swarming in active fluids and illustrates their profound influences on transport processes.

• Publisher: National Academy of Sciences
• Subject: Applied Physical Sciences / Bacteria / Banding / Computational fluid dynamics / Edge dislocations / Escherichia coli K12 - cytology / Escherichia coli K12 - physiology / Fluids / Models, Biological / Physical properties / Physical Sciences / Rheological properties / Rheometry / Shear bands / Shear Strength / Shear stress / Superfluidity / Swarming / Swimming / Symmetry / Transport processes / Viscosity / Vortices