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.
|Original language||English (US)|
|Number of pages||6|
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|State||Published - Jul 10 2018|
Bibliographical noteFunding Information:
ACKNOWLEDGMENTS. We thank K. Dorfman, Y.-S. Tai, and K. Zhang for help with bacterial culturing and J. Brady and Z. Dogic for discussions. This research is supported by Defense Advanced Research Projects Agency (DARPA) Young Faculty Award D16AP00120, the Packard Foundation, and National Science Foundation Chemical, Bioengineering, Environmental, and Transport Systems Award 1702352. X.X. acknowledges support from the National Natural Science Foundation of China (Grants 11575020 and U1530401).
- Active fluids
- Bacterial suspensions
- Shear banding