(358a) Nonlinear Velocity Profiles of Sheared Active Bacterial Suspensions

Bacterial suspensions under shear exhibit interesting rheology including a remarkable â??superfluid-likeâ? behavior with vanishing viscosities. Hydrodynamic theories have shown that such a bacterial â??superfluidâ? is associated with nonlinear shear banding flows. However, although the bulk rheology of bacterial suspensions has been experimentally studied, velocity profiles within sheared bacterial suspensions have not been systematically explored. Here, we experimentally investigate the microscopic dynamics of E. coli suspensions under planar oscillatory shear. Using high-speed confocal rheometry, we measure the velocity profile of sheared bacterial suspensions between two shear plates. We find that at low shear rates, high-concentration active bacterial suspensions exhibit an unusual nonlinear velocity profile in the â??superfluidâ? state. Such a nonlinear profile gradually transitions into a conventional linear velocity profile with increasing shear rates. We demonstrate that the nonlinear shear flow is a direct consequence of the collective motion of swarming bacteria. The phase diagram of sheared bacterial suspensions is systematically measured as functions of shear rates and bacterial concentrations. We quantitatively interpret the observed phase diagram based on the competition between the shear stress of fluid flows and the active stress of swarming bacteria. Our study provides new insights into rheology of bacterial suspensions and sheds light on shear-induced dynamics of active fluids.