(289h) Microscopic Dynamics of Bacterial “Superfluids” Under Planar Oscillatory Shear

Bacterial suspensions show interesting rheological behaviors including a remarkable "superfluid" flows with vanishing apparent viscosity. To reveal the microscopic dynamics of bacterial "superfluids", we systematically studied the shear profiles of E-coli suspensions using ultra-fast confocal microscopy. The flow fields of bacterial suspensions under planar oscillatory shears of different shear rates are measured along both the flow-vorticity and the flow-gradient planes. We found that concentrated bacterial suspensions exhibit strong non-homogeneous flow profiles at low shear rates, where shear rates vanish away from the moving shear plate. We characterized the shape of the nonlinear shear profiles at different applied shear rates and bacterial concentrations and activities. The shear profiles follow a simple scaling relation with the applied shear rates and the enstrophy of suspensions. We demonstrated that this scaling relation can be quantitatively understood by considering the ensemble average of the unusual shear banding predicted by the theory of active liquid crystals. Our experiments reveal a profound influence of shear flows on the locomotion of bacteria and provide new insights into the dynamics of active fluids.