Active mixing of swimming bacteria in a hyperbolic flow

We present experiments on the motion of smooth-swimming and tumbling bacillus subtilis bacteria in laminar flows. For most of the experiments the flow is a hyperbolic flow in a PDMS, microfluidic cross channel cell. Since the bacteria are self-propelled, they are able to swim across passive invariant manifolds that block the motion of passive tracers (including sessile bacteria). Theoretically, the motion of these active tracers is influenced by {\em swimming invariant manifolds} (SwIMs) that act as one-way barriers. We compare the motion of the trajectories to the SwIMs and their associated swimming fixed points, as predicted by the theory. We also discuss the effectiveness of the theory in explaining the behavior of tumbling bacteria. Time permitting, we will also present preliminary results for the motion of bacteria in a double-gyre (vortex) flow.