Active mixing of swimming bacteria in hyperbolic and vortex flows

We present experiments on the effects of imposed, laminar fluid flows on the motion of active (self-propelled) tracers. The active tracers are bacillus subtilis bacteria, including a wild-type strain and two variations, one with the GFP mutation and one with a smooth-swimming mutation for which the microbe doesn't tumble. The imposed flows are simple hyperbolic flows and vortex chain flows. We test theories that predict ``swimming invariant manifolds'' (SwIMs) that act as one-way barriers that impede the motion of active tracers in the flow. For the hyperbolic flows, we investigate the structure of the barriers as a function of the imposed flow magnitude. For the vortex flow, we investigate the effects of SwIMs that encircle the vortex centers. We also test predictions of chaotic trajectories of smooth-swimming tracers for time-independent, two-dimensional flows.