Active mixing, manifolds and barriers in imposed, laminar flows*

We present experiments and simulations on the motion of self-propelled tracers in imposed laminar fluid flows. The flows used are hyperbolic and vortex-dominated flows, generated in microfluidic (PDMS) cells and in laboratory-scale, magnetohydrodynamically-driven systems. The tracers are either brine shrimp (for large-scale flows) or bacillus subtilis (for microfluidic flows). Two types of bacillus subtilis are studied: a wild-type -- characterized by run-and-tumble trajectories in the absence of a flow -- and a mutated "smooth swimmer" strain in which the tumbling is suppressed. We analyze the results in conjunction with a theory that predicts the existence of "Swimming Invariant Manifolds" (SwIMs) that act as one-way barriers that impede the trajectories of self-propelled tracers. We explore how the shape and location of the SwIMs vary with the imposed flow, along with the different ways in which the swimming behavior of the organism affect these SwIMs.