Active mixing of motile bacteria in hyperbolic and vortex flows*

We present experiments and simulations on the motion of self-propelled microbes in imposed laminar fluid flows. The flows are either a hyperbolic flow in a microfluidic cross or T-cell, or a vortex-dominated flow. The microbes are bacillus subtillus, either wild-type -- characterized by run-and-tumble trajectories in the absence of a flow -- or 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 the bacteria. We explore how the shape and location of the SwIMs vary with the imposed flow, along with the different ways in which wild-type and smooth-swimming bacteria are affected by these SwIMs.