Barriers and chutes for mixing of active particles in a vortex chain flow

Self-propelled (active) particles in laminar fluid flows are blocked by one-way barriers called ``swimming invariant manifolds'' (SwIMs). The SwIM theory is a generalization of manifold approaches previously applied to the mixing of passive tracers. We verify the blocking behavior of SwIMs experimentally for algae swimming in a vortex chain flow. The SwIMs also form chutes that transport swimmers between vortices with a predicted inter-vortex flux that is consistent with the experiments, despite noise and nonuniformities in the swimming directions of the microbes.