Invariant manifolds as barriers to the motion of bacteria in vortex flows

We present experiments that study the motion of swimming bacteria (bacillus subtilis) in a time-independent vortex flow. The flow is a pair of vortices generated in a microfluidic cell composed of either a cross or an H-shaped channel. Experiments are done with both wild-type and a genetically-mutated ``smooth swimming'' \footnote{R. Rusconi, J.S. Guasto and R. Stocker, Nature Physics {\bf 10}, 212 (2014).} bacillus subtilis. We analyze the trajectories of these bacteria in terms of invisible barriers, based on a theory of ``burning invariant manifolds'' \footnote{J. Mahoney, D. Bargteil, M. Kingsbury, K. Mitchell and T. Solomon, Europhys. Lett. {\bf 98}, 44005 (2012).} that act as one-way barriers that impede the motion of reaction fronts in a fluid flow. We explore whether similar one-way barriers impede the motion of bacteria.