Active mixing of swimming microbes in laminar flows

We present experiments on hydrodynamic barriers that impede the motion of microswimmers in laminar flows. The flows studied are a hyperbolic flow in a cross channel, a vortex chain flow, and an oscillating channel flow. The active tracers are bacteria (bacillus subtilis) and motile eukaryotic microbes (chlamydomonas, tetraselmis and euglena).  The trajectories of these motile tracers in the flows are interpreted in terms of a theory of "swimming invariant manifolds" (SwIMs) that are manifested as one-way barriers in the fluid system. This theory is a generalization of a theory of "burning invariant manifolds" (BIMs) that have previously been shown to act as one-way barriers that block the motion of propagating reaction fronts in laminar flows.