Trapping of microswimmers in vortex flows

We theoretically investigate the trapping of rigid, ellipsoidal microswimmers in externally-driven two-dimensional vortex flows. Surprisingly, for swimmers that swim perpendicular to their elongation direction, we find that trapping depends non-monotonically on swimming speed. We identify certain stable periodic solutions of the swimmer equations of motion as the cause of trapping in individual vortices. A bifurcation analysis of these solutions explains the dependence of trapping on swimmer speed, shape, and swimming direction. We propose heteroclinic bifurcations between swimming fixed points as a general mechanism for the creation of stable swimmer trajectories.