Long-range active mixing of swimming microbes in a vortex chain flow

We present results of experiments that track the motion of swimming tetraselmis and euglena algae in a flow composed of a chain of alternating vortices. Trajectories of active particles in this two-dimensional flow can be chaotic, even though the flow is time-independent. Swimming invariant manifolds (SwIMs) can be calculated for the flow; these SwIMs not only act as barriers impeding the motion of the swimming microbes but also result in ``chutes'' that carry microbes between adjacent vortices. The flux of microbes between vortices in the experiments agrees with predictions based on these chutes. We also measure the growing variance of an ensemble for different microbe swimming speeds and interpret those results in terms of the SwIM geometry. Comparing the transport of elongated euglena versus the almost-circular tetraselmis microbes enables us to ascertain the role of microbe shape in the transport process.