Chaos, dynamic trapping, and transport of swimming microbes in a vortex chain flow.

We present experiments on chaotic motion of self-propelled (active) particles in a time-independent, two-dimensional vortex chain flow. We track Tetraselmis microbes and calculate the variance of a spreading distribution of these microbes in the flow. For small non-dimensional swimming speed v0, we find subdiffusion with variance ⟨x2⟩∼tγ with γ<1; transport is diffusive (γ=1) for larger v0. Subdiffusion for small v0 is due to dynamic trapping of microbes to islands of ordered trajectories surrounded by a sea of chaotic motion; these islands disappear for larger v0. We calculate Lagrangian-averaged trajectories (LATs) from the experimental data and use the LATs to measure trapping time probability distributions P(t). We find regimes with P(t)∼t-ν with ν<2 for small v0, consistent with the measured subdiffusion.