Cross-stream migration of spherical active particles
Dr. William Uspal (Max Planck Institute for Intelligent Systems, Stuttgart)

For natural microswimmers, the interplay of swimming activity and external flow can promote robust directional motion, e.g. propulsion against (upstream rheotaxis) or perpendicular to the direction of flow. These effects are generally attributed to their complex body shapes and flagellar beat patterns. We report on a strong directional alignment that occurs for spherical active particles exposed to flow near a surface. Using catalytic Janus particles as a model experimental system, we find that the particles are attracted to certain orientations that are nearly perpendicular to the direction of the flow, and they swim across the flow streamlines. In a deterministic theoretical model, we show how this alignment emerges from the interplay of shear flow and near-surface swimming activity. Adding the effect of thermal noise, we obtain probability distributions for the swimmer orientation that show good agreement with the experimental distributions. Our findings demonstrate that microswimmers can exhibit unexpected and robust behavior in confined flows, which occur in many envisioned microswimmer applications.