Interfacial flow around a pusher bacterium

Bacteria are prototypical active colloids that self-organize into coherent structures. However, their accumulation at fluid interfaces dramatically alters their behavior. We visualize the flow around swimming bacteria trapped on fluid interfaces by analyzing correlated displacements between bacteria and passive tracers. The observed flow fields are described by hydrodynamic multipoles for Stokes flow in an incompressible interface whose magnitudes and positions depend on the microbe's trapped configuration. Bacteria swim along noisy curved trajectories determined by the angle of their bodies with respect to the interface. By studying the motion of tracers entrained by their flow, we determine their enhanced diffusivity. Furthermore, we predict pair interactions that differ significantly from their bulk counterparts. This study identifies key factors that impact active colloidal dynamics at interfaces. These include the interplay of swimmer trapped configuration and far field flow, the importance of interfacial stresses and non-equilibrium fluctuations, and how these determine the dynamics of individual swimmers and their collective behaviors. These concepts can guide the design of microrobots for enhanced transport in multiphase systems.