(398e) Bacteria As Active Colloids on Fluid Interfaces

The advent of self-propelled colloids that move absent external force brings important and as yet largely untapped degrees of freedom to interfacial engineering. Bacteria are one example of active colloids that propel themselves and interact with fluid boundaries. Because of their directed motion, active colloids accumulate near interfaces. They can also adsorb directly and swim in an adhered state with complex trajectories that differ from those in bulk in both form and spatio-temporal implications. In this research, we image the displacement fields of passive colloidal tracers around the swimming bacteria over short lag times to approximate the hydrodynamic flow field, and we compare that flow field to predictions based on hydrodynamic multipoles for Stokes flow. The flow field of an interfacially trapped bacterium is well described by two hydrodynamic dipolar modes, which are associated with interfacial incompressibility. Building on our understanding of the hydrodynamic pair interactions of passive colloids with active bacteria, we calculate the path of tracer entrained by the flow of active swimmers that follow curly trajectories and straight paths. Presence of multiple swimmers and active noise on their swimming paths are also considered to quantify mixing on the interface. This work provides us new design rules for active interfaces to enhance transport in multiphase systems.