The motion of active colloids and their induced flow field at fluid interfaces

Colloid motion is altered by adsorption to fluid interfaces, allowing for boundary guidance or directed assembly. This is particularly interesting for colloidal motion driven by external fields (driven colloids) or for self-propelled objects (active colloids or swimmers). However, the influence of a fluid interface on these hydrodynamic phenomena and the 2-d flow field induced by the motion of these active colloids are relatively unexplored. Here we image the motion of passive tracer probes at the interface to discern displacement fields generated by (1) a microbead undergoing thermal motion, (2) a magnetic disk forced to translate at constant velocity, and (3) actively swimming bacteria. We also theoretically quantify the flows generated by interfacially trapped colloids by developing an appropriate flow singularity model for both driven and active colloids. We consider an ideal flat "clean" interface characterized solely by a uniform interfacial tension. We also consider an incompressible fluid interface, as can occur even for trace surfactant adsorption. Theoretical results are compared with experimental flow field. Our results will be useful in future work on the use of active colloids to direct and enhance transport at interfaces