(385g) Enhanced Transport Due to Driven and Active Colloids at Fluid Interfaces

We study enhanced transport by driven or active colloids trapped at fluid interfaces between immiscible fluids. This is an exciting system, as the colloids are trapped in a variety of configurations, and the interface is a two-dimensional fluid layer that is itself typically incompressible. The implications for enhanced or directed transport differ significantly from three-dimensional bulk suspensions of active/driven colloids. We assume that the colloids are adhered to the interface by a pinned contact line and thus fixed in a persistent orientation with respect to the interface. However, they are free to translate along the interface and rotate about an axis normal to the interface due to external forces and torques (for driven colloids) or via self-propulsion (for active colloids). Far-field flows generated by these colloids are of primary importance to long-ranged hydrodynamic interactions that drive transport of other passive material on or near the interface. We introduce a “library” of such flows, which vary depending on the mechanism of colloid motion (driven versus active), colloid orientation, and the properties of the interface itself. We then quantify transport of passive material by computing the “drift,” or fluid tracer displacements, arising from each of these modes for a colloid moving along a specified trajectory. We also estimate the resulting tracer diffusivity due to a very dilute layer of active or driven colloids moving along random trajectories of given length. We find conditions for viscous fluid interfaces in which lateral dispersion of passive material on or near the interface may be greatly enhanced by driven or active colloids. Our work is relevant to the determination of optimal mixing and transport strategies using colloids at interface.