Reorientation dynamics of microswimmers at fluid-fluid interfaces

Abstract

We show that a microswimmer trapped thermodynamically at fluid-fluid interfaces experiences a torque arising from the hydrodynamic boundary condition at the interface. This turns force dipoles corresponding to pullers perpendicular and pushers parallel to the interface. We demonstrate that in the general case, when there is a viscosity contrast across the interface, the steady state orientation is given by the interplay between the torques arising from the force dipoles and the self-propulsion which is sensitive to the viscosity difference between the two fluids.