  We investigate the individual and collective dynamics of torque-driven
particles, called microrollers, near fluid-fluid interfaces. We find that the
viscosity ratio across the interface controls the speed and direction of the
particles, their relative motion, the growth of a fingering instability, and
the self-assembled motile structures that emerge from it. By combining theory
and large scale numerical simulations, we show how the viscosity ratio across
the interface governs the long-range hydrodynamic interactions between
particles and thus their collective behavior.
