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.

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• Clustering
• Emergence of patterns
• Fingering instability
• Fluid-particle interactions
• Instability control
• Self-assembly
• Interfaces
• Magnetic colloids
• Self-propelled particles
• Collective dynamics
• Stokesian dynamics