We study, numerically, the collective dynamics of rotating nonaligning particles by considering a monolayer of spheres driven by constant clockwise or counterclockwise torques. We observe emergence of intriguing patterns: When rotors are far away, they move chaotically and strongly stir the ambient fluid. As rotors separation decreases (with increasing volume fraction), same-spin rotors spontaneously segregate and collectively move in traffic lanes or circulate in large vortices. When the rotor density gets close to maximum packing, the rotors jam into crystals that continuously melt, reassemble, and move.
The collective behavior emerges from the hydrodynamic interactions between the rotors: the flow stirred by each rotor drags other rotors into motion. For example, a pair of opposite-spin rotors cooperatively "swims’’, while a pair of same-spin rotors orbits around each other. The "hydrodynamic coupling’’ pervades the collective behavior of active suspensions – from living systems such as bacterial suspensions to inanimate matter made of rolling colloidal particles or tapered metal rods lying on a mechanically vibrated bed of smaller metal beads.