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Unstable fronts and stable “critters” formed by microrollers
We investigate a deceptively simple system, weakly magnetic colloids suspended in a fluid above a floor. When forced to rotate by the application of a rotating magnetic field, the microrollers exhibit a cascade of instabilities ; the formation of a shock front, an instability along the shock front, and the formation of fingers that detach and lead to moving stable autonomous clusters, “critters”. Once formed, the critters can be readily steered by the applied magnetic field. These persistent motile structures can form spontaneously from hydrodynamic interactions alone with no sensing or potential interactions conventionally associated with swarming and flocking. Our studies include experiments and large-scale 3D simulations. With power, speed and direction controlled remotely these discoveries may prove useful for transport, flow, vorticity and mixing control in fluidic systems.