Unstable fronts and stable "critters" formed by magnetic microrollers

Paul Chaikin (NYU) 

  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.