Critters: stable clusters born from an unstable front

Rotating colloidal particles near a floor creates strong advective flows, which lead to a rich variety of collective effects. Using a model experimental system in tandem with large-scale 3D simulations, we have shown that driving a compact group of microrollers leads to a flow instability, which produces fingers of a well-defined wavelength. Under the right conditions, stable structures we term "critters" can emerge from these fingers. We first identified these new structures in simulations, and have recently made promising steps towards producing them in the lab. Our simulations and experiments suggest that these critters are a stable state of the system, move much faster than individual rollers, and quickly respond to a changing drive. We believe that critters are unique in that they are clusters which form only with hydrodynamic interactions; no interparticle potentials are needed to create these structures. Furthermore, as compact, self-assembled clusters which can easily be remotely guided, critters may offer a promising tool for microscopic transport.