Stable clusters emerge from unstable fronts

Rotating colloidal particles near a surface create strong advective flows, which can lead to a rich variety of collective effects. It has recently been shown that long-lived compact motile structures, called ``critters'', emerge naturally from a fingering instability in this microroller system. We identified these new structures using large-scale 3D 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 are formed only with hydrodynamic interactions. Furthermore, as compact, self-assembled structures which can easily be remotely guided, critters may offer a promising tool for microscopic transport.