  Condensation of objects into stable clusters occurs naturally in equilibrium
and driven systems. It is commonly held that potential interactions, depletion
forces, or sensing are the only mechanisms which can create long-lived compact
structures. Here we show that persistent motile structures can form
spontaneously from hydrodynamic interactions alone with no sensing or potential
interactions. We study this structure formation in a system of colloidal
rollers suspended and translating above a floor, using both experiments and
large-scale 3D simulations. In this system, clusters originate from a
previously unreported fingering instability, where fingers pinch off from an
unstable front to form autonomous "critters", whose size is selected by the
height of the particles above the floor. These critters are a stable state of
the system, move much faster than individual particles, and quickly respond to
a changing drive. With speed and direction set by a rotating magnetic field,
these active structures offer interesting possibilities for guided transport,
flow generation, and mixing at the microscale.
