  At the surfaces of autophoretic colloids, slip velocities arise from local
chemical gradients that are many-body functions of particle configuration and
activity. For rapid chemical diffusion, coupled with slip-induced hydrodynamic
interactions, we deduce the chemohydrodynamic forces and torques between
colloids. Near a no-slip wall, the forces can be expressed as gradients of a
non-equilibrium potential which, by tuning the type of activity, can be varied
from repulsive to attractive. When this potential has a barrier, we find
arrested phase separation with a mean cluster size set by competing chemical
and hydrodynamic forces. These are controlled in turn by the monopolar and
dipolar contributions to the active chemical surface fluxes.
