Competing chemical and hydrodynamic interactions in autophoretic colloidal suspensions.

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. For bottom-heavy particles above a no-slip wall, the forces can be expressed as gradients of a nonequilibrium 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 interactions. These are controlled, in turn, by the monopolar and dipolar contributions to the active chemical surface fluxes.