Bulk Condensation by an Active Interface.

We present experiments, supported by mechanically detailed simulations, establishing bulk condensation of a hard-bead fluid by a tiny population of orientable motile grains that self-assembles into a moving polarized monolayer. In a quasi-1D geometry two such layers, oppositely aligned, immobilize the condensed nonmotile component. We account for our observations through a continuum theory with a naturally nonreciprocal Cahn-Hilliard structure, whose predicted trends as a function of packing fraction are consistent with our observations.