  We present experiments, supported by mechanically detailed simulations,
establishing bulk vapor-liquid 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 non-motile component. We account for our
observations through a continuum theory with a naturally non-reciprocal
Cahn-Hilliard structure, whose predicted trends as a function of packing
fraction are consistent with our observations.
