Roughness-induced filling.

We study adsorption of a fluid on a periodically corrugated substrate using the mean-field version of the effective Hamiltonian approach. We analyze the shape of the interface close to the wetting point of a planar substrate, and the free energy of the system as a function of temperature and amplitude of the corrugation for short-range and long-range interactions. We prove that the substrate roughness has no influence on the locus and order of the wetting transition, when the planar substrate of the same chemical composition as the corrugated one experiences critical wetting. For short-range interactions we observe the corrugation driven filling transition. We show analytically that a thin-thick first-order transition occurs when the corrugation amplitude of the substrate exceeds a critical value. The phase diagram of the adsorption on a sinusoidally corrugated substrate at the bulk liquid-gas coexistence is obtained.