Critical-point wetting near grain boundaries and defect planes

A Landau theory for wetting in systems with an internal defect plane is proposed and solved exactly. The systems can undergo a phase transition in which an interface depins from the defect plane (or a phase transition from partial to complete wetting of the defect plane), when the bulk critical point $T_c$ is approached. This phenomenon of critical-point wetting occurs if the defect plane remains ordered at $T_c$ (extraordinary transition) but is absent if the defect plane disorders at $T_c$ (ordinary and special transitions). This latter possibility has previously been found in Ising models with defect bonds and in lattice-gas models with a grain boundary between binary-alloy crystals.