Wetting near grain boundaries and defect planes, and its connection with wetting at walls and free surfaces

A Landau theory is studied for wetting and interfacial depinning in systems with internal defect planes, which can model grain boundaries. The defect plane divides the infinite system into two semi-infinite ones, each of which can undergo phase transitions of the wetting type. If both semi-infinite systems have the same critical temperature ${\mathit{T}}_{\mathit{c}}$, critical-point wetting does not occur unless the defect plane remains ordered at ${\mathit{T}}_{\mathit{c}}$. If the two semi-infinite systems have unequal critical temperatures, ${\mathit{T}}_{\mathit{c}}^{\mathrm{-}}$ and ${\mathit{T}}_{\mathit{c}}^{+}$, the phenomena are similar to those of wetting at a free surface or wall, and critical-point wetting is the rule. In the limit that ${\mathit{T}}_{\mathit{c}}^{\mathrm{-}}$ and ${\mathit{T}}_{\mathit{c}}^{+}$ approach each other, the line of tricritical wetting connects to the surface of defect-plane criticality and becomes the line of surface-bulk multicriticality of the defect plane. Implications for similar phenomena beyond Landau theory (e.g., in the Ising model) are outlined.