Faceting and wetting transitions of anisotropic interfaces and grain boundaries

A three-dimensional construction is presented that illustrates conditions u nder which anisotropic interfaces will be fully wetted, partially wetted, o r not wetted by a second phase. Recent experimental observations on the equ ilibrated morphologies of solid or fluid "wetting" phases along anisotropic interfaces and grain boundaries reveal features that are predicted-and, in some cases, required-by the construction. Theory distinguishes between cas es where surfaces are smoothly curved and where there are facets, edges, an d corners. In the latter case, the conventional comparison of the surface e nergy of the original surface with the sum of the surface energy of the two surfaces of the wetting layer leads to erroneous predictions. The correct predictions are obtained by comparing the Wulff shape of the original surfa ce with a carefully defined "sum" of Wulff shapes of the surfaces of the we tting layer, Where orientations that are wetted join with those that are no t, an abrupt change of orientation usually is present. Faceting on two hier archical levels can occur. Microscopic morphology changes along macroscopic ally curved surfaces follow well-defined rules that are predicted by the th eory, The analogy between the thermodynamics of surface faceting and phase transformations allows the well-known concepts of phase equilibria to be us ed to understand the predicted structures.