The Role of Diffusion and Faceting in Surface and Grain Boundary Wetting

In wetting experiments, the morphology of a substrate at its triple line with a sessile drop depends on the diffusion rate of the components of the solid. The thrust of the system to reach mechanical equilibrium at the triple line can cause the formation of a ridge, provided the diffusion rate is sufficiently high. There is much in common between the ridging of triple junctions in wetting experiments and the grooving of grain boundaries, the energy of liquid/vapour interface in the former case playing the role of grain boundary energy in the latter. The evolution of the shapes of the ridge and groove depends on the mechanism of diffusion. Mullins determined the shape of a GB groove by assuming isotropic free energies for the solid interfaces. We will show that anisotropy of solid/liquid and solid/vapour interfacial energies plays a major role in the shapes of both grooves and ridges, and may lead to such important phenomenon as rapid penetration of liquid metal along GBs.