Capillarity driven motion of solid film wedges

A solid film freshly deposited on a substrate may form a non-equilibrium contact angle with the substrate, and will evolve. This morphological evolution near the contact line is investigated by studying the motion of a solid wedge on a substrate. The contact angle of the wedge changes at time t = 0 from the wedge angle alpha to the equilibrium contact angle beta, and its effects spread into the wedge via capillarity-driven surface diffusion. The film profiles at different times are found to be self-similar; with the length scale increasing as t(1/4). The self-similar film profile is determined numerically by a shooting method for alpha and beta between 0 and 180 degrees. In general, we find that the film remains a wedge when alpha = beta. For alpha < beta, the film retracts, whereas for alpha > beta, the film extends. For alpha = 90 degrees, the results describe the growth of grain-boundary grooves for arbitrary dihedral angles. For beta = 90 degrees, the solution also applies to a free-standing wedge, and the thin-wedge profiles agree qualitatively with those observed in transmission electron microscope specimens. (C) 1997 Acta Metallurgica Inc.