CAPILLARITY DRIVEN MOTION OF SOLID FILM WEDGES

A solid film freshly deposited on a substrate may form a non-equilibri um contact angle with the substrate, and will evolve. This morphologic al evolution near the contact line is investigated by studying the mot ion of a solid wedge on a substrate. The contact angle of the wedge ch anges at time t = 0 from the wedge angle alpha to the equilibrium cont act angle beta, and its effects spread into the wedge via capillarity- driven surface diffusion. The film profiles at different times are fou nd to be self-similar; with the length scale increasing as t(1/4). The self-similar film profile is determined numerically by a shooting met hod 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 groove s for arbitrary dihedral angles. For beta = 90 degrees, the solution a lso applies to a free-standing wedge, and the thin-wedge profiles agre e qualitatively with those observed in transmission electron microscop e specimens. (C) 1997 Acta Metallurgica Inc.