Periodic mass shedding of a retracting solid film step

Abstract

A semi-infinite, uniform film on a substrate tends to contract from the edge to reduce the surface energy of the system. This work studies the two-dimensional retraction of such a film step, assuming that the film evolves by capillarity-driven surface diffusion. It is found that the retracting film edge forms a thickened ridge followed by a valley. The valley sinks with time and eventually touches the substrate. The ridge then detaches from the film. The new film edge retracts to form another ridge accompanied again by a valley, and the mass shedding cycle is repeated. This periodic mass shedding is simulated numerically for contact angle α between 30 and 180°. For smaller α, a small-slope late-time solution is found that agrees with the numerical solution for α=30°. Thus, the complete range of α is covered. The long-time retraction speed and the distance traveled per cycle agree quantitatively with experiments.