Dewetting

Dewetting is the process by which a liquid film on a solid substrate becomes unstable and retracts, eventually breaking up into droplets or patches and exposing the underlying surface. It is the opposite of wetting; while wetting describes the spreading of a liquid on a surface, dewetting describes the spontaneous reversal of a metastable or non-wetting thin film.

Two main mechanisms drive dewetting. Spinodal dewetting occurs when capillary forces and interfacial interactions render a uniform film unstable, so small perturbations grow and create a characteristic pattern that fragments into droplets. Heterogeneous, or nucleation, dewetting begins at defects, impurities, or substrate inhomogeneities where holes form and expand, leaving rims that may break up into isolated droplets. The stability of a thin film is governed by the disjoining pressure, related to the derivative of the effective film–substrate potential with respect to thickness.

The dynamics and morphology of dewetting depend on film thickness, liquid viscosity, surface tension, and substrate properties. Hole growth produces a raised rim around the opening; rims can merge or become unstable, and the film may coarsen into larger droplets over time. Spinodal dewetting often yields regular or labyrinthine patterns with a characteristic length scale set by thickness and interfacial forces.

Modeling and measurement typically use the thin-film equation under lubrication approximation, incorporating capillary and disjoining pressures. Experimental techniques include optical microscopy, atomic force microscopy, ellipsometry, and X-ray methods to monitor thickness and morphology.

Applications and control: dewetting is relevant in coatings, microfabrication, and patterning. It can be undesirable for protective coatings but useful for creating nanopatterns. Strategies to suppress or control dewetting include modifying substrate surface energy, increasing film thickness, crosslinking, or applying surface treatments to enhance wetting.