DEWETTING - NUCLEATION AND GROWTH OF DRY REGIONS

Dewetting of a solid surface covered by a film of nonwetting liquid either proceeds from a pre-existing dry patch or edge, or initiates from some film-thinning disturbance that continues to grow until the film ruptures. Local thinning can be caused by evaporation; by drainage due to gravity or capillarity-driven flow, especially from sharp surfaces; or by surface tension gradients, such as are caused by surface-active substances delivered by particles falling on the film. Once a nonwetting film has been sufficiently thinned, conjoining (negative disjoining) pressure can accelerate thinning until rupture. This catastrophic rupture is modeled here by solving the Navier-Strokes system approximated for thickness variations over distances long compared to the mean film thickness, and augmented with conjoining pressure. Rupture leads to film retraction and formation of a dry patch. These phenomena are visualized with the aid of moire topography. Of special interest are local spreading disturbances where airborne particles fall on the film surface: craters or dry patches often nucleate. Implications for coating operations are discussed.