Dewetting

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Dewetting is one of the processes that can occur at a solid-liquid or liquid-liquid interface. Generally, dewetting describes the rupture of a thin liquid film on the substrate (either a liquid itself, or a solid) and the formation of droplets. The opposite process - spreading of a liquid on a substrate - is called wetting. The factor determining the wetting and dewetting is the so-called spreading coefficient S

S=S - F - SF (1)

where S and F are the surface energies of the substrate and film, respectively, and SF is the interfacial energy between film and substrate. When S>0, the surface is considered wettable, and if S<0, dewetting occurs. Wetting and dewetting are important processes for many applications, including adhesion, lubrication, painting, printing, and protective coating. For most applications, dewetting is an unwanted process, because it destroys the applied thin film. In most dewetting studies a thin polymer film is spun onto a substrate. Even in the case of S<0 the film does not dewet immediately if it is in a metastable state, e.g. if the temperature is below the glass transition temperature of the polymer. Annealing such a metastable film above its glass transition temperature increases the mobility of the polymer chain molecules and dewetting takes place. When starting from a continuous film, an irregular pattern of droplets is formed. The droplet size and droplet spacing may vary over several orders of magnitude, since the dewetting starts from randomly formed holes in the film. There is no spatial correlation between the dry patches that develop. These dry patches grow and the material is accumulated in the rim surrounding the growing hole. In the case where the initially homogeneous film is thin (in the range of 100 nm), a polygon network of connected strings of material is formed, like a Voronoi pattern of polygons. These strings then can break up into droplets, a process which is known as the Rayleigh-Taylor instability. At other film thicknesses, other complicated patterns of droplets on the substrate can be observed, which stem from a fingering instability of the growing rim around the dry patch.