Dewetting
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In fluid mechanics, 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 for a drop of oil placed on a liquid substrate (water here) with ambient gas, is the so-called spreading coefficient S
where γgw is the gas-water surface tension, γgo is the gas-oil surface tension and γow is the oil water surface tension (measured on the fluids before they are brought in contact with each other)
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. Surfactants can have a significant effect on the spreading coefficient. When a surfactant is added, it would decrease the interfacial tension, thus increasing the spreading coefficient (i.e. making S more positive) and allowing wetting to occur.
