Dewetting

In fluid mechanics, dewetting is one of the processes that can occur at a solid–liquid, solid-solid or liquid–liquid interface. Generally, dewetting describes the process of retraction of a fluid from a non-wettable surface it was forced to cover. The opposite process—spreading of a liquid on a substrate—is called wetting. The factor determining the spontaneous spreading and dewetting for a drop of liquid placed on a solid substrate with ambient gas, is the so-called spreading coefficient S: Circular hole formed in a 100 nm thick film of polystyrene. The blue color of the film is due to Structural coloration and depends on the film's thickness.AFM height profile of a dewetting hole's rim. Notice that the material that is removed by dewetting is accumulated at the rim around the hole; initial film thickness (height): 100 nm.Voronoi tessellation pattern of polygons achieved by the coalescence of dewetting holes.If given enough time, this network of polygons decays into separate droplets.Rim instability in the case of a thicker (200 nm) polystyrene film. In fluid mechanics, dewetting is one of the processes that can occur at a solid–liquid, solid-solid or liquid–liquid interface. Generally, dewetting describes the process of retraction of a fluid from a non-wettable surface it was forced to cover. The opposite process—spreading of a liquid on a substrate—is called wetting. The factor determining the spontaneous spreading and dewetting for a drop of liquid placed on a solid substrate with ambient gas, is the so-called spreading coefficient S: S   = γ SG − γ SL − γ LG {displaystyle S =gamma _{ ext{SG}}-gamma _{ ext{SL}}-gamma _{ ext{LG}}} where γ SG {displaystyle gamma _{ ext{SG}}} is the solid-gas surface tension, γ SL {displaystyle gamma _{ ext{SL}}} is the solid-liquid surface tension and γ LG {displaystyle gamma _{ ext{LG}}} is the liquid-gas surface tension (measured for the mediums before they are brought in contact with each other). When S > 0 {displaystyle S>0} , the spontaneous spreading occurs, and if S < 0 {displaystyle S<0} , partial wetting is observed, meaning the liquid will only cover the substrate to some extent. The equilibrium contact angle θ c {displaystyle heta _{ ext{c}}} is determined from the Young-Laplace equation. Spreading 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 liquid film. Dewetting can be inhibited or prevented by photocrosslinking the thin film prior to annealing, or by incorporating nanoparticle additives into the film. Surfactants can have a significant effect on the spreading coefficient. When a surfactant is added, it's amphiphilic properties cause it to be more energetically favorable to migrate to the surface, decreasing the interfacial tension and thus increasing the spreading coefficient (i.e. making S more positive). As more surfactant molecules are absorbed into the interface, the free energy of the system decreases in tandem to the surface tension decreasing, eventually causing the system to become completely wetting. In biology, by analogy with the physics of liquid dewetting, the process of tunnel formation through endothelial cells has been referred to as cellular dewetting.