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 :  
where  is the solid-gas surface tension,  is the solid-liquid surface tension and  is the liquid-gas surface tension (measured for the mediums before they are brought in contact with each other).
When , the spontaneous spreading occurs, and if , partial wetting is observed, meaning the liquid will only cover the substrate to some extent.
The equilibrium contact angle 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, its 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.