Dewetting

When S > 0, the spontaneous spreading occurs, and if S < 0, partial wetting is observed, meaning the liquid will only cover the substrate to some extent.

Spreading and dewetting are important processes for many applications, including adhesion, lubrication, painting, printing, and protective coating.

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).

Annealing such a metastable film above its glass transition temperature increases the mobility of the polymer-chain molecules and dewetting takes place.

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.

The driving force for dewetting is the minimization of the total energy of the free surfaces of the film and substrate as well as of the film-substrate interface.

[7] The dedicated heating stage in SEM has been widely used to accurately control sample temperature through a thermocouple to observe the in-situ behaviour of the material, and can be recorded as a video format.