Stability of liquid coatings

Liquid horizontal films are always rupture resistant when their thickness exceeds a certain value hi, given by  

Most liquid films which are formed during film-coating are metastable or instable. This especially involves coating with aqueous suspensions. With ceramic membrane support coatings with a wet coating as thin as 1000 nm, instability can already occur at relatively low contact angles. 

A film can only break up into droplets after a disturbance the film locally thins to less than t)q)ically 1000 nm (see Fig. 6.40). In this region the interaction force (van der Waals, electrical double layer, for example) between the liquid-solid and liquid-air surface of the film becomes important. Attraction forces can rupture the thin film and a dry patch is nucleated. Such a film is called a non-wetting film. When the interaction between the two film interfaces is repulsive the so-called disjoining pressure (see also p. 162) of the film, i.e. the pressure difference between the film and bulk liquid, is negative. In the other case of negative disjoining pressures, it may also be called conjoining pressure. 

6 — PREPARATION OF ASYMMETRIC CERAMIC MEMBRANE SUPPORTS BY DIP-COATING 

High Low High surface surface surface tension tension tension 

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