Thick films - spreading of one liquid on another

Nowadays, LB films are also used to cover electronical devices of special optical, electronical, or mechanical properties. Often, however, such hybrid systems between organic and inorganic compounds are not stable over a long time. For reviews on this topic see Refs. [631,632], 

What happens, when we put a drop of liquid B on the surface of liquid A The two liquids are assumed not to mix and the density of B should be smaller than the density of A. That is, for example, the case for many oils on water. There are two possibilities either liquid B spreads or it forms a lens with a defined edge. 

Liquid B spreads spontaneously on liquid A, if the spreading coefficient, also called spreading pressure 

A complication now arises. 7a and 7b are values for the pure liquids. In a real situation, at least a tiny amount of molecules A is dissolved in liquid B, and vice versa. Therefore we should actually use 7a b) and 7B a) instead of ja and 7b- Here, 7a(b) is the surface tension of the liquid A, in which molecules B are dissolved up to saturation. For simplicity, we continue using the first notation. 

If the spreading coefficient is smaller than zero, then a well defined drop of liquid B forms on the surface of A. The shapes of the surface of liquid A, the liquid B surface, and the liquid A-liquid B interface are determined by the Laplace equation. The angles 0i, 02, and 3 are boundary conditions (Fig. 13.14). They are related by 

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