Thin liquid film formation, stages

FIGURE 5.36 Main stages of formation and evolution of a thin liquid film between two bubbles or drops (a) mutual approach of slightly deformed surfaces (b) at a given separation, the curvature at the center inverts its sign and a dimple arises (c) the dimple disappears, and eventually an almost plane-parallel film forms (d) due to thermal fluctuations or other disturbances the film either ruptures or transforms into a thinner Newton black film (e), which expands until reaching the final equilibrium state (f). 

The main stages in the formation and evolution of the thin liquid film between two equal size approaching foam bubbles as shown in Figure 4 can be summarized as ... 

Fig. 2 A schematic for spin coating thin films. The coating solution is either (A) dripped on the substrate while it is at rest or (B) dripped on while the substrate is spinning (C) spin-off stage, forming a thin liquid coating on the substrate (D) solvent evaporation results in self-assembly and thin film formation.

In spite of this, there is no accurate description accounting for the formation of a liquid film at the surface of a solid starting from an adsorbed film, or conversely of the appearance of an adsorbed film in the final stages of evaporation of a liquid film. In fact, the two major models hitherto developed for multilayer adsorption, namely the model originally proposed by Brunauer, Emmett and Teller (BET) and the one proposed separately by Frenkel, Halsey and Hill (FHH), apply to the description of a thin adsorbed phase the former and of a liquid film the latter. 

When moving a bubble toward a planar solid-liquid interface, the liquid film between the solid-liquid and the liquid-gas interface is thinning due to a radial flow of the liquid. When the bubble is still a large distance away from the surface, the distance of closest approach is in the axial center. Then, however, the central part approaches slower and slower and is overtaken by the periphery. Thus, the film is most often not plane parallel. It thins faster at rim than in the central part, which causes the formation of the so-called dimple (Figure 6.12) [686-688, 690]. In the last stage, the central part also approaches the solid surface until it has reached the... 

Two processes take place simultaneously in the emulsion formation a) dispersion of one liquid in another b) coalescence, i.e. irreversible fusion of two or more droplets and formation of a larger one. The coalescence proceeds in several subsequent stages [54] formation of a thin film as result of droplet contact, thinning of the film, and its rupture when achieving a critical thickness. The role of the surfactant concentration in the formation of emulsions can be demonstrated using as an example sunflower oil emulsions stabilized by potassium monododecyl phosphate (see Fig. 6.8). 

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