Drainage Processes in Foam Films

Any freshly generated foam him that survives will now be subject to a capillary pressure exerted by the curved surfaces of the adjacent Plateau borders. That pressure will tend to suck liquid out of the foam him. The resultant process of him drainage is surprisingly complex. 

The simplest description of foam him drainage is obtained if the him is supposed cylindrical with immobile plane-parallel surfaces. Such behavior is represented by the Reynolds equation [37] 

In the case of relatively large foam hlms 100 microns, where the air-liquid surface is close to a condition of immobility, drainage occurs in an axisynunetric manner bnt with a non-nniform thickness. A thick region develops in the center of the him, the so-called dimple, while a thinner region surrounds this dimple— the so-called barrier ring [38]. A schematic cross-section of a him with a dimple is depicted in Fignre 1.6a. The behavior of such a him, formed inside a cylindrical glass cell (the 

Thin barrier ring forms that inhibits drainage from dimple 

FIGURE 1.6 Axisymmetric drainage of foam film with immobile air-water surfaces, (a) Dimple and barrier ring that form, (b) Defining maximum dimple size where barrier ring is absent and where film is formed in a cylindrical Scheludko cell (references [13,39] see Section 2.3.1). Radius of curvature, of Plateau border air-water surface is approximated by radius, R, of cell—is radius of curvature of air-water surface of the dimple. (Reprinted with permission from Joye, J. et al., Langmuir, 8, 3083. Copyright 1992 American Chemical Society.) 

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