Disjoining Forces and Foam Film Stability

In consequence, any perturbation of the film thickness that produces thin and thick regions will tend to grow spontaneously because molecules in the thin regions will transfer to the thick regions. However, any perturbation of a film to produce thick and thin regions must also inevitably increase the surface area of the film. This will increase the number of molecules in the relatively weak attractive force field close to the air-liquid surface. Such an increase in surface area will therefore be resisted by an opposing force—the surface tension [51,52], 

FIGURE 1.10 Plot of disjoining pressure isotherm for a plane-parallel air-water-air film with only van der Waals interactions. Here disjoining pressure, n LA, is given by IlALAffO =.Ah/6jc/i where Ah is Hamaker constant of 3.7 x 10 J. (From IsrealachviUi, J.N. Intermolecular and Surface Forces with Applications to Colloidal and Biological Systems, Academic F ress, London, 1985 [50].) 

The rate of growth of the perturbation will increase with increasing wavelength X for X X it because the damping effect of the capillary pressure will decrease. However, 

FIGURE 1.11 Sinusoidal thickness perturbations in thin liqnid foam film. 

FIGURE 1.12 Schematic diagram of disjoining pressnre isotherm exhibiting two stable regions where common black foam films and Newton black films can be formed, respectively (see text for full explanation). IIala air-Uquid-air (ALA) foam film disjoining pressure and h is film thickness. 

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