Foam stability

The formation of the Newton Black Film (NBF) in single vertical soap films with varied electrolyte concentrations was studied. A ‘slow’ and a ‘rapid’ regime for NBF formation could be identified. These results were interpreted in terms of a modified Scheludko–Vrij model. In analogy with the DLVO theory for coagulation kinetics, we consider the effect of the disjoining pressure on the spontaneous growth of thickness fluctuations in thin films. The repulsion between the film surfaces and the elasticity of the films have a damping effect on these fluctuations. The formation of the NBF by a spontaneously growing fluctuation explains why surfactants stabilise foams and can make them persistent.