Effect of surface forces and surfactant adsorption on the thinning and critical thickness of foam films

The new experimental results on thinning and rupture of microscopic foam films, obtained by the dynamic method of Scheludko and Exerowa, are analyzed with respect to the observed discrepancies with the theoretical predictions. The possible reasons for the deviations from the frequently used Reynolds’ equation, describing the thinning rate of planar circular horizontal liquid films with non-deformable and tangentially immobile surfaces, are discussed. The applicability of the theoretical description of the thinning process (accelerated drainage), advanced in a previous study, is tested here experimentally. The effect of surface tension and surface mobility on the thinning rate and critical thickness of rupture is established. The relative effects of different factors are estimated through specially designed experiments. They comprehend: bulk and surface diffusion of the surfactant; evaporation of liquid from the film; variation of the film radius during the process of drainage; inhomogeneity of the film thickness, etc. It is concluded that the influence of the surface forces and the surfactant concentration is significant for the thinning and rupture, and respectively for the life-time of the microscopic films, while the influence of evaporation is negligible under the conditions of the experiment.