On Dimple Formation in Foam Films

Dimple formation has been studied numerically in foam films formed in the conventional Scheludko-Exerowa cell and in a miniature cell recently developed by O. D. Velev et al. (J. Colloid Interface Sci. 175, 68 (1995)). Results of the simulations provide a criterion for dimple formation in the conventional cell in the absence of disjoining pressure. Because the films in the miniature cell are very thin, disjoining pressure effects are important even during the liquid withdrawal step when the film is initially formed. If the disjoining pressure is repulsive, film thickness remains nearly uniform and constant as observed experimentally by the above authors. In other cases the simulations predict that dimple formation will occur in the miniature cell but that it will not be detectable by the usual interference techniques because film thickness is less than one-fourth the wavelength of visible light.