Marginal Regeneration in Thin Vertical Liquid Films

TLDR: Experiments and simulations prove that marginal regeneration cannot be the result of thickness fluctuations, and that it is due to surface tension gradients between the film and its borders.

Abstract: Marginal regeneration is the rate-determining drainage mechanism in mobile vertical liquid films stabilized with surfactants. Mysels, Frankel, and Shinoda explained this process from (thermal) thickness fluctuations, like capillary waves. The Laplace underpressure in the Plateau border would exert a larger force on a thick, rather than on a thinner film element. This force unbalance would make film elements of different thicknesses move in opposite directions so that they are exchanged at the border. However, experiments and simulations prove that marginal regeneration cannot be the result of thickness fluctuations. Our alternative view is, that marginal regeneration is due to surface tension gradients between the film and its borders. Drainage of film elements into the lower Plateau border causes a local excess of surfactant, and thereby local differences in surface tension. This causes film elements to flow and generates the thickness differences between the absorbed and emerging film elements. The rates of the Marangoni flows reflect the surface dilational properties. This Marangoni effect is a consequence of the compression of the film surface when a film element flows into the lower Plateau border. Marginal regeneration is then a mechanism which returns the surfactant back into the film.