Elasticity of thin liquid films

Abstract

The elasticity of a film drawn from a surfactant solution has been determined by measuring simultaneously the increase in surface tension and the corresponding relative change in thickness of a film element. The film element was extended by increasing the weight of the film suspended from the element. The surface tension increase was found by assuming that the weight of the film is supported completely by the vertical gradient in surface tension. The decrease in thickness was measured from the displacement of the interference fringes, corrected for the contribution of drainage. Typical surfactant solutions were investigated, at concentrations both below and above the critical micellar concentration, in films of thickness between 0.5 and 2.5 × 10 −4 cm. The measured elasticity decreases with increasing thickness of the film element, as predicted by theory. The relation between elasticity and composition is found to be in satisfactory agreement with the theory, for films containing a pure surfactant. The effect of an added “minor component” on the film elasticity is used to calculate the composition of the film, which turns out to be different from the composition of the solution from which the film was generated.