Foamability of oil mixtures

In the absence of stabilization mechanism of films of pure liquids, foams do not form in pure liquids. Froth formation is observed in liquid mixtures, among which, oil mixtures, and that effect is well documented in the literature. The higher film lifetimes has however not been understood in non-volatile liquids and in the absence of surface-active molecules. We have experimentally studied the foaming of binary mixtures of liquids, and found the foam lifetime depends not only on the nature of the liquids but also on the proportions of the two liquids. We suggest a stabilizing mechanism of thin films of liquid mixtures based on non-linear variations of the mixture’s surface tension with its composition, analogous to Gibbs’ elasticity defined for surfactant solutions. We show that the variations of foam lifetime with mixture composition is quantitatively described by the model. Finally, we define an effective disjoining pressure for thin films that can be fully computed from the variations of the surface tension of any liquid mixture which surface tension does not vary linearly with composition.