Foam stability: Part IV. Kinetics and activation energy of film rupture

Abstract

AbstractHigh‐speed cinematography of collapsing lamellae of an aqueous Tee‐pol solution reveals that spontaneous rupture always takes place in the thinnest part of the lamella. The rate of hole expansion is dependent on the thickness of the lamella; its value is found to be in the order of some hundreds of cms per second. Dupré's assumption that the released surface energy will be completely converted into kinetic energy of the liquid appears to be essentially correct. Film rupture in a foamed aqueous solution of ammonium oleate is shown to proceed at a rate which is at least of the same order of magnitude as that in the isolated lamellae.Deformation of a foam lamella occurs locally when a hole is about to be formed. The deformation and the very first beginning of hole expansion require an energy of activation which is calculated with the aid of a geometrical model. The energy is more than 1,000 kT units in a “black” film of an aqueous soap solution. This amount of energy has to be supplied in order to increase the interfacial area. The activation energy is proportional to the square of the film thickness and its value is hardly influenced by the magnitude of the electric double‐layer interaction.The formation and relative stability of “black spots” in films of much larger thickness than the spot itself is shown to be the result of an equilibrium, existing at a thickness between 100 and 150 Å, of the three forces coming into play: London‐Van der Waals attraction, double‐layer repulsion and surface tension.