Foam stability: Part V. Mechanism of film rupture

Abstract

AbstractLamellae of pure liquid and low‐molecular solutions are unstable because of the absence of a limiting thickness. Rapid collapse of these lamellae may start at relatively large thickness (∼ 500 Å) under influence of the London‐van der Waals attractive forces. Small quantities of surface active ions will prevent this collapse mechanism to occur in aqueous lamellae. A sufficient amount of activation energy for rupturing a “stable” lamella may be supplied by a local depression of surface tension in a small volume element of the film. The minimum surface area of such an element is calculated as a function of the film thickness, and its dependence on the rheological properties of the superficial layers of liquid (plasticity, viscosity and elasticity) is explained.Some results are given of an experimental investigation of the influence of a local rise in temperature on the lifetime of isolated lamellae of a Tee‐pol solution. The destabilizing effect of local heating is interpreted as a result of the decrease of surface tension in the warmer portion of the film. The importance of local temperature variations for the instability of foams is discussed. A calculation is given of the magnitude, frequency and average lifetime of spontaneous temperature fluctuations in liquid lamellae under adiabatic conditions. It is shown that these fluctuations are able to affect the stability of very thin lamellae only (thickness of the order of some tens of Å).A general explanation of the action of anti‐foaming agents is given, based on the occurrence of local depressions in surface tension. Finally the effect of mechanical disturbances on the probability of film rupture is considered.