Mechanisms of action of mixed solid-liquid antifoams. 1. Dynamics of foam film rupture

Antifoams (usually consisting of a mixture of hydrophobic solid particles a nd oils) are widely used in different technological applications to prevent the formation of excessive foam. Uncertainty still exists in the literatur e about the actual mechanisms by which these substances destroy the foam. T o elucidate this problem, we have performed microscopic observations on the process of foam film destruction by means of a high-speed camera. Horizont al and vertical foam films (obtained from solutions of the surfactant sodiu m dioctyl sulfosuccinate) were studied in the presence of antifoam particle s containing silicone oil and hydrophobized silica. The observations show t hat in this system the antifoam particles destroy the foam lamella by the f ormation of unstable oil bridges, which afterward stretch and eventually ru pture, due to uncompensated capillary pressures across the different interf aces. These bridges can be formed either from initially emulsified antifoam droplets, which enter both surfaces of the foam film during its formation and thinning, or from oil lenses which float on the bulk air-water interfac e even before the foam film is formed. We show that the presence of an oil layer having a thickness of several nanometers, prespread over the foam fil m surfaces, is very important for the process of lamella destruction, becau se this layer substantially facilitates the entry of the oil drops on the f ilm surface and the formation of unstable bridges. The process of oil-bridg e stretching, which is usually not considered in the standard mechanisms of antifoam action, is theoretically analyzed in the second part of this stud y.