Drainage and rupture of thin foam films in the presence of ionic and non-ionic surfactants

Abstract

The thin film pressure balance technique was used to determine the overall magnitude of the surface forces in foam films stabilized by flotation reagents such as sodium dodecyl sulfate and polypropylene glycol at high NaCl concentrations. The Stefan–Reynolds lubrication approximation was used to estimate the forces from measured film thinning rates while the capillary wave model of Valkovska, Danov and Ivanov was used to calculate the forces from measured critical rupture thicknesses. It was found that at very low surfactant concentrations commensurate with typical flotation reagent dosage, the overall forces were attractive and up to one order of magnitude stronger than the Lifshitz–van der Waals forces. The forces became smaller with increasing surfactant concentration. It was also found that the forces obtained from the capillary wave theory were indifferent to changes in film radii and surface mobility, in contrast to the Reynolds lubrication approximation. For comparison, other film drainage models considering film surface mobility and hydrodynamic corrugation were used to fit the present experimental thinning curves obtained at very low surfactant concentrations. They also showed that the overall attraction forces were several times stronger than the Lifshitz–van der Waals forces.