The decay of saline surfactant foams: a study at macroscopic and bubble scales

Abstract How stable are saline, surfactant foams? While the effect of surfactants on stabilizing bubbles in foam is well understood, the effect of salt is not. Saline foams are potentially very important in industrial applications, including enhancing oil recovery, yet there is only a limited understanding of their long-term stability. We report an experimental methodology to simultaneously study the time evolution of foam, at varying salinity, at both macroscopic and bubble scales. With our dual-scale measurements, we track the dynamics of foams made with air and aqueous solutions containing non-ionic surfactant and salts over long times. In macroscopic foam height measurements, we observe that at long times, the foam height decreases logarithmically for all salinities, with the height reduction rate being significantly lower at the highest salinity, i.e. the most saline foam persists the longest. Bubble-scale measurements of bubble size, carried out near the bottom of the foam are far more sensitive, providing early warning of the fate of the foam.