Foam stabilization in salt solutions: The role of capillary drainage and Marangoni stresses

The long-standing question of why foaming is easier in seawater than in freshwater remains unresolved. We hypothesize that this phenomenon can be explained by Marrucci's 1969 theory, which predicts that as a foam film thins, its interfacial area increases, leading to a rise in salt concentration due to the Gibbs surface excess. This concentration gradient induces Marangoni stresses, causing a flow reversal, film thickening, and enhanced foam stability.

To test this hypothesis, we performed single-bubble interferometric experiments using electrolyte solutions with varying concentrations of phosphate salts. The film formation and drainage dynamics were tracked using color interferometry, enabling precise thickness measurements.

In deionized water, bubbles rupture within seconds due to rapid dimple collapse. However, in phosphate salt solutions, bubble lifetimes increased significantly, lasting several minutes. While previous theories have attributed foam stability to evaporation-driven salt concentration gradients, our results reveal that capillary drainage dominates film thinning, yet Marangoni-driven influx is still observed. We demonstrate that Marrucci's theory has been incorrectly dismissed, and the predicted critical film thickness at which fluid influx occurs closely matches our findings, as well as previous sodium chloride experiments. Additionally, we extend the validity of Marrucci's theory to non-aqueous fluid mixtures, underscoring its broader relevance to colloidal and interfacial science.