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

TL;DR

This study explains why foaming is easier in seawater than freshwater by demonstrating that Marangoni stresses, driven by salt concentration gradients, stabilize foam films, supported by interferometry experiments and theoretical analysis.

Contribution

The paper validates Marrucci's theory on Marangoni stresses in foam films, showing its relevance in salt solutions and extending its applicability beyond aqueous systems.

Findings

Marangoni stresses cause flow reversal and stabilize foam films.

Salt concentration gradients induce Marangoni-driven influx in thin films.

Marrucci's theory accurately predicts critical film heights and stability conditions.

Abstract

The long-standing question of why foaming is easier in seawater than in freshwater remains unresolved. In this study, we address this issue through precise interferometry single bubble experiments, demonstrating that the theory proposed by G. Marrucci (1969) provides a compelling explanation. Electrolyte solutions with varying concentrations of phosphate salts were used to study film formation and drainage, with thickness tracked by interferometry. In deionized water, bubbles rupture within seconds due to repaid dimple collapse. However, in phosphate salt solutions, bubbles persisted for several minutes. While surface tension gradients from evaporation-driven salt concentration gradients have been thought to create Marangoni stresses, our results show that despite film thinning being capillary-dominated, Marangoni-driven influx can be observed. Marrucci's theory explains this by showing that an increased interfacial area as the film thins, leads to higher salt concentration in the film due to Gibbs surface excess. This concentration gradient induces Marangoni stresses, causing flow reversal, increased film thickness, and enhanced foam stability. We show that Marrucci's theory has been incorrectly dismissed, and the predicted critical heights where fluid influx occurs closely match our findings and other studies using sodium chloride. Additionally, we extend the theory's applicability to foam films in non-aqueous film mixtures, highlighting its broader relevance.