Understanding frothing of liquid mixtures: A surfactant-like effect at the origin of enhanced liquid film lifetimes

• Published in: Physical review letters Vol. 125
• Main Authors: Tran, H.-P, Arangalage, M, Jørgensen, L, Passade-Boupat, N, Lequeux, F, Talini, L
• Format: Journal Article
• Language: English
• Published: American Physical Society 2020
• Subjects: Condensed Matter; Physics; Soft Condensed Matter
• ISSN: 0031-9007; 1079-7114

The formation of froth in mixtures of liquids is well documented, in particular in oil mixtures. However, in non-volatile liquids and in the absence of surface-active molecules, the effect increasing liquid film lifetimes had not been identified. We suggest a stabilizing mechanism resulting from the non-linear variations of the surface tension of a liquid mixture with its composition. We report experimental lifetimes of froths in binary mixtures and we show their variations are well predicted by the suggested mechanism. We demonstrate it prescribes the thickness reached by films before their slow drainage, thickness which correlates well with froth lifetimes for both polar and non-polar liquids. Foams are metastable dispersions of gas in liquids. In the past decades, the processes at the origin of their finite lifetimes have been extensively studied in surfactant solutions [2]. Among the destabilizing mechanisms, one results from the capillary drainage of liquid films in the Plateau borders where three liquid films meet. The pressure drop associated with the curved gas/liquid interfaces at the Plateau borders induces a capillary suction of the liquid, leading to film thinning. In presence of surfactants adsorbed at the interfaces, thinning can be opposed by a repulsion between both film interfaces, either of steric or charge-induced nature. In addition, film rupture can be delayed by the surface tension gradient originating from local extension of the film surface. A local decrease in surfactant concentration generates an increase in surface tension, driving a Marangoni flow opposing drainage [3]. A measurement of that effect is the Gibbs elasticity, which associated modulus relates the excess surface tension to the relative increase of surface area [4, 5]. However, the influence of Gibbs elasticity on foam stability is still an open question [6]. Marangoni flows are also invoked to explain the large influence of contaminants on the stability of liquid films, even in the absence of purposely added surfactants [7]. However, in pure liquids of low surface tensions such as oils, contaminant-induced effects are small and the lifetime of films is so short that generally no foaming is observed in pure non-polar