The Gibbs-Marangoni stress and nonDLVO forces are equally important for modeling bubble coalescence in salt solutions

- Models based on only the Gibbs-Marangoni stress over-predict film rupture by 156%.
- Analysis of the deficiencies using the new model for film rupture in saline water.
- Important role of soluble gases in nonDLVO attraction in salt solutions.

Some salts can inhibit bubble coalescence above a critical concentration. The initial models for prediciting the critical salt concentrations were developed based on the effects of the Gibbs-Marangoni stress and either the London-van der Waals or Casimir-van der Waals attraction on the drainage and rupture of thin liquid films between bubbles. The later models considered only the Gibbs-Marangoni stress and discarded the van der Waals forces. Here, we present new experimental results of film drainage experiments using halide salts to show that both the Gibbs-Marangoni stress and colloidal forces are equally important. Specifically, our experimental results show that if only the Gibbs-Marangoni stress is considered, the predicted film rupture thickness can differ from the experimental results by as much as 92-156%. The initial models also significantly under-predict the experimental results for the critical salt concentration and film rupture thickness. To examine these deficiencies we use a new mathematical model recently developed to obtain both the salt concentration and rupture thickness of film concurrently, thus removing the inherent ambiguity of the previous models that need the rupture thickness to calculate the salt concentration or vice versa. Indeed, the new modelling approach requires a nonDLVO attractive force together with the Casimir-van der Waals attraction and the Gibbs-Marangoni stress to predict the experimental results for both the salt concentration and rupture thickness of film, which are critical to understanding bubble coalescence in salt solutions.

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