Rupture of Thin Liquid Trilayer Films with Soluble Surfactants: Fundamentals and Applications to Droplet Coalescence

Understanding the stability of thin liquid trilayer films is of direct relevance to applications such as multilayer coatings and polymer processing. The stability of trilayer films may also provide insights into multiphase processes, such as the rupture of the thin film formed between two droplets during coalescence. Often, these applications involve additives and surfactants, which can play a key role in the stability of the film. In this work, we study surfactant-laden thin film rupture by considering a model problem involving a thin liquid trilayer film where rupture of the inner layer is driven by van der Waals forces. Surfactant is soluble in either the outer layers or the inner layer, corresponding to surfactant soluble in the droplets or the continuous phase. Lubrication theory is applied to derive coupled evolution equations describing the perturbations to the interface positions and the surfactant concentrations. Our findings reveal that surfactant better stabilizes the film when soluble in the inner layer, which is consistent with experimental observations, where emulsions tend to be more stable when surfactant is in the continuous phase rather than in the droplets.