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 can also be used to provide insights into emulsion dynamics, such as the rupture of the thin film formed between two droplets during coalescence. Often, emulsions are laden with surfactants and other additives, which can be present in one or both phases as well as at the interfaces between the liquids. In experimental studies, complicating factors such as variations in droplet sizes, curvatures, and collision processes make it difficult to specifically isolate the influence of surfactant transport on droplet coalescence and film rupture. The present work addresses this issue by systematic consideration of a model problem involving a thin liquid trilayer film. Surfactant is soluble in either the outer layers or the inner layer, corresponding to surfactant soluble in the droplets or the continuous phase. Rupture of the inner layer is driven by van der Waals forces. Lubrication theory is applied to derive coupled nonlinear 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, and the stabilizing effect is more pronounced when the outer layers are thicker. These findings are consistent with experimental observations involving emulsions, where emulsions tend to be more stable when surfactant is in the continuous phase rather than in the droplets, with the distinction being more pronounced when droplets are larger.