Surfactant-surfactant interactions govern unusual Marangoni spreading on a soap film

When a liquid spreads over another liquid, the presence of surfactants in one or both liquids can add to the complexity of the spreading dynamics. In this work, we investigate the influence of the surfactant-surfactant interaction on the Marangoni spreading. We systematically study the spreading behavior of various surfactant solutions with different molecular structures while keeping the macroscopic parameters of the solutions, such as surface tension and viscosity, equal. To simplify the system and to remove the influence of adsorption kinetics of the surfactants in the liquid phases and remove the effect of diffusion, we employ a unique configuration of “droplet on a soap film”. Our results show that when the surface tension gradient between the droplet and the soap film is not significantly large, the molecular structures and interactions between the surfactants in the droplet and the soap film dramatically influence the spreading scenario. Although, in general, the surface tension difference between the two solutions governs the spreading dynamics, for specific surfactants solutions, unusual spreading occurs that cannot be explained only by the surface tension differences.