(381j) Dynamics of Surfactant Spreading on Deep Viscous Films

The spreading of surface active species (surfactants) on the surface of a liquid film is key to natural and biomedical systems such as cell propulsion , drug delivery in pulmonary airways, and surfactant replacement therapy for respiratory distress syndrome. The process is also central to important technological systems including biofilm inhibition and interfacial transport in microfluidic devices.

This work discusses in detail results from a rigorous numerical model capable of accurately simulating Marangoni flows generated by gradients in surfactant at an interface. By simultaneously solving the full system of governing equations, the multi-scale model enables both a detailed examination of the microscopic physical mechanisms of surfactant transport, and a comprehensive picture of the macroscopic free-surface flow. By varying the thickness of the film over about three decades, we characterize the power law describing the influence of film thickness in deep purely-viscous films. In addition, the crossover time and drop radius at which the early spreading dynamics in a deep viscous film transition to the late thin-film dynamics is also identified. In the process, we also establish the limits of applicability of the theoretical scaling solutions for thin films , and calculate a necessary correction to accurately describe the late thin-film dynamics on deep viscous films.

New findings from this research not only enhance the fundamental understanding of the physics of Marangoni flows, but also the ability to accurately predict the behaviour of Marangoni flows and the associated transport of surface-active species, which is critical to the understanding of important natural and biomedical processes.