Dynamic evolution of thin liquid films over curved substrates
Keywords: Chemical Engineering.

We encounter thin liquid films on a daily basis. The micron-thick tear film coating the cornea ensures the clarity of our vision. Pulmonary surfactants coating the air-liquid interface in the alveoli enable us to breath. Lubricant foaming in a gear box can lead to machine degradation and unsafe operations. Textile dye solution films drying unevenly can leave undesirable marks on the fabric. A fundamental understanding of thin liquid film stability is key to optimizing the compositions of the base liquid to suit our needs and applications. In this research, we examine the effects of liquid composition and substrate materials on the dynamics of thin liquid films by building heat and mass transfer models and conducting interferometric experiments via the dynamic fluid-film interferometer. The geometry of a thin liquid film dictates that the liquid-air interactions are critical in affecting the dynamics of the films. Many such interactions can lead to an uneven distribution of surface tension, creating Marangoni flow. The interplay between the Marangoni flow and other physical forces such as capillarity and gravity govern the dynamics of most thin liquid films. A key part of the work focuses on one of the simplest systems that captures all of the major physical forces at play in a thin liquid film: evaporating binary silicone oil films over a glass dome or an air bubble. The binary silicone oil composes of two silicone oils with different viscosity, surface tension, and evaporation rate. Evaporation and the curvature of the substrates create a thin film with varying thickness and composition, thereby surface tension. In the experimental system, a thin film is formed by forcing the curved substrate upward until the apex of the substrate penetrates the initially flat air/liquid interface. The forcing is then stopped and the evolution of the film is recorded by a camera positioned directly above the thin film. Combining experimental observations and theoretical modeling, we elucidate the mechanisms behind the resulting dramatic thin film dynamics. For a binary silicone oil film over a glass dome, at low volume fractions of the less evaporative species (< 0.3%), the liquid film remains axisymmetric and is stabilized by van der Waals interactions and Marangoni flows [1]. At higher concentrations (> 0.35%), the increase in Marangoni flow leads to a film that is more susceptible to ambient disturbances, resulting in asymmetry breakage events [2]. Faster dynamics are observed for an oil film over an air bubble, due to the reduction in resistance to flow. At low volume fractions of the more evaporative species (< 0.1%), the liquid film oscillates axisymmetrically as a result of the evaporation-sustained Marangoni flow and changes in the film curvature and capillary pressure. At higher volume fractions, the liquid film forms a distinctly uneven film thickness and oscillates in a non-axisymmetric manner [3]. With physical insights gained from the binary silicone oil systems, we have proceeded to examine aqueous surfactant films. We find that the lifetime of a surfactant thin film draining over an air bubble has four general dynamic stages. (1) Axisymmetric dimple formation is first observed due to the forcing motion of an air bubble approaching an initially flat air/liquid interface. (2) When the forcing stops, the axisymmetric thin film immediately succumbs to ambient disturbances and non-axisymmetric film thickness is observed. During this stage, the dimple undergoes non-axisymmetric discharge and the maximum film thickness is reduced. (3) In the wake of the discharge event, asymmetrical flow dominates and the film further thins to nanoscopic thickness that leads to bubble coalescence. The transition from an axisymmetric film to a non-axisymmetric film is a crucial step that leads to drastic decrease in film thickness that affects the thin film life time. Therefore, our work focuses on examining the factors that contribute to the formation of the asymmetric film during the second stage of film evolution. [4] [1] Rodríguez-Hakim, M., Barakat, J. M., Shi, X., Shaqfeh, E. S., & Fuller, G. G. (2019). Evaporation-driven solutocapillary flow of thin liquid films over curved substrates. Physical Review Fluids, 4(3), 034002. [2] Shi, X., Rodríguez-Hakim, M., Shaqfeh, E. S., & Fuller, G. G. (2021). Instability and symmetry breaking in binary evaporating thin films over a solid spherical dome. Journal of Fluid Mechanics, 915. [3] Shi, X., Fuller, G. G., & Shaqfeh, E. S. (2020). Oscillatory spontaneous dimpling in evaporating curved thin films. Journal of Fluid Mechanics, 889. [4] Shi, X., Fuller, G. G., & Shaqfeh, E. S. (2022). Asymmetric evolution of surfactant films over an air bubble. Submitted