Abstract
Thin liquid films are central to everyday life. They are ubiquitous in modern technology (pharmaceuticals, coatings), consumer products (foams, emulsions) and also serve vital biological functions (tear film of the eye, pulmonary surfactants in the lung). A common feature in all these examples is the presence of surface-active molecules at the air-liquid interface. Though they form only molecular-thin layers, these surfactants produce complex surface stresses on the free surface, which have important consequences for the dynamics and stability of the underlying thin liquid film. Here we conduct simple thinning experiments to explore the fundamental mechanisms that allow the surfactant molecules to slow the gravity-driven drainage of the underlying film. We present a simple model that works for both soluble and insoluble surfactant systems in the limit of negligible adsorption-desorption dynamics. We show that surfactants with finite surface rheology influence bulk flow through viscoelastic interfacial stresses, while surfactants with inviscid surfaces achieve stability through opposing surface-tension induced Marangoni flows.
Highlights
Stability and drainage of thin surfactant films is relevant across various disciplines: industrial applications including engineered foams and emulsions [1], fundamental physics of bubbles [2,3,4], bio-foams in aquatic animal nests [5], and physiological systems including the human tear film [6] and pulmonary surfactants [7]
Deformation of the interface leads to interfacial stresses that need to be accounted for in the hydrodynamic model capturing the evolution of this thin film
To account for the surfactants at the air-liquid interface, we use the Boussinesq-Scriven model for the interface, which characterizes the surfactant layer with a surface tension (σ), surface shear viscosity, and surface dilatational viscosity
Summary
Stability and drainage of thin surfactant films is relevant across various disciplines: industrial applications including engineered foams and emulsions [1], fundamental physics of bubbles [2,3,4], bio-foams in aquatic animal nests [5], and physiological systems including the human tear film [6] and pulmonary surfactants [7]. The majority of past literature has looked at the stability of thin films in presence of soluble amphiphiles, including drainage from horizontal films [8, 9], drainage of vertical films based on Frankel’s law [10, 11] and film stability in fiber coating experiments [12]. The problem of drainage in presence of insoluble surfactants has been studied relatively less due to experimental challenges; the majority of investigations by Naire and coworkers focused on mathematical models to study the drainage of vertical thin films in the presence of insoluble. There is a need for a simple experimental platform that can systematically compare both soluble and insoluble surfactants, with varying surface rheologies and quantify the drainage dynamics using a simple theoretical model
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