Interfacial mechanisms for stability of surfactant-laden films

RESEARCH ARTICLE Interfacial mechanisms for stability of surfactant-laden films M. Saad Bhamla1*, Chew Chai2, Marco A. Àlvarez-Valenzuela3, Javier Tajuelo4, Gerald G. Fuller2 1 Stanford University, Department of Bioengineering, Stanford, 94305, United States of America, 2 Stanford University, Department of Chemical Engineering, Stanford, 94305, United States of America, 3 Universidad Carlos III de Madrid, Department of Mechanical Engineering, Leganes, 28911, Spain, 4 Universidad Nacional de Educación a Distancia (UNED), Departamento de Fı́sica Fundamental, Madrid, 28040, Spain * a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 OPEN ACCESS Citation: Bhamla MS, Chai C, Àlvarez-Valenzuela MA, Tajuelo J, Fuller GG (2017) Interfacial mechanisms for stability of surfactant-laden films. PLoS ONE 12(5): e0175753. https://doi.org/ 10.1371/journal.pone.0175753 Editor: Jay D. Schieber, Illinois Institute of Technology, UNITED STATES Received: November 8, 2016 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. Accepted: March 30, 2017 Published: May 17, 2017 Copyright: © 2017 Bhamla et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: Individual data-sets for all experiments are available through FigShare: https://doi.org/10.6084/m9.figshare.c.3736687.v1. Funding: The authors received no specific funding for this work. However, J. T. acknowledges a personal grant from UNED’s Researchers Formation Program and partial support from MINECO (Grant FIS2013-47350-C5-5-R). Competing interests: The authors have declared that no competing interests exist. Introduction Stability and drainage of thin surfactant films is relevant across various disciplines: industrial applications including engineered foams and emulsions [1], fundamental physics of bubbles [2–4], bio-foams in aquatic animal nests [5], and physiological systems including the human tear film [6] and pulmonary surfactants [7]. However, the drainage rate of these thin films depends critically on the mechanism through which these films are stabilized, which in turn is strongly coupled to the chemical composition of the surfactants. 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]. In comparison, 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 PLOS ONE | https://doi.org/10.1371/journal.pone.0175753 May 17, 2017 1 / 14 Interfacial mechanisms for stability of surfactant-laden films Fig 1. Experimental platform. Schematic (A) and photograph (B) of the drainage platform. For the insoluble surfactant experiments, the glass dome is initially submerged in the PBS-filled Langmuir trough (white, teflon container) and DPPC is spread at the air-liquid interface. DPPC is then compressed to the desired surface pressure using a single Delrin barrier and the surface pressure is monitored using a paper Wilhelmy balance (1). For the soluble surfactant experiments, the Langmuir trough is filled with SDS solution of desired concentration. In both cases, the measurement commences once the glass dome is elevated through airliquid interface with a computer controlled motorized stage (2). A high speed interferometer (black tube) captures the thickness of the draining films as a function of time at the apex of glass dome. https://doi.org/10.1371/journal.pone.0175753.g001 surfactants [13–15]. Past work by Joye et al. also presents numerical simulations and linear stability analysis to explore the role of surface rheological parameters [16, 17]. However, 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. Here we utilize a simple setup (Fig 1) to measure the drainage dynamics of surfactantladen aqueous films. Thin films of liquid are created by elevating an initially submerged curved glass substrate through the air-liquid interface at controlled velocities (Ve). A highspeed interferometer enables measurement of the varying film thickness at the apex of the film. We employ two heavily-studied commercial surfactants: 1, 2-dipalmitoyl phosphatidylcholine (DPPC), an insoluble surfactant that forms viscoelastic interfaces [18, 19] and sodium dodecyl sulfate (SDS), a soluble surfactant that forms inviscid interfaces [20]. We also show by surface flow visualization that the viscoelasticity of DPPC films resists surface deformation and creates an immobile interface at high surface pressures, while the SDS films are more fluid-like and yield extremely mobile interfaces. The remarkably different surface properties between DPPC and SDS allow us to systematically explore the role of surface mobility on drainage dynamics. PLOS ONE | https://doi.org/10.1371/journal.pone.0175753 May 17, 2017 2 / 14 Interfacial mechanisms for stability of surfactant-laden films Results and discussion Theoretical hydrodynamic model for draining films with complex interfaces Consider a hemispherical glass dome that is raised through a bulk of liquid that results in the capture of a thinning liquid film with a complex surfactant-laden interface. Deformation of the interface leads to interfacial stresses that need to be accounted for in the hydrodyna (...truncated)