Interfacial mechanisms for stability of surfactant-laden films.

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

Surface flow visualization.

<p>Photograph (A) and schematic (B) of the surface visualization setup. Instead of a glass substrate, an air bubble is elevated through air-water interface. Thin film color interference patterns are clearly visible under diffused white-light illumination, due to enhanced refractive index mis-match.</p

DPPC drainage experiments.

<p>(A, B) Dimensionless variable (1/<i>H</i>² = (<i>h</i>₀/<i>h</i>)²) as a function of rescaled time (<i>τ</i>) for DPPC at 5 mN m⁻¹ and 25 mN m⁻¹ at various elevated velocity (<i>V</i>_<i>e</i>) ranging from 1–10 mm s⁻¹. (C) Summary of the value for the fitting parameter <i>α</i> of DPPC at various surface pressures. (D) Summary of the initial height capture of the aqueous film laden with DPPC at different surface pressures. The standard deviation is calculated from three independent trials.</p

SDS surface visualization.

<p>Snapshots of interference patterns observed for SDS at 0.13 cmc and 5 cmc. The images are attained using the surface flow visualization setup. The colormap is a guide to relate individual vibrant color to its corresponding thickness. The dark black spot in the center of each frame is the reflection of the camera and the white bright ring at the periphery is the edge of the glass capillary. The scale bar shown is 0.25 mm.</p

Characteristic experimental data-set for DPPC film draining at 7 mN m⁻¹ in the LE-LC plateau.

<p>(A) The film thickness (h) as a function of rescaled time (<i>τ</i>) at elevated <i>V</i>_<i>e</i> = 10 mm s⁻¹. The parameter <i>b</i> corresponds to fitting parameter <i>α</i>. (B) Summary of the <i>α</i> values as a function of elevation velocity <i>V</i>_<i>e</i> = [1–10] mm s⁻¹.</p

Surfactant stability mechanisms.

<p>Schematic summarizing the two different stabilizing interfacial mechanisms for surfactant films: Viscoelastic interfaces create <i>immobile</i> films that reduce drainage through surface stress dissipation, while surface inviscid surfaces create <i>mobile</i> interfaces and create surface-tension induced Marangoni flows that counter the bulk-flow direction.</p

SDS drainage experiments.

<p>[A, B] Dimensionless variable (1/<i>H</i>² = (<i>h</i>₀/<i>h</i>)²) as a function of rescaled time (<i>τ</i>) for SDS at 0.13 cmc and 5 cmc at various elevated velocity (<i>V</i>_<i>e</i>) ranging from 1–10 mm s⁻¹. [C] Summary of the value for the fitting parameter <i>α</i> of SDS at 0.13 cmc and 5 cmc. [D] Summary of the initial height capture of SDS film at 0.13 cmc and 5 cmc. The standard deviation is calculated from two independent trials.</p