Interfacial mechanisms for stability of surfactant-laden films

TL;DR

This paper investigates how surfactants stabilize thin liquid films by examining interfacial mechanisms, revealing that surface rheology and Marangoni flows play key roles in preventing film drainage.

Contribution

It introduces a simple model applicable to both soluble and insoluble surfactants, elucidating their distinct stabilization mechanisms.

Findings

Surfactants with finite surface rheology induce viscoelastic interfacial stresses.

Inviscid surfactants stabilize films via Marangoni flows.

The model explains stability in various surfactant systems.

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 molecularly-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. 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.