Soap film stability: a nanoscale description – SOLSTICE

Soap films are of great interest, whether as nanometer-thick membranes or as foam building blocks. It is therefore essential to understand their stability. The formulation of soapy solutions is essential to adjust this stability by modifying the thermodynamic and rheologic properties of the liquid/air interfaces. Furthermore, numerous studies prove that the structure and dynamics of adsorbed surfactants are keystones to understand these macroscopic changes. If the link between micro and macro scale is commonly admitted, it is still not fully revealed. This link is usually done through a state equation, relation between the surface and bulk concentrations in surfactants and the surface tension. However, this equation is established at equilibrium and takes little account of electrostatic effects and spatial organization of the surfactants. Direct experimental investigations of the adsorbed layer structure on the macroscopic properties of the interface, and particularly soap film stability, is still lacking. The main cause is the lack of surface-specific tools (to neglect bulk contributions in the measured signals) which are also sensitive to the spatial organization of the surfactants.

SOLSTICE thus aims to develop a molecular description of soap film stability, with Second Harmonic Generation (SHG) experiments. It consists basically on the annihilation of two photons at the fundamental frequency ? to create a harmonic photon at 2?. This elementary process is forbidden in centrosymmetric media: it is thus a tool of choice to tackle questions addressed in SOLSTICE project. Several configurations of SHG technique will be implemented to probe the spatial organization of the surfactants and their dynamic properties on planar interface and soap films. A peculiar attention will be devoted to the evidence and study of spatial heterogeneities, as molecular aggregates or interfacial concentration gradient at the origin of Marangoni stresses. In this project, we will study model films made of standard surfactants used to form foams.

Studies will first be done on soapy interfaces, i.e. interface at rest between air and aqueous surfactant solution that contains or not some additive salts and air. Changing the solution formulation, we will tune the interaction between surfactant and their affinity for the surface. SHG measurements will probe the consequences on surface concentration, effective charge, organization and dynamics of the adsorbed surfactants. These microscopic observables will be correlated to the surface tension and the rheological properties of the interface. Link between surface structure and macroscopic properties of the interface will thus be discussed at rest. Then, we will question the dynamic properties of the surfactants (adsorption kinetics, diffusion on the interface) in regards with the response of soapy interface to external solicitations. We will notably probe, with a direct and non-intrusive technique, the surfactant concentration gradient at the interface. The question of soap film stability will eventually be addressed with challenging SHG experiments on a thin liquid film.

Tools and methods developed in SOLSTICE project will have a broader impact than for soap film stability. They can be useful when adsorption and organization of molecules at interfaces govern macroscopic phenomena, as among others the displacement of active camphria object at air/water interface or electrokinetic phenomena close to (electronically or chemically) reactive interface studied in the laboratory.