Supramolecular Structural Forces in Stratifying Foam Films and Micelle Aggregation Number

Liquid foams are complex fluids, mostly formed by gas bubbles dispersed within a surfactant solution. The lifetime of foams depends critically on stability and drainage of thin liquid films that separate gas bubbles. It is well-established that the monotonic decrease in film thickness observed experimentally can be qualitatively described using lubrication approximation, where pressure is contributed by capillary and DLVO forces (electrostatic plus dispersion). However, foam films containing micelles, colloidal particles or polyelectrolyte-surfactant mixtures exhibit step-wise thinning or stratification. In this study, we use experiments and theory to investigate the influence of non-DLVO forces, including supramolecular oscillatory structural forces, on drainage and stratification of thin foam films (<100 nm). Using a Scheludko-type cell, we experimentally study the stratification kinetics of horizontal foam films formed by aqueous sodium dodecyl sulfate (SDS) solutions, and carefully determine how the concentration of  surfactants influences the stepwise thinning process for micellar solutions. We elucidate how quantitative characterization of stratification provides a method for measuring dynamic disjoining pressure, as well as for estimating micelle size and interactions. The concentration-dependent aggregation number extracted from our experiments match-up reasonably well with values obtained by other techniques including scattering and fluorescence.