Thermodynamics of Stratification and Supramolecular Oscillatory Structural Forces in Micellar Foam Films
2019-02-01T00:00:00Z (GMT) by
Foams consist of a three-dimensional network of foam films that separate gas pockets, and the foam films intersect to form thicker channels called Plateau borders. Rupture of the thin foam films, after fluid drainage into Plateau borders, can result in coalescence of bubbles, eventually leading to foam breakdown. Foam film drainage is driven by suction pressure contributed by the capillary pressure difference between plane parallel film and the Plateau border. Foam films are often stabilized by surfactants that adsorb on the liquid-air interface, and above the critical micelle concentration, form self-assembled structures called micelles. The foam films show monotonic decrease in thickness for low surfactant concentrations, and for ionic surfactants, the Laplace pressure can be balanced by disjoining pressure contributed by combination of van der Waals and electrostatic double layer forces (called DLVO forces). However, micellar foam films undergo drainage via stratification, characterized by a step-wise fashion decrease in thickness and the coexistence of thick-thin flat regions, where step-size is correlated with intermicellar distance. The confinement-induced layering of supramolecular structures like micelles results in additional non-DLVO, supramolecular oscillatory structural forces that can counterbalance the Laplace pressure at multiple flat thicknesses. In this study, we visualize, analyze and characterize the drainage via stratification in micellar foam films, with the aim of elucidating the critical role played by thickness-dependent contribution of supramolecular oscillatory structural forces to disjoining pressure. The nanoscopic thickness transitions and variations are analyzed using interferometry digital imaging optical microscopy (IDIOM) protocols that allow pixel-wise thickness mapping with exquisite spatiotemporal resolution (thickness < 1nm, time < 1 ms, and in-plane < 1000nm). We investigate the influence of surfactant and added electrolyte concentration on stratification in micellar foam films formed with sodium dodecyl sulfate (SDS), an ionic head-tail surfactant, used on a daily basis, and focus of related studied on salt-free solutions before. We then elucidate the effect of surfactant type and chemistry on stratification by picking four bile salts, that have a steroid, planar structure, with varied number of hydroxyl groups and size of hydrophilic unit. We characterize the variation in step size, number of steps, as well as the shape and size of nanoscopic, nonflat structures such as mesas, and determine how each variation can be described in terms of the influence of supramolecular oscillatory structural forces. Finally, we show that the thickness-dependent oscillatory free energy potential drives an additional, hitherto, unreported mechanism for film drainage and rupture, that we christened as spinodal stratification, for it results in the formation of thick hills and thin gullies that are arranged in spinodal-like patterm.