Contrasting drainage via stratification in horizontal vs. vertical micellar foam films

Understanding and controlling the drainage kinetics of freestanding thin films is an important problem that underlies the stability, lifetime and rheology of foams. Ultrathin micellar foam films exhibit stratification due to confinement-induced structuring and layering of micelles that contributes non-DLVO supramolecular oscillatory structural force contribution to disjoining pressure. Stratification in micellar foam films is manifested as stepwise thinning over time, and it leads to the coexistence of flat domains with discretely different thicknesses. In this contribution, we use Interferometry Digital Imaging Optical Microscopy (IDIOM) protocols to visualize and analyze thickness transitions and variations associated with stratification in micellar foam films made with sodium dodecyl sulfate (SDS). We contrast the drainage and stratification dynamics in horizontal and vertical foam films. We experimentally study the emergence of thickness fluctuations near the borders and within thinning films, and study how buoyancy, capillarity and gravity driven instabilities and flows, are affected by the variation in bulk and interfacial physicochemical properties, that are effectively dependent on the choice of constituents, including added electrolytes.