(65f) Surface Forces Sculpt Nanoscopic Mesas in Micellar Foam Films

A complex interplay of curvature-dependent capillary pressure and thickness-dependent disjoining pressure influences topography of ultrathin supported and freestanding films of soft matter. The confinement-induced structuring and layering of supramolecular structures like micelles provide non-DLVO oscillatory structural contributions to disjoining pressure. In micellar foam films, the oscillatory structural disjoining pressure drives drainage via stratification, associated with coexisting thick-thin flat regions and stepwise thickness evolution. Stratification in micellar foam film proceeds by nucleation and growth of thinner domains at the expense of surrounding thicker film. Often brighter halos arise around growing domains, before creating a necklace with one, two or more bright spots. Even though such brighter regions were attributed in publications to a possible formation of thicker non-flat regions including nanoridges, the experimental and theoretical characterization of nanoscopic topography have remained longstanding challenges. Here, we show the use of IDIOM (interferometry digital imaging optical microscopy) protocols for visualizing and analyzing the nanoscopic thickness transitions in stratifying micellar foam films, with exquisite spatial (thickness < 10 nm, lateral < 1 μm) and temporal resolution (< 1 ms). We discern the nanoridge at the moving front and analyze the topographical instability leading to formation of nanoscopic mesas that can grow and often coalesce with other mesas along the moving front. Finally, we provide a theoretical model based on thin film equation amended with supramolecular oscillatory structural disjoining pressure and show that the shape and size evolution of nanoridges and mesas can be captured quantitatively.