Surface Forces Sculpt Nanoscopic Mesas in Micellar Foam Films

A complex interplay of curvature-dependent capillary pressure and thickness-

dependent disjoining pressure influences the 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

the 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.