Pressures and Flows in Foams and Ultrathin Foam Films

Freestanding liquid films or foam films containing dispersed supramolecular structures, including self-assembled structures (like micelles) and macromolecules (like polymers), arise in cleaning, personal care, food, and pharmaceutical foams. The confinement-induced structuring and layering of these supramolecular structures in ultrathin foam films (thickness, h < 100 nm) provide a non-DLVO, oscillatory structural disjoining pressure. The interplay of capillary pressure and oscillatory disjoining pressure drives drainage via stratification, associated with stepwise thinning and coexisting thick-thin regions. Stratification proceeds by nucleation and growth of thinner domains at the expense of surrounding the thicker film. The expansion speeds up after spots thicker than the surrounding appear at the moving front, and the formation, shape and size of spots remain largely unexplored. Here, using IDIOM (interferometry, digital imaging, optical microscopy) protocols developed recently for visualizing and analyzing the nanotopography of stratifying foam films, we show that the spots are like mesas, with thickness in 10s of nm but width in microns. A tutorial introduction to IDIOM protocols is included to facilitate future studies on thickness mapping with exquisite spatiotemporal resolution (thickness < 10 nm, lateral < 1 μm, temporal < 1 ms). The shape and size evolution of nanoscopic mesas growing along the moving front is characterized, and we show that scaled shapes are identical for mesas irrespective of timespan after formation or surfactant concentration. We develop a theoretical model based on the thin film equation amended with disjoining pressure and show the width, height and shape of mesas follow similarity solutions quantitatively. The coalescence of mesas is analyzed for the first time to show a coalescence preference with ratio of azimuthal position shifts in moving frame showing quadratic dependence on inverse of mesa sizes. Lastly, we show that the foam films with a dilute amount of neutral polymers to micellar surfactant solutions exhibit all signatures of stratification. We present the first comprehensive evidence for stratification in neutral polymer-surfactant complexes. We determine that for surfactant concentrations beyond the excess micelle point, polymer addition and complexation impact the nanotopography but not the step-size, implying the amplitude of disjoining pressure changes, but the periodicity remains unchanged.