Nanoscopic Thickness Transitions and Domain Growth Dynamics in Stratifying, Micellar Foam Films

The stability, rheology and applications of foams, emulsions and colloidal sols depend on the hydrodynamics and thermodynamics of thin liquid films that separate bubbles, drops and particles respectively. Thin liquid films containing micelles, colloidal particles, liquid crystals or polyelectrolyte–surfactant mixtures exhibit step-wise thinning or stratification, often attributed to the layer-by-layer removal of the aforementioned supramolecular structures. Stratification proceeds through emergence and growth of thinner circular domains within a thicker film, and the domain expansion dynamics are the primary focus of this study. Domain and associated thickness variation in thin foam films (h < 100 nm) made from sodium dodecyl sulfate (SDS) micellar solutions are examined using a porous plate cell with a novel technique we call Interferometry Digital Imaging Optical Microscopy (IDIOM). We show that the domain expansion dynamics exhibit two distinct growth regimes: a transition from a constant diffusivity to a constant velocity regime is realized when a topological instability occurs at the contact line between the growing thinner isolated domain and the surrounding thicker film. We show a similar transition is also realized after a section of the expanding domain coalesces with the Plateau border. Though several studies have focused on the expansion dynamics of isolated domains that exhibit a diffusion-like scaling, the change in expansion kinetics observed after domains contact with the Plateau border has not been reported and analyzed before. We have developed a theoretical model that incorporates both hydrodynamic and thermodynamic effects for capturing domain expansion kinetics in stratifying foam films. Ultimately, a better understanding of stratification and drainage kinetics, and the role played by supramolecular structural forces, will enable design and creation of foams that drain more rapidly (are fragile and unstable) or more slowly (are stable and long lasting) as desired for applications like food and beverages, oil recovery, pharmaceuticals and cosmetics.