Dynamics of Stratifying Foam Films
2017-02-17T00:00:00Z (GMT) by
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. Freestanding thin liquid films (foam films) containing micelles, nanoparticles, polyelectrolyte-surfactant complexes or smectic liquid crystals undergo drainage in a discontinuous, step-wise fashion termed stratification. Stratification proceeds by growth of thinner domains at expense of thicker surrounding films, and involves the coexistence and evolution of domains and nanostructures of discretely different thickness. We developed a novel Interferometry Digital Imaging Optical Microscopy (IDIOM) protocol, and characterized the landscape of stratifying free-standing thin films with high spatial (1 nm in thickness and 0.5 μm laterally) and temporal resolution (1 ms) for the first time. We experimentally observed that the expansion dynamics of a single thinner domain exhibit two distinct growth regimes, with characteristic scaling laws. Initially, the radius of the isolated domains grows with a constant diffusivity with R ∝ t1/2. In contrast, after a section of the expanding domain coalesces with the surrounding Plateau border, the contact line between domain and the thicker film moves a constant velocity, i.e. R ∝ t. The change in dynamics brought about after coalescence with the Plateau border is reported and analyzed for the first time. A similar transition from a constant diffusivity to a constant velocity regime is also realized when a topological instability occurs at the contact line between the growing thinner isolated domain and the surrounding thicker film. Utilizing IDIOM protocol, we visualize nanoridges formed around the growing domains for the first time and show that these ridges are susceptible to the topological instability that leads to the formation of white spots and scaling transition observed in domain growth. We characterize the nanoridge shape evolution and instability using the IDIOM protocols and we show the non-DLVO, supramolecular oscillatory surface forces underlie both nanoridge growth and instability. Lastly, we developed a nonlinear thin film equation, amended with supramolecular oscillatory surface forces contributed by micelles confined in thin films, to describe dynamics of stratification. Our scaling analysis and numerical solutions capture the shape and growth dynamics of asymmetric nanoridges that flank the growing domains, and we find that the nanoridge growth and instability modulate the domain expansion dynamics as well as stratification dynamics.