(343b) Supramolecular Oscillatory Surface Forces Drive Instabilities in Stratifying Foam Films
AIChE Annual Meeting
2019 AIChE Annual Meeting
Engineering Sciences and Fundamentals
Interfacial and Nonlinear Flows
Tuesday, November 12, 2019 - 12:45pm to 1:00pm
Ultrathin films exhibit stratification due to confinement-induced structuring and layering of small molecules in simple fluids, and of supramolecular structures like micelles, lipid layers and nanoparticles in complex fluids. Stratification proceeds by the formation and growth of thinner domains at the expense of surrounding thicker film, and we characterize flows and instabilities that drive the formation of nanoscopic terraces, ridges, hills, valleys and mesas within the micellar freestanding films. The detailed mechanisms underlying stratification and various flow instabilities are still under debate, and are resolved in this contribution by addressing long-standing experimental and theoretical challenges. Thickness variations in stratifying films are visualized and analyzed using interferometry, digital imaging and optical microscopy (IDIOM) protocols, with unprecedented high spatial (thickness < 100 nm, lateral ~500 nm) and temporal resolution (< 1 ms). Using IDIOM protocols we developed recently, we characterize the shape and the growth dynamics of nanoridges and mesas that flank the expanding domains in micellar thin films. We show that topographical changes including nanoridge & mesa growth, and the overall stratification dynamics, can be described quantitatively by nonlinear thin film equation, amended with supramolecular oscillatory surface forces. Stratification typically involves the spontaneous formation and growth of thinner, darker, circular domains or thicker, brighter mesas, and mechanistically, domain expansion appears similar to hole growth in polymer films undergoing dewetting by nucleation and growth mechanism. Dewetting polymer films occasionally phase separate into thick and thin regions forming an interconnected, network-like morphology by undergoing spinodal dewetting. However, the formation of thickâthin spinodal patterns has never been reported for freestanding films. In this contribution, we also show that the thickness-dependent oscillatory contribution to free energy that arises due to confinement-induced layering of micelles can drive the formation of such thick-thin regions by undergoing a process we term as spinodal stratification.