Surface Forces and Stratification in Micellar Foam Films

Ultrathin films of soft matter containing supramolecular structures like micelles, nanoparticles, smectic liquid crystals, lipid bilayers, and polyelectrolytes undergo drainage via stratification, manifested as step-wise thinning in interferometry-based measurements of average thickness. We focus primarily on stratification in micellar foam films formed with an aqueous solution of small molecule surfactants, including sodium dodecyl sulfate (SDS) above the critical micelle concentration (CMC). Foam films typically consist of fluid sandwiched between two surfactant-laden surfaces ~ 5 nm - 10 microns apart. The drainage in films occurs under the influence of viscous, interfacial, and intermolecular forces, including disjoining pressure. In reflected light microscopy, the stratifying films (thickness < 100 nm) display regions with distinct shades of grey, implying that domains and nanostructures with varied thickness coexist in the thinning film. Understanding and analyzing such nanoscopic thickness transitions and variations had been a long-standing experimental challenge due to the lack of technique with the requisite spatio-temporal resolution and theoretical challenge due to the absence of models for describing hydrodynamics and thermodynamics in stratified thin films. We show that the nanoscopic thickness variations in stratifying films can be visualized and analyzed with an unprecedented spatial (thickness ~ 1 nm, lateral ~500 nm) and temporal resolution (< 1 ms) using IDIOM (interferometry digital imaging optical microscopy) protocols that we developed recently. Stratification proceeds by forming thinner domains that grow at the expense of surrounding films. Using the exquisite thickness maps created using IDIOM protocols, we provide the first visualization of nanoridges and mesas that form at the moving front around expanding domains. We contrast the step size measured in stratification studies with intermicellar distance obtained from scattering measurements. Most significantly, we develop a self-consistent theoretical framework, a nonlinear thin film equation model that explicitly accounts for the influence of non-DLVO supramolecular oscillatory surface forces and the physicochemical properties of surfactants. Finally, we elucidate how surfactant type and concentration can be manipulated and controlled for the molecular engineering of micellar foams. The ongoing efforts are directed at exploring the influence of surfactants and defoamers/ antifoamers in bioprocessing, proteins and lipids in food engineering, and biologically sourced/derived surfactants on drainage via stratification in foam films, and integrated examination of foamability, and foam stability.