(221o) Molecular & Macromolecular Engineering of Foams

We pursue an understanding of molecular and macromolecular principles that determine the three desirable attributes – stability, lifetime and rheology— of foams. Drainage in foam film formed by surfactant concentrations above the critical micelle concentration proceeds in a nonmonotonic, step-wise fashion called stratification in contrast to the monotonic thinning exhibited by films containing no micelles. In reflected light microscopy, stratifying films 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 have been 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 in stratified thin films. Stratification proceeds by formation of thinner domains that grow at the expense of surrounding films. Using interferometry, digital imaging and optical microscopy (IDIOM) protocols that we developed recently, we show that nanoridges mesas that form and grow at the moving front around expanding domains can be visualized and analyzed with an unprecedented spatial (thickness ~ 1 nm, lateral ~500 nm) and temporal resolution (< 1 ms). We show the complex spatio-temporal evolution of nanoridges, mesas and domains can be modeled quantitatively by amending the nonlinear thin film equation with supramolecular oscillatory surface forces. Finally, we investigate the influence of adding polymers and proteins to micellar foam films, paying close attention to the additional influence of enhanced extensional viscosity and viscoelasticity. We elucidate the influence of choice of polymers (flexibility, extensibility, concentration and charge), surfactants (CMC, charge, structure) as well as the effects of nonlinear viscoelasticity and disjoining pressure on the stability and drainage of stratifying foam films, and highlight the challenges and opportunities for molecular and macromolecular engineering of foams with desirable and controllable rheological properties, processability and stability.