Drainage Via Stratification in Foam Films Made with Polymer-Surfactant Complexes

Freestanding films of soft matter exhibit stratification due to confinement-induced structuring and layering of supramolecular structures like micelles. In many cosmetics, foods, pharmaceutical and petrochemical applications, often polymers are added to surfactant solutions as rheology modifiers. Interaction between neutral polymer like PEO and monomers and micelles of an ionic surfactant like SDS results in the formation of polymer-surfactant complexes, and changes both interfacial properties and bulk shear rheology response. The influence of such polymer-surfactant complexes on foam formation, stability, drainage and lifetime are not well-understood and motivate this study. In this contribution, we show foams formed with PEO-SDS mixtures exhibit three features of stratification: step-wise thinning, co-existance of thick thin regions and formation of nanoscopic topological features like nanoridges and mesas. The nanoscopic thickness variations and transitions are characterized using interferometry, digital imaging and optical microscopy (IDIOM) protocols, with unprecedented high spatial (thickness < 100 nm, lateral ~500 nm) and temporal resolution (< 1 ms). We supplement foam film characterization studies with characterization of extensional rheology response using Dripping-onto-Substrate (DoS) rheometry protocols we developed, and with the characterization of structure and interactions of polymer-surfactant complexes using small angle x-ray scattering.