Freestanding films of soft matter exhibit stratification due to confinement-induced structuring and layering of supramolecular structures like micelles. In many cosmetics, foods, pharmaceuticals, and petrochemical applications, often polymers are added to surfactant solutions as rheology modifiers. Interaction between neutral polymers like PEO and monomers and micelles of anionic surfactants 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-existence of thick thin regions, and formation of nanoscopic topological features like nanoridges and mesas. The nanoscopic thickness variations and transitions in foam films are characterized using interferometry, digital imaging, and optical microscopy (IDIOM) protocols, with unprecedented high spatial (thickness &lt; 100 nm, lateral ~500 nm) and temporal resolution (&lt; 1 ms). We characterize the variation in surface tension, shear viscosity, and extensional relaxation time as a function of surfactant concentration at fixed polymer concentration. By complementing with tensiometry and rheometry measurements, we seek an understanding of the influence of added polymer on forces, flows, and fluxes that drive drainage via stratification, and model drainage via thin film equation amended with thickness-dependent disjoining pressure.

Drainage via stratification in foam films made with polymer-surfactant complexes

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