Foam Film Drainage and Nanoscopic Topography of Micellar Sodium Caseinate Solutions

Proteins in food, cosmetic and pharmaceutical foams influence the interfacial and bulk properties of formulations. In this study, we characterize the drainage of protein-based foams as well as single foam films, and contrast their behavior with micellar foams formed with small molecular surfactants above the critical micelle concentration. Micellar foam films undergo drainage via stratification manifested as step-wise thinning in the plots of average film thickness over time. Stratification in micellar foam films is accompanied by formation of coexisting thick-thin regions visualized in reflected light microscopy as exhibiting distinct grey regions as intensity is correlated with thin film interference. We critically examine the drainage of protein foam films to determine how and when stratification can be observed, and evaluate the connection between drainage of single foam films and bulk foams.We find that unlike flat domains in micellar SDS solutions with a sharp and circular moving front, and non-flat features we identify as nanoridges and mesas, the micellar NaCas solutions display non-flat domains, that grow by creating non-circular domains.