Flow and fracture phenomena in adsorbed protein layers at the air/water interface in connectin to spreading oil droplets

Abstract: Oil spreading at the air/water interface was studied for protein-stabilized emulsion droplets added under the surface of a spread protein layer. The initial transition from entered oil lens to spread oil layer can be thought of as a wetting transition, which is a surface-tension-controlled phenomenon. The essentially irreversible nature of protein adsorption allows manipulation of the air/water surface tension by compression and expansion of the air/water surface such that the wetting transition can be induced. The initial wetting initiates a cooperative spreading process; it is this spreading process that is the subject of investigation. From the morphology of the spreading emulsion, clear differences in the flow behavior of different protein films can be observed. The proteins investigated represent a series exhibiting an increased tendency to form a coherent protein film at the air/water interface in the order -casein <-lactoglobulin <soy glycinin. In the case of -casein, the film flows and oil spreads in a radial fashion. The -lactoglobulin and soy glycinin films on the other hand fracture during expansion and oil spreads in the cracks in the protein film, making the broken structure visible. This observation serves as strong visual evidence for the inhomogeneity of protein films during large-scale deformation.