Skin formation on liquid surfaces under non-equilibrium conditions.

Skin formation on liquid surfaces is a well known phenomenon, especially when liquid surfaces of foodstuffs are subjected to mechanical disturbances. From the rheological point of view, skin formation indicates that a rigid surface layer, having a kind of network structure, is formed. This structure gives rise to pronounced dilational as well as pronounced shear properties. Proteins are found to promote such layers on aqueous solutions. In spite of the fact that skins have been observed frequently, the physical properties of these layers are not well described in literature. In our investigation we have been able to make these skins under well defined physical conditions. Skins of proteins such as BSA, β casein and egg white have been made under well defined physical conditions on the surface of aqueous solutions. Their surface dilational properties have been measured under different experimental conditions. The results demonstrate that depending on the method used pure elastic or viscoelastic behavior can be found. This can be explained by assuming that the proteinaceous surface layer is an elastic gel filled with water. In a dynamic measurement viscous behavior is found due to the flow of water through the protein network. In a one-stroke compression experiment, when the flow of the water is finished, only the elastic properties of the protein network are measured. [ABSTRACT FROM AUTHOR]