Compression and shear surface rheology in spread layers of β-casein and β-lactoglobulin

We investigate the surface viscoelasticity of β-lactoglobulin and β-casein spread surface monolayers using a recently discovered method. Step compressions are performed, and the surface pressure is measured as a function of time. This is a common experiment for surface monolayers. However in our experiments the pressure is recorded by two perpendicular sensors, parallel and perpendicular to the compression direction. This enables us to clearly measure the time relaxation of both the compression and shear moduli, at the same time, in a single experiment, and with a standard apparatus. β-Lactoglobulin and β-casein monolayers are interesting because of their importance in food science and because they exhibit universally slow dynamical behavior that is still not fully understood. Our results confirm that the compressional modulus dominates the total viscoelastic response in both proteins. Indeed for β-casein we confirm that the shear modulus is always negligible, i.e., the layer is in a fluid state. In β-lactoglobulin a finite shear modulus emerges above a critical concentration. We emphasize that in Langmuir trough dynamic experiments the surface pressure should be measured in both the compression and the perpendicular directions. Anisotropy in the surface pressure shows that β-casein monolayers are in a fluid state, whereas in β-lactoglobulin a finite shear modulus emerges above a critical concentration.