| A new type of interfacial shear rheology apparatus with high sensitivity was build. It is based on a floating magnetic rod moved by an external electromagnetic field. The displacement of the rod is detected with a linear CCD camera and the rheological properties (interfacial shear dynamic storage modulus and interfacial shear dynamic loss modulus (G′ and G ″)) of the system can be determined. The rheometer was used to study the behavior of β-casein layers at air-solution interface. The layers were formed from solutions with different protein concentration and had different aging times.; At early stages (0–14 h) dynamic loss modulus of the layers was bigger than the dynamic storage modulus, which is consistent with the existence of a viscoelastic liquid layer at the surface. Layers, formed from low protein concentration solutions (1 × 10−6 to 1 × 10−3 wt %), kept their liquid characteristics even at long aging times (60 h).{09}Layers, formed from high β-casein concentration solutions (5 × 10−3 to 2 × 10−2 wt %), on the other hand, showed growth of the moduli and at ∼15 h G′ become larger than G″ at a 11 the frequencies studied (0.2–6.3 rad/sec). The observed behavior is consistent with physical gelation of the layer described by the percolation theory for sol-gel transition.; AFM measurements of the layers, transferred to a solid support showed that the layers had large variability in the thickness and structure. Thickness of the layers showed values (0 to 3 nm) and trends (growth with the increase of β-casein concentration and time) similar to the literature data. A trend for increase of the thickness at long times (48 h) was observed. The aging of the interface was accompanied with the formation of some nanoparticles (d = 20–30 nm), that were hard and for high concentrations were with high surface concentration and tended to form extended aggregates. |
