Rheological properties of human saliva

From measurements with a Couette-type viscometer provided with a guard ring it was shown that at the saliva-air interface a protein layer is adsorbed. Measurements of the surface shear modulus of this layer on saliva of 7 healthy subjects were performed at a frequency of about 70 Hz and a temperature of 25 degrees C. For a surface age of about 1.5 h the surface shear modulus and the surface viscosity were in the order of 1 Nm-1 and 10(-3) Nm-1 s, respectively. From ellipsometric measurements it was found that the thickness of the protein layer was approx. 100 nm and, using this value, it could be concluded that the shear modulus and the dynamic viscosity were in the order of 10(7) Pa and 10(4) Pa s, respectively. The layer appeared to be fragile. Even shear deformation amplitudes of 4 x 10(-5) are too high to assure linearity. The complex viscosity (eta = eta' - i eta") of the bulk liquid of human submandibular saliva below the absorbed layer was measured in the frequency range 70 Hz-200 kHz with 3 torsional resonators, each for a different frequency, and a Ni-tube resonator. It was concluded, that the real part of the complex viscosity (eta') decreases from 1.1 mPa s at 70 Hz to a value of 0.95 mPa s at high frequencies. Except at the lowest frequency (70 Hz), the value of eta" was too small to be detected.