Application of surface and normal ultrasonic waves for measuring the parameters of technical fluids: I. Shear viscosity measurements

The effect of the viscosity of a fluid on the propagation of the zero mode of a horizontally polarized normal wave in a thin (compared to the wavelength) waveguide immersed in the fluid is studied. It is shown that, to a first approximation in the ratio of the shear impedance of the fluid to the shear impedance of the waveguide, the wave amplitude decays exponentially as a function of the distance at a damping coefficient proportional to the square root of the shear viscosity of the fluid. The decrease in the wave amplitude induced by the shear viscosity of the fluid is numerically estimated, and the results obtained point to the possibility of developing a method for its measurement at a high accuracy. This method was developed and tested on a waveguide in the form of an aluminum ribbon 0.3 mm thick and 14 cm long at a frequency of 2 MHz. The decrease in the signal amplitude when the waveguide is immersed in distilled water (tabulated shear viscosity is 1.05 $\times$ 10$^{-3}$ Pa s) is found to be 0.42 dB as compared to the amplitude in the unloaded waveguide, which can be considered as the sensitivity of the experimental setup. A method for calibrating the sensor is described. The shear viscosities of solutions of saccharose in distilled water are measured, and the experimental results agree well with the theoretical estimates.