Propagation of acoustic waves in plates bordered with viscous liquid

This paper is concerned with the influence of a viscous liquid on the propagation of acoustic waves in thin plates of lithium niobate. The characteristics of the three lowest order wave modes that such a plate can support (zeroth order antisymmetric Lamb wave A(o) symmetric Lamb wave S-o, and quasi-shear-horizontal wave QSH(o)) are investigated. It is assumed that the liquid is isotropic such that its viscous properties are described by two independent components of viscosity tensor eta(11) and eta(44) It is found that the attenuation of the waves depends primarily on the shear component of viscosity eta(44). The influence of expansion component eta(11) is negligible. The attenuation per unit length is proportional to (eta(44))(1/2) and f(1.5), where f is the frequency of the wave. Under identical conditions, the lowest value of attenuation is for the S, mode, while the highest is for the A(o) mode. For example, for h/lambda = 0.025 (h = plate thickness, lambda = acoustic wavelength) and eta(44) = 0.003 Ns/m(2), the attenuation in dB/cm at a frequency of 1 MHz for A(o), S-o, and QSH(o) modes is 13.5, 0.024, and 0.073, respectively. The above results indicate that the S-o wave is most suitable for use in devices operating in contact with a viscous liquid. The results also show that by using the three different wave modes, one can develop a viscosity meter with a very wide measurable range of viscosity.