Quasi-longitudinal wave mode waveguide sensing concentration of liquid solution

This paper describes a novel quasi-longitudinal acoustic wave mode waveguide (QLWMW) for sensing the concentration of liquid solution with different dielectric constant. The waveguide, operating at a frequency of 18.6 MHz, consists of a 128° Y cut lithium niobate of 450 mm thick substrate along the direction perpendicular to the X axis of crystal. The variation of the acoustic wave velocity in the waveguide is demonstrated, to be very sensitive to the dielectric constant of a liquid in contact with the back surface of waveguide, by the experimental measurement of the dielectric constant of pure liquids, such as toluene, 2-propanol, acetone, ethanol, acetonitrile, methanol, glycerol and distilled water. The experimental frequency shift as a function of the percentage (in weight) of ethanol and methanol solution are also shown in this paper