Selective Detection of Liquid Viscosity Using Acoustic Plate Waves with In-Plane Polarization

Using plates of weak piezoeletcric crystal (quartz) loaded with various liquids, it is shown that along with common modes, whose sensitivity towards different liquid parameters comparable with each other, there are some uncommon modes, whose amplitude responses towards viscosity eta are much larger than towards temperature T and electric conductivity sigma. The search of the modes with the selective properties is accomplished by varying plate thickness h, crystal orientation, wave length lambda, and mode order n. It is found that all modes possessing the property are characterized by small surface-normal displacement, avoiding wave radiation into adjacent liquid, large in-plane displacements, enhancing viscous coupling the modes and liquids, and small electro-mechanical constant, reducing electro-acoustic interaction. Basing on the modes, the sensor prototypes with selective operation are developed and tested for eta from 1 to 1500 cP, sigma from 0 to 1.2 S/m, and t from 0 to 55 degrees C. Because of operation at ultrasonic frequency (tens MHz) the prototypes have different sensitivities in various eta-ranges: 0.3 dB/cP for 1-20 cP, 0.12 dB/cP for 20-100 cP, and 0.015 dB/cP for 100-1500 cP. Viscosity responses of the prototypes become comparable with their electric outputs only for eta < 2 cP. Temperature responses are almost zero in air, but when plate is coated with liquid they increase depending on liquid properties, allowing measurements of the temperature dependence of the liquid viscosity.