MEMS viscosity sensor

Quartz shear resonators are employed widely as sensors to measure Newtonian viscosities of liquids. Perturbation of the electrical equivalent circuit parameters of the plate resonator by the fluid loading permits calculation of the mass density-shear viscosity product. Use of doubly rotated resonators does permit additional information to be obtained, but in no case can the viscosity and mass density values be separated. In these measurements, the resonator surface is exposed to a measurand bath whose extent greatly exceeds the penetration depth of the evanescent shear mode excited by the active element. Here we discuss the more complicated situation where the separation between the resonator and a confining wall is less than the penetration depth of the fluid occupying the intervening region. The resonator perturbation in this case is a sensitive function of the separation. This important fact permits extreme miniaturization, since for temperatures and pressures ordinarily encountered with gases and liquids, and for frequencies between 10 MHz and 1 GHz, the penetration depth varies from micrometers to nanometers. Micro-electro-mechanical (MEMS) versions of viscometers and associated types of fluid sensors are thereby enabled.