Fluid viscosity nanosensor

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 only of the mass density - shear viscosity product. 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 interesting situation where the separation between the resonator and a confining wall is less than the penetration depth of the fluid occupying the intervening region. This modality allows separate and unique determinations of shear viscosity and mass density. Moreover, because evanescent shear wave penetration depth usually is nanometers to micrometers, extreme miniaturization is enabled.