Resonator sensors for fluid properties

Mechanical resonators in various vibration modes and different excitation mechanisms are used as sensors for measuring mass loading or fluid pro- perties such as the viscosity and density. In this presentation the focus is on devices with elec- trodynamic excitation and readout with reso- nance frequencies in the low kilohertz range. The resonance frequency of the sensor is crucial for measuring complex fluids with frequency depen- dent linear viscoelastic properties. These proper- ties are relevant in bodily fluids with a significant content of polysaccharides, like hyaluronic acid in synovia (joint liquid). The electrodynamic excitation allows us to design devices, e.g., of low resonance frequency and a dominant acoustic shear-wave interaction with the sample fluid. Our work consists of modeling the frequency response as electrical equivalent circuits of the coupled system, finite elements analysis of the fluid-structure interaction, and lin- king the two domains with reduced order impe- dance models. We derived an expression for the fluid impedance consisting of a set of device spe- cific parameters describing the dependence on viscosity and mass density. Under quite general conditions, these parameters are only depending on the vibrational mode shape and independent of the fluid properties and frequency. An efficient numerical computation of these para- meters allows the tuning of the sensor geometry to a specific range and sensitivity during the de- sign step. Several examples of implemented sen- sor devices are presented together with an evalu- ation of the performance in realistic conditions.