PMMA/ZnO/glass Love wave sensor for liquids sensing

The propagation of Love modes along ZnO/glass-based structures has been modeled and analysed aimed at the design of a sensor able to operate in liquid environment. The Love modes propagation was modeled by numerically solving the system of coupled electro-mechanical field equations and Navier–Stokes equations. The ZnO layer was considered isotropic but its elastic constants were considered numerically equal to the stiffened elastic constants of the 30° tilted piezoelectric counterpart. The sensor velocity and attenuation sensitivities to the changes of liquid viscosity and mass loading were calculated for different ZnO layer thicknesses and  the peak sensitivity was achieved at the ZnO thickness to/wavelength ratio h/λ=0.05. In order to further enhance the sensor sensitivities, the PMMA/ZnO/glass structure was investigated. The phase and group velocities and the attenuation of the acoustic wave propagating along the PMMA /30° tilted c-axis ZnO/glass structure contacting a viscous non-conductive liquid were calculated for different PMMA and ZnO guiding layer thicknesses, added mass thicknesses,  and liquid viscosity and density. The three sensor responses (the wave phase and group velocity, and attenuation changes) were calculated for different environmental parameters and related to the sensor velocity and attenuation sensitivities. The resulted sensitivities to liquid viscosity and added mass were optimized by adjusting the ZnO and PMMA guiding layer thickness corresponding to a sensitivity peak. The present analysis is meaningful for the manufactures and applications of the PMMA-ZnO-glass structure Love wave sensors for the detection of liquids properties, such as viscosity, density and mass anchored to the sensor surface.