Piezoelectric Microcantilevered Plate Resonators Utilizing Flexural-Based 2-D Modes for Individually Sensing Density and Viscosity in Liquids

This article investigates a piezoelectric microcantilevered wide-plate resonator for liquid sensing. The higher order flexural mode featuring 2-D vibration of the resonator is exploited to enable precise measurements under viscous liquid conditions. The analytical model based on fluid–structure interaction successfully explains this nonconventional flexural mode and resonance characteristics, which provides an optimization design method for the resonator. For on-chip fully electrical interfaces, the piezoelectric aluminum nitride (AlN) film is employed in combination with tailored electrode design to allow self-actuation and self-sensing capabilities when submerged in liquid media. The experiments demonstrated that the density and dynamic viscosity of various liquids can be individually determined from their high-linear relationships with the resonant frequency and quality factor of the resonator. The proposed piezoelectric resonator exhibited excellent sensing characteristics, resulting in accuracies of 0.48% for density with a sensitivity of 27.94 kHz/(g·cm⁻³) and 3.37% for viscosity. In addition, the minimum detectable changes were evaluated to be 1.79 × 10⁻⁴ g·cm⁻³ and 9.29 µPa·s for the liquid density and viscosity, respectively. The results presented here show the outstanding potential of 2-D vibrational modes using microplate resonators for achieving high accuracy and compact design in liquid monitoring applications.