Boosting Resonant Sensing in Fluids: A Surprising Discovery

Micro-mechanical resonators are widely used in modern sensing technology due to their high quality-factor (Q), enabling sensitive detection of various stimuli. However, the performance of these resonators in fluid environments is limited by significant viscous and acoustic radiation losses that reduce their Q. Here, we present a paradigm-shifting discovery that challenges the conventional wisdom of resonant sensing in fluids. We report an optimal volume of fluid over a 2D micro-resonator that increases the Q by up to 1000% compared to that in air. We have conducted precise experiments on piezoelectric, circular, membrane-type micro-resonators of 4 mm diameter fabricated using MEMS technology on silicon-on-insulator (SOI) wafers. The top side of the resonator was filled with different volumes of fluid and its Q was measured through resonance tracking by actuating the resonator with an appropriate voltage. We found the existence of an optimal volume of fluid that maximized the Q. We argue that this phenomenon is a result of a balance between the enhancement of kinetic energy of the resonator due to mass loading of the fluid and the energy dissipation through viscous and acoustic radiation losses in the fluid medium. This remarkable enhancement in Q substantially improves the sensitivity of the resonator, with important implications for diverse applications such as biosensing, chemical detection, and environmental monitoring. Our findings challenge the prevailing understanding of resonant sensing in fluids and open up new avenues for the development of highly sensitive and accurate sensors.