Comparison of quartz tuning forks and AlN-based extensional microresonators for viscosity measurements in oil/fuel mixtures

In-situ monitoring of the physical properties of liquids is of great interest in the automotive industry. For example, lubricants are subject to dilution with diesel fuel as a consequence of late-injection processes, which are necessary for regenerating diesel particulate filters. This dilution can be determined by tracking the viscosity and the density of the lubricant. Here we report the test of two in-plane movement based resonators to explore their capability to monitor oil dilution with diesel and biodiesel. One of the resonators is the commercially available millimeter-sized quartz tuning fork, working at 32.7 kHz. The second resonator is a state-of-the-art micron-sized AlN-based rectangular plate, actuated in the first extensional mode in the MHz range. Electrical impedance measurements were carried out to characterize the performance of the structures in various liquid media in a wide range of viscosities. These measurements were completed with the development of low-cost electronic circuits to track the resonance frequency and the quality factor automatically, these two parameters allow to obtain the viscosity of various fluids under investigation, as in the case of dilution of lubricant SAE 15W40 and biodiesel.