Viscosity and density decoupling method using a higher order Lamb wave sensor

Viscosity and density are two important physical parameters of liquid. Such parameters are widely used for label-free chemical detection. Conventional technologies employ acoustic wave sensors to detect viscosity and density. In these sensors, the liquid under test directly contacts with the surface of the sensor. The produced acoustic wave in the sensor leaks to the adjacent liquid layer, causing a shift in the resonance frequency of the sensor. However, such sensors are not able to separately measure the viscosity and density because these two parameters jointly affect the shift of frequency. Although some indirect methods for decoupling these two parameters have been investigated, either dual-device or simultaneous measurement of frequency and attenuation is required. In this paper, a novel AlN based acoustic wave sensor is developed for decoupling viscosity and density. Multiple higher order modes of Lamb waves are generated in this sensor and employed to interact with the adjacent liquid under test. The frequency change of two unique modes (mode C and mode D) has been found in a linear relationship with viscosity and density, respectively. With this unique feature, viscosity and density of a liquid can be distinguished by a single device, which is promising for potential industrial applications, label-free chemical detection and clinical diagnosis.