Dynamically Tunable Planar Microwave Resonator Sensor Using Liquid Metal Alloy

Microwave resonant sensors are limited in their current form to a static resonant frequency, which can lead to misidentification when two materials or mixtures with similar dielectric properties at a specific frequency range are tested. This work presents a frequency-tunable resonator with a highly sensitive interdigitated capacitor region for material sensing. The sensor utilizes liquid metal in a microfluidic channel to tune the resonant frequency of the resonator over a wide, continuous spectrum by changing the effective capacitance of the sensor. The designed and fabricated sensor demonstrated an operating resonant frequency that could be tuned between 2.1 and 2.8 GHz for both solid and liquid sensing. The resonant amplitude and quality factor of the developed sensor were - 25 dB and 101, respectively, and did not show significant degradation in response to various volumes of liquid metal injection. Sensitivity varied from 36 to 6 MHz per unit permittivity due to the increased reactive loading introduced by the liquid metal. This approach using liquid metal, for the first time, was directed toward materials with identical dielectric properties at specific frequencies. Experimental validation of the unique sensor and standard liquid mixtures confirmed that the frequency variability enabled similar dielectric materials at a specific frequency to be distinctly identified by tuning the resonant frequency.