A Microfluidic Antenna-Sensor for Contactless Characterization of Complex Permittivity of Liquids

In this article, a microfluidic antenna-sensor based on a microwave transmission model is proposed for the contactless characterizing of the complex permittivity of liquid under test (LUT). The antenna-sensor comprised a cavity-backed slot antenna (CBSA) and a substrate-integrated waveguide re-entrant cavity resonator (SIW RECR) embedded with microfluidics. The microwave signal is fed into the SIW RECR through the coplanar waveguide (CPW) feed line. Then, the corresponding signal responded by the SIW RECR, which carries the liquid dielectric information, is radiated outward by the slot antenna and then captured by the external receiving horn antenna. Finally, the sensing signal is utilized to wirelessly characterize the complex permittivity of LUT. In addition, the equivalent circuit model of the SIW RECR CBSA sensor (CBSAS) without microfluidics is established to investigate the dielectric sensing mechanism. Then, a predictive model for predicting the complex permittivity of LUT is developed by a multivariable regression fitting method and a LabVIEW-based real-time test system. The predictive model is calibrated and verified by ethanol–water mixtures and methanol–water mixtures, respectively. The maximum errors of the extracted real and imaginary permittivity are 3.5% and 5.63%, respectively, which demonstrate the strong ability of the SIW RECR CBSAS to wirelessly characterize the complex permittivity of LUT. In addition, an uncertainty analysis is performed to determine the reliability of the measured results. The microfluidic antenna-sensor combined with the real-time test method provides a promising strategy for wireless dielectric measurement.