Abstract
In this study, a novel non-invasive and contactless microwave sensor using a square substrate integrated waveguide (SIW) re-entrant cavity is proposed for complex permittivity measurement of chemical solutions. The working principle of this sensor is based on cavity perturbation technique, in which the resonant properties of cavity are utilized as signatures to extract the dielectric information of liquid under test (LUT). A winding microfluidic channel is designed and embedded in the gap region of the cavity to obtain a strong interaction between the induced electric field and LUT, thus achieving a high sensitivity. Also, a mathematical predictive model which quantitatively associates the resonant properties of the sensor with the dielectric constant of LUT is developed through numerical analysis. Using this predictive model, quick and accurate extraction of the complex permittivity of LUT can be easily realized. The performance of this sensor is then experimentally validated by four pure chemicals (hexane, ethyl acetate, DMSO and water) together with a set of acetone/water mixtures in various concentrations. Experimental results demonstrate that the designed sensor is capable of characterizing the complex permittivities of various liquids with an accuracy of higher than 96.76% (compared with the theoretical values obtained by Debye relaxation equations), and it is also available for quantifying the concentration ratio of a given binary mixture.
Highlights
Over the past two decades, accurate complex permittivity measurement of materials/liquids in the microwave region has aroused considerable enthusiasm as it shows great potential in many areas such as biomedical, chemical, agriculture and food grading [1,2,3,4]
The sensor design in this work consists of two parts, namely the substrate integrated waveguide (SIW) re-entrant cavity resonator design and the microfluidic subsystem design
The maximum relative error between the measured complex permittivities using our sensor and the theoretical values obtained by Debye relaxation equations is only 3.24%
Summary
Over the past two decades, accurate complex permittivity measurement of materials/liquids in the microwave region has aroused considerable enthusiasm as it shows great potential in many areas such as biomedical, chemical, agriculture and food grading [1,2,3,4]. A variety of resonators have been proposed and utilized for material characterization, including split-ring resonators (SRRs) [5,6,7], cavity resonators [8,9], metamaterial-based resonators [10,11], half/quarter wavelength resonators [12,13] and dielectric resonators [14] Among these resonators, the re-entrant cavities have attracted significant interest since they offer the advantages of high Q-factor and highly concentrated electromagnetic fields, with the result that a number of complex permittivity. In order to realize the interaction between the liquid and the induced electric fields and the extraction of liquid dielectric properties, resonators are usually immersed in a mass of liquid specimens [13] or loaded by a micro capillary filled with samples [25] Such methods, suffer from wastefulness of specimens or poor sensitivity. It is compatible with lab-on-a-substrate approach thanks to the utilization of SIW technology
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