A Radio-Frequency Microfluidic Dielectric Sensor Based on Coupled Stepped-Impedance Resonators

A novel radio-frequency microfluidic dielectric sensor based on coupled stepped-impedance resonators (SIRs) is introduced in this study. The sensor utilizes two SIRs with weak electrical coupling to improve the quality factor, which is critical for accurate dielectric characterization. The sensing mechanism relies on variations in the dip frequency of the reflection coefficient and insertion loss when test liquids are loaded into the microfluidic channel. These variations are directly related to the liquid's complex permittivity. To validate the proposed sensor, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$7-\mu \mathrm{L}$</tex> water-ethanol mixtures with ethanol volume fractions ranging from 20% to 80% were used as test liquids. The dielectric constant (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\varepsilon_{r}$</tex>) and loss tangent (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\tan \delta$</tex>) of the mixtures were measured and compared with results obtained using a commercial dielectric probe. The proposed sensor demonstrated high accuracy, with maximum errors of −5. 0 9% for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\varepsilon_{r}$</tex> and 1.6 5% for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\tan \delta$</tex>. In addition, this sensor only occupies an approximately 10% sensor area as compared to that using coupled split-ring resonators. These results highlight the sensor's potential for precise and efficient dielectric characterization in microfluidic applications.