High-Sensitivity Dual-Band Microfluidic Microwave Sensor for Liquid Dielectric Characterization

The sensitivity and resolution are the crucial parameters for microwave (MW) microfluidic sensors in monitoring the concentration of binary liquid mixtures at low concentrations. This work proposes a miniaturized, reusable, high-sensitivity dual-frequency metamaterial microfluidic MW sensor with no-load resonance frequency points of 2.75 and 8.31 GHz for measuring the dielectric properties of liquid samples. The sensor comprises a microstrip transmission line (MTL) loaded with complementary split ring resonator (CSRR), which incorporates a bent groove structures to generate strong electric field (E-field) confinement within the CSRR. The polydimethylsiloxane (PDMS) microfluidic channels are designed based on the E-field distribution, allowing the loaded liquid samples to interact fully with the E-field. Given that the liquid sample’s complicated permittivity influences the magnitude of frequency shift and peak attenuation, a mathematical model is established according to the changes in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S} _{{21}}$ </tex-math></inline-formula> for different concentrations of ethanol-aqueous solutions and is experimentally validated. The results indicate that using the variation in the difference between two resonant frequencies to obtain the liquid permittivity can eliminate environmental factors to a certain extent, accurately estimate the complex permittivity of the liquid, and thus achieve dual-band sensing of chemical dielectric properties. The average value of sensitivity and the size of the proposed sensor are 149.2 MHz/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta \varepsilon '$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$40\times 30\times 0.813$ </tex-math></inline-formula> mm3, respectively. This sensor provides beneficial support for dual-band sensing measurements of material dielectric characteristics.