A compact dual-mode microstrip sensor with curved-notch SRR for microfluidic permittivity characterization

This paper presents a dual-mode microwave microfluidic sensor based on symmetric curved-notch split-ring resonators (CN-SRRs) for liquid permittivity characterization. The sensor consists of a microstrip splitter/combiner loaded with two identical CN-SRRs and aligned polydimethylsiloxane (PDMS) microfluidic channels. The curved notch enhances the local electric-field confinement in the sensing region, while the PDMS channel confines the liquid under test at the high-field region to improve loading repeatability and field-liquid interaction. By controlling the loading liquids of the two microfluidic channels, the sensor operates in either a frequency-shift detection mode or a differential amplitude-detection mode. The fabricated sensor has a compact footprint of , and the unloaded resonance is designed at . For ethanol-water mixtures, the sensor achieves a frequency sensitivity of with a measured frequency shift of over the ethanol concentration range from to . In the differential amplitude-detection mode, an amplitude sensitivity of ( ) is obtained over the concentration range from to . The experimental results are in good agreement with the simulations. Compared with single-mode SRR sensors, the proposed CN-SRR sensor combines frequency-shift detection and amplitude-based differential sensing in one compact structure, providing improved operational flexibility for different measurement scenarios.