A dual functional SSPP based microwave sensor for detecting volumetric concentration of alcohol mixtures and paper thickness

This work presents a compact and novel dual-functional microwave sensor based on Spoof Surface Plasmon Polaritons (SSPP), designed for multi-component liquid analysis and thickness sensing. The proposed sensor, with dimensions of 16 $\times$ 14 mm, features a dual-layered structure comprising a grooved ring resonator excited by an SSPP-based transmission line. The sensor operates at multiple sharp resonant modes of 4.32 GHz, 8.17 GHz, and 11.56 GHz, enabling wide-spectrum and multi-band sensing capabilities. For liquid mixture analysis, the sensor effectively detects the volumetric concentration of multiple alcohols blended in varying proportions while maintaining a constant total mixture volume. Predictions based on resonance frequency shifts, extracted as critical input features, are made using machine learning algorithms employing multivariable regression, achieving an impressive overall average root mean squared error (RMSE) of 0.1156%. In addition to liquid analysis, the sensor demonstrates exceptional performance in thickness sensing, achieving a maximum sensitivity of 3.64 GHz/mm in the lower band and 3.35 GHz/mm in the upper band. These characteristics highlight its ability to detect minute variations in material properties with high precision. This dual-functional sensor provides a reliable and versatile solution for liquid mixture analysis and thickness sensing, representing a notable advancement in multi-functional microwave sensing technology.