Sensitive Microfluidic Sensor With Weakly Coupled Dumbbell Defect-Ground-Structure Resonators for Volume Fraction Determination in Liquid Mixtures

This letter presents a highly sensitive reflective-mode phase-variation permittivity sensor implemented by means of a pair of weakly coupled dumbbell defect-ground-structure (DB-DGS) resonators, the sensing elements, excited by a microstrip line. Weak magnetic coupling between the DB-DGS sensing resonators generates two closely spaced resonances, the separation of which can be adjusted by the coupling level. Since the phase of the reflection coefficient experiences an excursion of 360° between such frequencies, the phase slope can be made arbitrarily high. Thus, by tuning the device within that frequency range, the phase response of the sensor is highly sensitive to variations in the dielectric constant of any material surrounding the DB-DGS resonators, e.g., a liquid under test (LUT). This configuration, with the sensing elements etched in the ground plane, avoids any interference between the driving line and the LUT, or any mechanical accessory required for operation, thereby enhancing the sensor's practical application. Specifically designed for detecting small concentration changes in mixtures of water and isopropanol, the sensor achieves a peak sensitivity of 293°/% and a resolution of less than 0.1% with the presented setup.