Microwave microfluidic sensor for dielectric characterization of liquids using phase and amplitude variations

This research article presents the design and verification of a microwave sensor for measuring the complex permittivity of microfluidic dielectric samples. Unlike the conventional sensors, where the measurement is performed based on the frequency shift and quality factor change, the operation principle in the proposed design is the variation of the reflection phase and amplitude. This efficiently simplifies the measurement by removing the requirement for a frequency sweeping equipment since phase and amplitude variations can be simply detected at a single frequency. The sensor is designed using a coplanar waveguide (CPW) terminated with a series L C resonator realized with a stepped impedance (SIR) structure. A microfluidic channel is realized in the capacitive gap of the resonator. This enables loading microfluidic liquids onto the most sensitive part of the sensor. Injecting the liquids into the microfluidic channel produces variations in the phase and amplitude of the reflected signal. Such variations are used to determine the complex permittivity of the test sample by developing a mathematical sensing model. The designed sensor is fabricated and tested with water-ethanol solutions for calibration and developing a mathematical model for complex permittivity sensing. The sensing model is then verified using water-methanol solutions. The sensor offers an average sensitivity of 2 . 04 1 ∘ showing almost two-fold improvement compared with the previously reported designs in the literature. • The proposed sensor offers simple and low-cost complex permittivity measurement of microfluidic samples at single frequency. • The sensor offers twofold higher sensitivity compared with the previously published phase variation microfluidic complex permittivity sensors in the literature. • The manuscript presents analytical step-by-step procedure for the design of the sensor operating at the desired frequency.