A miniaturized 2.4 GHz re-entrant cavity has been designed, manufactured and tested as a sensor for microfluidic compositional analysis. It has been fully evaluated experimentally with water and common solvents, namely methanol, ethanol, and chloroform, with excellent agreement with the expected behaviour predicted by the Debye model. The sensor’s performance has also been assessed for analysis of segmented flow using water and oil. The samples’ interaction with the electric field in the gap region has been maximized by aligning the sample tube parallel to the electric field in this region, and the small width of the gap (typically 1 mm) result in a highly localised complex permittivity measurement. The re-entrant cavity has simple mechanical geometry, small size, high quality factor, and due to the high concentration of electric field in the gap region, a very small mode volume. These factors combine to result in a highly sensitive, compact sensor for both pure liquids and liquid mixtures in capillary or microfluidic environments.

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In this paper, a new type of microwave microfluidic sensor is presented, using three separate half wave microstrip resonators with a common junction. The sensor comprises three parallel resonators with resonant frequencies of 2.5, 3.5, and 4.5 GHz. This sensor exhibits all the usual advantages of microwave resonant methods for dielectric liquid characterization, such as high precision, accuracy, and sensitivity, but here also over a wideband of frequencies. The resonators have common input and output ports. The sensor was tested using several types of liquids, including water, methanol, ethanol, and chloroform, and experimental results are in excellent agreement with the simulation results.

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