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Abstract-We present a method of making low loss split-ring resonators for microfluidic sensing at microwave frequencies using silver coated copper wire. We show that a simple geometric modification and the use of square cross-section wire give greater electric field confinement in the capacitive region of the resonant sensor. We use a combination of theoretical analysis, finite element simulations and empirical measurements to demonstrate the subsequent increases in the sensitivity of these split ring resonators for complex permittivity measurements of some common solvents.

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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A dual mode, microstrip, microfluidic sensor was designed, built, and tested, which has the ability to measure a liquid's permittivity at 2.5 GHz and, simultaneously, compensate for temperature variations. The active liquid volume is small, only around 4.5 µL. The sensor comprises two quarter ring microstrip resonators, which are excited in parallel. The first of these is a microfluidic sensor whose resonant frequency and quality factor depend on the dielectric properties of a liquid sample. The second is used as a reference to adjust for changes in the ambient temperature. To validate this method, two liquids (water and chloroform) have been tested over a temperature range from 23°C to 35°C, with excellent compensation results.

An electronically adaptive coupling technique is presented for a microwave microstrip resonator to improve the efficiency of liquid heating in a microfluidic system. The concept is based on feeding the resonator with two synchronized inputs that have a variable phase shift between them. A Wilkinson power divider and phase shifter were designed and fabricated for this purpose. Both simulation and measurement (using chloroform as an exemplar liquid) demonstrated that the adaptive coupling can be used to optimize the heating efficiency of the liquid.

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