Abstract
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.
Highlights
The re-entrant microwave cavity (RMC) is a very attractive sensor for dielectric characterisation of small liquid volumes due to the high concentration of electric field in its gap region.The structure is easy to manufacture, and retains a high quality (Q) factor (~3000) even when machined from aluminium
We present a new approach by developing a miniaturized re-entrant cavity for studying the interaction between dielectric fluids and the electric field within the RMC’s gap region
COMSOL Multiphysics software has been used in the design and in measurements, to compare theoretical and experimental results
Summary
The re-entrant microwave cavity (RMC) is a very attractive sensor for dielectric characterisation of small (mL to μL) liquid volumes due to the high concentration of electric field in its gap region.The structure is easy to manufacture, and retains a high quality (Q) factor (~3000) even when machined from aluminium. Unlike for the electric field, the associated magnetic field magnitude is small and spread over a much larger volume, leading to low surface loss on the exposed metal surfaces, the high Q. These desirable features (high Q, high concentration of electric field in the gap) contribute to a high performance dielectric sensor, as will be demonstrated. In particular, are of high interest in applications where no physical contact is possible or the use of active devices is impractical. Such microwave sensors can be fully compatible with, and so can be embedded within, lab-on-a-chip type approaches [3]
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