In this paper, a new type of microwave microfluidic sensor is proposed to detect and determine the dielectric properties of common liquids. The technique is based on perturbation theory, in which the resonant frequency and quality factor of the microwave resonator depend on the dielectric properties of the resonator. A microstrip split-ring resonator with two gaps is adopted for the design of the sensors (i.e., a double split-ring resonator). This resonator is both compact and planar, making it suitable for a lab-on-a-chip approach. Several types of solvents are tested with two types of capillaries to verify sensor performance. At 3 GHz, very good agreement is demonstrated between simulated and measured results.

We present a method of making low-loss split-ring resonators (SRRs) 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 SRRs for complex permittivity measurements of some common solvents.

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.

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.

In this article, a highly sensitive and compact microwave sensor with quality factor (Q ≈ 3000) is proposed for identifying and sensing liquid samples using a central gap ring resonator (CGRR). The high Q is a result of surface current redistribution. Our sensor is designed to operate at 2.5 GHz with an active gap region, where the sample flows, in which the electric field is at its maximum amplitude. The sensor was simulated, fabricated, and tested for a new application to identify the condition of oil used as a coolant and an insulating material in high-voltage transformers. The efficiency of oil in this application depends on its insulation capability and water content. Since both of these factors correlated with the oil's dielectric constant, the CGRR sensor showed very good sensitivity for detecting the ratio of water in oil from the measurements of Q change (and its link to the oil's breakdown voltage) involving fresh and used oil samples.

A microwave microstrip sensor incorporating a split ring resonator is presented in this paper for microsphere detection and dielectric characterization within a microfluidic channel. Split ring resonator (SRR) sensor of three different radii, but with approximately equal gap dimensions to change their sensitivity, were designed and fabricated, of resonance frequencies 2.5, 5.0 and 7.5 GHz. To validate the SRR sensors, two sizes of polystyrene microspheres were tested, of diameters 15 and 25 μm. Measurements of changes in resonance frequency and insertion loss of the odd SRR mode were related to the dielectric contrast provided by the microspheres and their host solvent, here water. COMSOL Multiphysics was used to model the sensors, and good agreements were found between the simulated and measured results.