New Radio-Frequency Liquid Permittivity Measurement System Using Filter-Based Microfluidic Sensor

A new radio-frequency (RF) liquid permittivity measurement system using a filter-based microfluidic sensor is proposed in this article. Integrating the filter-based sensor with an RF source, a power divider, and a gain/phase detector (GPD) can form a new permittivity measurement system without using a bulky and expensive vector network analyzer (VNA) to acquire measured data. Since two coupled resonators with a strong coupling synthesize the filter-based sensor, it can provide a stronger electric-field concentration in the sensing area and improve the sensor’s sensitivity. The 8- $\mu \text{L}$ water–ethanol mixtures with ethanol volume fractions (EVF) ranging from 10% to 90% in 10% increments are used as the test liquids to evaluate the proposed sensor and system. Compared with the measured results using the commercial dielectric probe kit, the maximum errors of $\varepsilon _{r}'$ and $\varepsilon _{r}''$ using the filter-based sensor with a VNA are −4.34% and 13.24%, respectively. At the same time, $\varepsilon _{r}'$ and $\varepsilon _{r}''$ errors are 8.28% and 14%, respectively, using the proposed permittivity measurement system. Since the proposed permittivity measurement system can flexibly synthesize the sensing area size, control the electric-field intensity, and get rid of a VNA, it makes it easier to satisfy the requirements of various microfluidic sensing scenarios.