Broadband Microwave Microfluidic Coupled-Line Sensor With 3-D-Printed Channel for Industrial Applications

In this article, a novel microwave microfluidic coupled-line sensor allowing for monitoring of proportional quantities and determination of complex permittivity of liquid dielectric samples through simple analytical formulas is proposed. The application of broadside coupling configuration enabled the sample to be placed in-between coupled strips, thus a high sensitivity can be obtained as the electric field for differential excitation is confined within the sample's volume. Moreover, the use of 3-D printing technology allowed for the realization of an insert with the microfluidic channel and dedicated liquid inlet/outlet. The sensitivity analysis of such a sensor is provided along with the system calibration technique, which is straightforward because of a fixed and well-defined geometry of the sensor-sample setup. An exemplary, general-purpose sensor along with a cal-kit was realized in a stripline structure and manufactured using a combination of laminate and PolyJet 3-D printing technologies. The broadband measurement results are provided for a water-ethanol solution with variable solvent content up to 12 GHz and for various alcohols featuring frequency-dependent complex permittivity up to 5 GHz. The sensor's performance is assessed and discussed.