Microwave Microlitre Lab-on-Substrate Liquid Characterisation based on SIW Slot Antenna

A microwave microlitre binary liquid mixture concentration detection sensor with potential biological analysis is presented. The microwave lab-on-substrate sensor is fabricated using a substrate integrated waveguide (SIW) slot antenna. The microfluidic channel encapsulating liquid under investigation is located on top of the antenna slot at a quarter wavelength from the short-circuited end of the SIW. The radiated electric nearfield interaction with the liquid mixture exhibits different relationships between the complex permittivity of the liquid mixtures versus the resonant frequency and return loss, discriminating types and percentages of mixed liquid. The sensor was initially demonstrated with three types of samples: deionised water, methanol and air. A resonant frequency shift of 110MHz was measured to discriminate between air and deionised water while we obtained a 20MHz resonant frequency shift between air and methanol. Furthermore, the sensor was used to assess deionised water-methanol mixtures with methanol fractional volumes of 0 to 1 in 0.2 steps. The microwave-microfluidic sensor is contactless, uses readily available materials, cost effective and offers fast and accurate liquid characterisation.