An RF Ring Resonator across Fluidic Media for Dielectric Sensing and Air-bubble Detection

Design, fabrication, and characterizations of an RF ring resonator used as a dielectric sensor, air-bubble detector and droplet sizer inside fluidic media are presented. The resonator with a split in its design has been analyzed and optimized using numerical methods and design guidelines are established to select the most sensitive resonance mode. A tube with an inner diameter of 800 µm placed over the 350 µm split is used to bring liquids and test samples. Different types of liquids (Isopropyl alcohol and de-ionized water) are easily distinguished by frequency shifts in the resonance peak. For instance, a 5 MHz frequency shift between air and water is observed. Furthermore, uniform oil droplets (0.6mm and 2.8mm long) were passed through the tube and measured to demonstrate the transient response of the sensor in detecting droplet sizes.