Microwave Resonators Enhanced with 3D Liquid-Metal Electrodes for Microparticle Sensing in Microfluidic Applications

In electrical sensors, effect of particle position on the signal magnitude can be mitigated through achieving uniform electric field at sensing region. 3D electrodes based on conductive solutions have been used before to generate uniform electric field in microfluidic platforms, where the ionic conductivity of the electrolyte was sufficient for impedance measurements at low frequencies (typically lower than 50 MHz). Due to low ion mobility, electrolyte liquids cannot be used as electrodes at microwave frequencies (>1 GHz). We used Galinstan, a metal that is liquid at room temperature, to fabricate 3D liquid electrodes connected to a microfluidic channel integrated microwave resonator.  A mixture of 20 μm and 30 μm diameter polystyrene particles were measured and classified based on their size without any calibration for particle position by generating uniform electric field.These results demonstrate the utility of liquid electrodes in enhancing the electrical control of microwave resonant sensors.