Millimeter-Wave Dielectric Characterization of Liquids-in-Flow Using Spherical Cavity Resonator

A millimeter-wave (mmWave) sensor for characterization of microliter volumes of liquid-in-flow is demonstrated. The proposed sensor design is a spherical cavity resonator operating in its fundamental electric dipole mode which has its electric field intensity focused at its center through which a pipe containing the liquid is introduced. This leads to a highly sensitive design based on electromagnetic perturbation of the resonating energy. The working principle of the design is verified in accordance with the Debye dispersion model of distilled water over a wide range of temperatures spanning from near ice to almost vapor states around 35 GHz. Furthermore, low-viscous chemicals are tested. The sensor outperforms most of the other designs presented in the literature for liquids-in-flow in terms of sensitivity and range of complex permittivity deciphered. The unique advantages of spherical-shaped cavities over other typologies at mmWave frequencies have been outlined.