Effects of polarization-charge shielding and electromagnetic resonances on water behavior under microwave heating

Abstract

Microwave heating of water-rich solvents is a widely used processing technique in research and applications. High-quality outcome requires a uniform temperature environment; which, in turn, depends on the balancing of a variety of effects taking place during the heating. Here, we show that two inherent effects, namely, polarization-charge shielding and electromagnetic resonances, play a critical role in shaping up the field pattern in the heated water sample. Polarization-charge shielding produces an internal electric field sensitive to the sample size, shape, and orientation. Internal electromagnetic resonances result in a widely varying electric field, while also allowing much deeper field penetration than the attenuation length to allow large-scale treatment. The key to temperature uniformity, thus, lies in an optimized thermal flow to balance the non-uniform energy deposition. These complicated processes are examined in simulation and interpreted physically. It is shown that a spherical sample is most favorable for obtaining a high temperature uniformity mainly because of its rotational symmetry. This conclusion is significant in that prevailing sample vessels are mostly non-spherical.