Temperature dependence of microwave resonances in saline aqueous spheres [Invited]

Heating via the absorption of microwaves in water is the underlying physical mechanism of microwave food processing. Here we show that while bulk heating can contribute at all sizes and shapes, water’s high refractive index at 2.45 GHz means that resonant heating can also contribute—and perhaps even dominate—at particular shapes and sizes, modifying heating rates and internal field distributions. The temperature dependence of the dielectric susceptibility of water gives rise to an evolving resonant landscape as an aqueous object heats up. In water-laden spheres, these changes include shifts in the resonant conditions that can lead to self-tuning, where the system stabilizes on resonance, and/or thermal runaway, where the system steadily drifts away from resonance. Salt content likewise alters the dielectric properties of water, in terms of both absorption and refraction. High salinity, for example, both broadens and weakens the optical resonances, promoting more uniform heating rates across a range of sizes.