Linking microwave heating of aqueous spheres to morphology-dependent resonances

It was recently suggested that the sparking of grape dimers in microwave ovens is due to the interaction of morphology-dependent resonances in aqueous spheres. However, evidence for microwave resonances in individual grape-sized aqueous spheres has remained weak and is open to interpretation. In this work, we provide new experimental evidence for size-dependent resonances in hydrogel spheres via calorimetric measurement of the electromagnetic energy absorbed by hydrogel spheres under microwave irradiation. Using finite-element simulations, we predict the resonant behavior of grape-sized aqueous spheres and further explore the differences between mode intensities in free-space and various in situ positions of a microwave oven. The lowest morphology-dependent resonance—a magnetic dipolar mode—is experimentally confirmed, appearing at the predicted diameter of ∼1.35 cm. Finally, experimental evidence for higher order modes in larger spheres is suggestive but remains unresolved.