What is it about?

Sparks fly when a pair of grapes are irradiated in a microwave oven. Why is this dazzling parlor trick only ever seen in pairs of touching water-laden spheres, but never in individual orbs? Previous work suggested that constructive interference between size-dependent resonances in each sphere was key, but direct evidence of such resonances has been lacking. In this work, we combine conceptually-simple but surprisingly effective coffee-cup calorimetry, with thermal imaging and computer simulations to map internal microwave resonances in hydrogel water beads. While there is evidence of microwave resonances in a wide range of aqueous spheres, the clearest and most intense resonance is experimentally found to coincide in grape-sized orbs.

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Why is it important?

This is a multifaceted study that ties together seemingly disparate areas of research; from food sciences to microwave engineering to nanomaterials. While the results most immediately confirm that grape-sized water-laden spheres are optically resonant in microwave ovens, the work will easily be extended to learn how to best use a microwave as a cooking utensil. For microwave engineers, this work contributes to an understanding of how water may impact microwave frequency communications technologies, such as Bluetooth. Moreso, the phenomenon at play here is analogous to what we see in some nanoscopic materials irradiated by visible light. Since we cannot easily resolve the nano world, this work shows simple and accessible methods for studying nanophotonic-related phenomena at the macro scale.


This study began as an "afternoon experiment" six years ago. At the time, I found the results surprising and confusing. Five years later, my lab's understanding of microwave resonances in water-laden objects like grapes had matured and we realized that our previously-discarded results offered a shockingly good confirmation of the existence of microwave resonances in cm-sized fruit. The fact that sophisticated optical theory can be studied with the kind of coffee-cup calorimetry experiment you could find in any high school science fair is particularly satisfying.

Aaron Slepkov
Trent University

Read the Original

This page is a summary of: Linking microwave heating of aqueous spheres to morphology-dependent resonances, AIP Advances, November 2022, American Institute of Physics,
DOI: 10.1063/5.0122773.
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