Public Release: 

How grapes produce sparks in microwave ovens

Proceedings of the National Academy of Sciences

Researchers report a physical explanation for a popular internet parlor trick. Irradiating grape halves in a household microwave oven produces plasma--a parlor trick that can be found online. However, a physical explanation of this phenomenon is lacking. Aaron Slepkov and colleagues combined thermal imaging with computer simulations of electromagnetic fields to elucidate the physics underlying the production of sparks from microwave-irradiated grapes. Typical demonstrations use grape halves, connected by a strip of skin, but the authors found that whole grapes and skinless hydrogel beads are similarly capable of producing plasma, as long as there is physical contact. Thermal imaging and computer simulations of isolated irradiated beads revealed that temperatures and energy densities were maximal in the center, indicating the existence of morphology-dependent microwave resonances. When two beads were in contact, a bonding-like interaction between the resonant modes of each bead led to the formation of a hot spot at the point of contact, much smaller than the irradiating wavelength, with substantially greater temperature and energy density than in the isolated beads. The absorption of microwaves by water serves to suppress internal modes and allow hot spots to form over a range of bead sizes. According to the authors, such a system could be useful for experimental research in nanophotonics.

Article #18-18350: "Linking plasma formation in grapes to microwave resonances of aqueous dimers," by Hamza K. Khattak, Pablo Bianucci, and Aaron D. Slepkov.

MEDIA CONTACT: Aaron D. Slepkov, Trent University, Peterborough, CANADA; tel: 705-748-1011 x-6216; 705-874-1816; e-mail: aaronslepkov@trentu.ca

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