DIY science enthusiasts know that, if you put a halved grape into a microwave with just a bit of skin connecting the halves, it'll produce sparks and a fiery plume of ionized gas known as a plasma. There are thousands of YouTube videos documenting the effect. But the standard explanation offered for why this occurs isn't quite right, according to a new paper in the Proceedings of the National Academy of Sciences. And its authors only needed to destroy a dozen microwaves to prove it.
"Many microwaves were in fact harmed during the experiments," admitted co-author Hamza Khattak of Trent University in Canada. "At one point, we had a microwave graveyard in the lab before disposing of the many early iterations in electronic waste."
Co-author Aaron Slepkov first became interested in the phenomenon when, as an undergraduate in 1995, he noticed there was no formal (i.e., scientifically rigorous and peer-reviewed) explanation for how the plasma was being generated. Once he'd finished his PhD and established his own research group at Trent University, he started doing his own experiments (microwaving grapes for science) with one of his undergraduate students. They used thermal imaging and computer simulations of both grapes and hydrogel beads in their experiments.
In order to capture the process on camera, certain modifications to the microwaves were needed, including drilling holes and removing the door, the better to peek inside while the machine was active (the opening was carefully covered with mesh to prevent leakage). And of course, the experiment itself can be hazardous to microwaves: running the oven nearly empty produces a ton of damaging unabsorbed radiation. That's why the group plowed through 12 microwave ovens over the course of the study.
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The wrath of grapes: A tale of 12 dead microwaves and plasma-spewing grapes