Scientists Produce Rigorous Study Of Why Grapes Spark In The Microwave

Hydrogel balls sparking due to microwave radiation. (Photo: Khattak et al, PNAS)

A paper published Monday in a well-known science journal begins with the following sentence: “It is a truth universally acknowledged that a pair of grape hemispheres exposed to intense microwave radiation will spark, igniting a plasma.” A universally acknowledged truth indeed... but what causes this microwave marvel?

If you’re not familiar, putting grapes into a microwave to make sparks has become a popular YouTube trick. This new research from Canadian scientists shows that worthwhile advances can come from anywhere, even by studying something sort of silly.

“This is a regime that hasn’t been significantly studied before,” one of the paper’s authors, Pablo Bianucci from Concordia University in Montreal, told Gizmodo.

The trick usually shows two grape halves connected by a thin sliver of skin. After a few seconds of being microwaved, they begin to spark. Though various explanations exist online, researchers wanted to study the phenomenon more rigorously.

The researcher imaged both sliced grapes and hydrogel beads—made from a material that absorbs lots of water—as they sparked in the microwave. They realised quickly that the grape skin wasn’t required in order to get the sparks, as evidenced by the sparking in the hydrogel beads, held together only by their weight and the shape of the dish they sat in, according to the research published in the Proceedings of the National Academy of Sciences.

The specific geometry of two touching water-filled circular objects in an electromagnetic field creates resonances concentrated at the point where the spheres or half-spheres intersect. This becomes a very small hotspot with a high energy density, enough to create plasma out of the ions in the region where the objects touch.

Is the research worth publishing in a journal as high-profile as PNAS? The paper’s editor, University of Illinois chemistry professor Catherine Murphy, certainly thought so. “The fact that they were rigorous enough to pass through the process of peer review is a testament that they’re doing a good job on the technical end,” she told Gizmodo.

But the paper is far more than a gimmick, Murphy said. This sort of research on directed energy could find important use in other directed-energy systems, such as explosives or high-intensity laser pulses. Additionally, the paper presents a way to image electric fields in these sorts of physical setups, and could lead to advances in photonics more generally.

Plenty of quirky things could yield useful scientific results if you apply rigorous thinking and conduct a well-thought-out experiment, as demonstrated by this work. Also, so we’re clear: If you accidentally burn your house down microwaving grapes, it’s not my fault.

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