Why sparks fly when you microwave grapes - Science Magazine

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By Alex FoxFeb. 18, 2019 , 3:15 PM

YouTubers have gone grape crazy. In a plethora of internet videos, kitchen scientists have cut a grape almost in half—leaving just a strip of skin connecting the two sides--and stuck it in the microwave. In seconds, sparks erupt. Now, physicists think they know why this happens.

Here’s the common explanation: water-heavy grapes trap the wavelengths of energy microwave ovens emit because the waves are roughly the same size as the diameter of grapes. That energy starts charging up electrolytes inside the fruit, which then flow from one half of the grape to the other—using the strip of skin like an electrical wire and gaining energy as they go. The current quickly burns through the skin, causing the charged electrolytes to try to jump from one half of the grape to the other, supercharging the surrounding air into a bright flare of plasma—the same light-emitting state of matter responsible for the sun’s rays and fluorescent lighting.

To test this hypothesis, the researchers put grapes into microwaves and watched what unfolded with thermal cameras. Early on, the scientists found that a pair of grapes could also produce plasma, as long as they were kept within 3 millimeters of each other. If grapes can produce plasma without the skin strip, the researchers say, then the energy that produces the plasma must build up another way.

The thermal cameras revealed a hotspot between the grapes from a buildup of electromagnetic energy—not inside the grapes where the internet’s explanation would predict. This led the physicists to a new explanation: When two grapes are close to each other in a microwave, the waves they absorb bounce back and forth in the tiny space between them, creating an increasingly powerful electromagnetic field. This continues until the electromagnetic field becomes so powerful that it supercharges nearby electrolytes that then shoot out in a brief explosion of fiery plasma, the researchers report today in Proceedings of the National Academy of Sciences.

Aside from damaging microwave ovens, the authors say their findings could, with the right materials, one day be extended to trap and concentrate visible wavelengths of light for use in nano-scale microscopy.

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