These physicists have finally understood why the grapes in the microwave ignite



A grape meets a hydrogel bead and sparks fly.
A grape meets a hydrogel bead and sparks fly. All images courtesy of Hamza Khattak

"It's a truth universally recognized that a pair of grape hemispheres exposed to intense microwave radiation would cause a spark, igniting a plasma. "

You will notice that the quote above is a riff of Jane Austen's famous frontline. Pride and Prejudice. But if sparks can fly between Elizabeth Bennett and Mr. Darcy, it's nothing compared to the flaming chasms that follow when two halves of a grape get close enough to touch each other while turning in a micro oven -ondes. The quote above is actually the first line of another great book published yesterday by Proceedings of the National Academy of Sciences.

Two grapes meet and sparks fly.
Two grapes meet and sparks fly.

In the article "Linking plasma formation in grapes to microwave resonances of aqueous dimers"., " Physicists Hamza K. Khattak, Pablo Bianucci and Aaron D. Slepkova have deployed many grapes and several microwaves in the name of science. Microwave a grape almost cut in half to watch the central inflamed is a popular pastime and very well documented. For decades, this burning parlor trick has been filmed and shared on the Internet, where it has mystified observers. The researchers set out to solve the mystery.

The main theory was that when two halves of a grape are microwaved, the skin bridge that connects them acts as an antenna. The current flowing through the antenna warms up to the formation of a plasma. (Plasma, by the way, is an ionized gas that occurs when atoms are heated to the point of releasing their electrons.Lightning is a plasma, just like the sun.)

Do not try this at home.
Do not try this at home.

This turned out not to be the case, says author of the study Pablo Bianucci. A physicist at Concordia University, he responds all day to calls from journalists interested in their passionate discoveries. His research colleague, Slepkova, discovered the grape phenomenon on the Internet in 1995 and studied it together as a side project in 2015. But it was only when undergraduate student Khattak joined the team in 2017, said Biannuci. "Advanced very quickly." Using regenerated microwaves to take pictures, Slepkova and Khattak started using grapes in the microwave. According to the paper, the team used "thermal imaging techniques and computer simulations" to examine what was happening.

Plamsa in fusion igniting from a grape.
Plamsa in fusion igniting from a grape.

It turned out that cutting the grapes was not necessary, any more than the skin bridge. Even two whole peeled grapes side by side could cause fireworks. In fact, any grape-like object, including blueberries, cherry tomatoes, quail eggs and hydrogel beads, does the same – a fact that researchers have consistently proven by microwaving each. in turn.

This idea led the team to a new theory. In what Bianucci calls "a very lucky coincidence," the grapes are mainly made of water, which significantly reduces the wavelength of microwaves. From about 12 centimeters in the air, they go about one to two centimeters in the water: a little smaller than your average grape. "The microwaves can get stuck inside the grapes," says Bianucci. If a grape is microwaved, a hot spot is formed in the center from the trapped microwaves. But when two grapes are close enough, the waves can "move" from one to the other. "These jumps cause a very strong electromagnetic field between the grapes," he says. When the field is strong enough to ionize the sodium and potassium ions of the grapes, the result is a tiny fireball.

The researchers even removed the doors of the microwave to take better pictures.
The researchers even removed the doors of the microwave to take better pictures.

The discovery of the team lasted several years and resulted in the death of 12 microwaves. (Even heavily modified microwaves "do not like to be empty" with the exception of two grapes.) This may seem like a slight search, and the newspaper acknowledges that a piece of fruit burned in a microwave oven is exciting and memorable. "But there are also implications in the real world. As Bianucci explained to World of physics, these results could be useful in the field of nanophotonics (the study of light on a very small scale). Meanwhile, anyone interested in a free light show can micromanage (cautiously), knowing that we have a better idea of ​​what is going on there.

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