Why a grape turns into a fireball in a microwave

It's a pleasure for the crowd, as long as you avoid melting your kitchen appliances. But eventually, even after millions of views on YouTube and probably dozens of microwaves burned, no one knew exactly why the fireball was forming. The most common online explanations usually say that the grape halves act as an antenna and somehow direct the microwaves on the small skin bridge to ignite the initial spark. But no one did the math to prove it. After several summers in which grape-shaped objects have been microwaved and simulated in the microwave, a trio of physicists in Canada may have finally understood.

Physicists point out that people were distracted by the fireball: "exciting and memorable" but "of secondary interest," they write in the newspaper. The fireball is simply a nice hot drop of electrons and loose ions called plasma. The most interesting science is contained in the steps leading to plasma, they say. The real question is how does the grape heat enough to produce the plasma?

So they used their microwave oven, grape after grape. They made replicas of grapes from plastic beads soaked in water. They found a way to heat the grapes and their replicas in the microwave, at the edge of the plasma formation, without ever letting the sparks spin. They even replaced the door of their microwave with a more transparent mesh to be able to film the grapes more clearly.

Their findings: the grape looks less like an antenna than a trombone, although it is for microwaves rather than for sound. When you play a paperclip, you insert vibrating air into it. The trombone only supports vibrations of a particular wavelength – the musical note you hear – depending on the location of the slide. Only certain wavelengths, called standing waves, fit perfectly into the trombone. When vibrating air of different wavelengths enters the trombone, the standing waves add constructively, while the other wavelengths cancel each other out. In other words, the trombone amplifies the standing waves and cuts all the others.

Grapes, by the way, have the perfect size to amplify the microwaves emitted by your kitchen appliance. The device pushes the microwaves into the two halves of grapes, where the waves bounce off and add constructively to focus the energy on one point of the skin. Sometimes the two halves of grapes concentrate the energy at the same tiny point. This intense energy jostles atoms and molecules in this place, heating them so much that they can no longer hold their electrons, which turns them into a plasma – and a fireball.

A common misconception is that the microwave acts on the grapes from the outside, such as defrosting frozen meat, said physicist Pablo Bianucci of Concordia University, who worked on simulations of grapes included in an article published today in the Proceedings of the National Academy of Sciences. Instead, it's important to think of the microwaves that interact with the grapes as a whole, with the wave entering the fruit and bouncing around it like an excited puppy.

They also found that the shape of the two halves of grapes and the small bridge of skin that connected them were not important. You can make the fireball by microwaving, for example, two whole grapes sitting side by side. And that does not even need to be a grape. "Anything the size of a grape will work if it's watery enough," says physicist Hamza Khattak, a graduate student at McMaster University, who did a lot of the microwave. They demonstrated this point by making fireballs from gooseberries in the microwave, large blackberries and even quail eggs.

But it was not just a snack time for these physicists. The work also has more serious applications, says Bosi. Her experiences with grape fireballs began and ended with the 2011 YouTube video, but not her curiosity. "I am impressed by the scientific depth of the document," wrote Bosi in an email. He notes in particular that the authors have proposed mathematical rules to describe the hotspot of grapes. They could possibly reduce these rules to a smaller scale, to create similar sensitive areas in nanoparticles, for example. Scientists use heated nanoparticles to make very accurate sensors or to facilitate chemical reactions, says Bianucci.

Nevertheless, the case is not completely closed. For the duration of their microwave, they noticed that two grapes placed side by side were bumping repeatedly, one after the other. They do not know why this is happening and they will study that later, says Bianucci.

For these studies, they will likely stick to replicas of plastic grapes rather than to the fruit. "It's because they do not go bad," says Khattak. Maybe they could try raisins.

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