Physics + MathPhysics & Math

# How Does Microwaving Grapes Create Plumes of Plasma?

ByNOVA NextNOVA Next

If you’ve got a couple grapes and a microwave, it’s technically possible to make plasma in the comfort of your own home. The catch? There’s a very real possibility neither the microwave nor the grapes will survive the encounter.

For those unwilling to set their kitchens ablaze, however, there’s good news. For one, it’s not terribly hard to sate your curiosity on YouTube—but more importantly, after decades of speculation, a team of researchers has finally puzzled out the physics behind this mind-bending phenomenon so you don’t have to (please don’t). Your kitchen—and your landlord—will thank you.

Here’s the deal. In most online iterations, an intrepid citizen scientist will slice a grape in half, leaving just a thin, connective bridge of skin, and nuke the split fruit on high. After a couple seconds, the center of butchered grape will begin to belch out fiery, amorphous little sparks that ricochet through your microwave. Voilà: DIY plumes of piping hot plasma (as an aside, this is probably the point where the reaction should be shut down).

This plasma, of course, is not the plasma of blood, but the state of matter (as in solid, liquid, gas, plasma) that’s like a gas, but consists of charged, or ionized, atoms whose electrons have been stripped away from their positively charged nuclei. The result is a swarm of subatomic particles that clash and collide, often emitting roiling blobs of light and heat that can resemble molten fire.

Plasma is naturally found in lightning, the Earth’s ionosphere, and the Sun’s corona, but can also be artificially generated by exposing a gas to blistering temperatures or an electromagnetic field—basically, something that can infuse the gas with enough energy to jostle electrons loose from their atoms.

So what business does plasma have roaring out of nuked grapes?

This question plagued physicist Aaron Slepkov of Trent University in Canada for two decades. Slepkov first witnessed the phenomenon while surfing a website called “Fun with Grapes” in 1995. But while videos and blog posts of microwavable plasma abounded, it seemed there were no rigorous, scientific explanations for the physics behind the frivolity. So many years later, when Slepkov started up his own research group, he and his trainees, including study author Hamza Khattak, decided to put some theories to the test. The scorched fruits of their labor are published today in the journal PNAS.

One myth was quickly busted: a split grape wasn’t a necessary component of the blaze; in fact, the phenomenon wasn’t grape-specific at all. Sparks flew just fine with intact grapes—as well as with gooseberries, particularly buxom blueberries, and even self-contained beads of salted water—as long as there were two of them, and they were touching.

The key, it seems, is cramming the energy present in microwaves into a very tiny space—the point of contact between the objects in question. In your garden-variety microwave oven, microwaves have a wavelength of about 12.5 cm. But adjoining grapes (which are full of water that can absorb said microwaves) can concentrate the energy within into a region where the two spheres touch, which is no more than a couple millimeters wide. This creates a very strong, very condensed electric field at their interface—a pocket of ammo powerful enough to liberate negatively-charged electrons from, say, the salts naturally present in grapes and other fruits. And the results are explosive.

A single grape by itself can’t do the trick, though. In these cases, the energy simply concentrates at the center of the grape. But if joined by a willing dance partner, the “hotspot” in each grape gravitates towards the other, until the two synergize in a blaze of glory.

Ultimately, microwaving your way to plasma is actually a pretty flexible feat, as long as you’re mindful of size, says study author Pablo Bianucci, a physicist at Concordia University in Canada. With microwaves of this wavelength, typical grapes have a pretty ideal diameter. Scaling up to anything too much bigger than a grape—like a tomato—won’t concentrate the energy into a tight enough space (for that, you’d need to scale up the wavelength too). Conversely, undershooting the size will prevent the spheres from absorbing enough energy to begin with.

“This really shows that there’s an explanation for everything,” says Lydia Kisley, a physicist and nanoscience expert at Case Western University who was not involved in the study. “Physics can be used and applied to everyday phenomena. All these theories that were developed with pencil and paper can actually be applied to something you throw into your microwave.”

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And beyond piecing together the physics behind parlor tricks, the results could have implications for broader studies on plasma and light, says Julie Biteen, a biophysicist and chemist at the University of Michigan who was not involved in the study. One example is nanophotonics, or the study of light on the nanometer scale—another instance in which wavelengths are condensed into extremely small spaces. Nanophotonics can typically be visualized only with expensive microscopes. But the grape-microwave combination offers a way to tinker with these phenomena on a larger scale, with affordable everyday appliances.

Replicating these effects with visible light will require some rejiggering, Bianucci says. But it’s a logical and exciting next step.

In the meantime, it seems there are finally some answers to the mysteries behind the fiery wrath of these particular grapes. It’s worth noting, though, that the results didn’t necessarily come easy: The road to publication was littered with casualties—including a series of variably sized fruits and a dozen or so microwaves, each solemnly christened with a name to honor its sacrifices in the name of science (among the fallen microwaves were George I, George II, Jesús, Albert, and Thomas). One thing hasn’t changed: Plasma is a fickle and dangerous beast, not to be underestimated.

Even Bianucci is loath to try this at home. “I’m waiting until my microwave gets really out of commission,” he says.

With plasma in the picture, “you have to be careful about not melting a hole in the top of your microwave,” Khattak says. “I mean, you could give this a try, but I wouldn’t recommend it.”

For the record, neither does NOVA.

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