02/22/2019

Why Do Grapes Spark In The Microwave?

7:45 minutes

Video of plasma created by irradiation in a commercial microwave oven of whole (uncut) grapes forming a dimer on a watch glass. Credit: Hamza K. Khattak, Pablo Bianucci, and Aaron D. Slepkov, via PNASPlease do not try at home.

From tenured physicists to home experimenters, many researchers have been plagued by a question—why do grapes spark when you microwave them? More than a few microwaves have been destroyed to answer this top physics question.

A team of researchers decided to rigorously test this question so you don’t have to. They found that grapes—either one split in half or two touching—are able to concentrate the energy of the microwave into two “hotspots.” When the hotspots touch, they create a plasma, or an ionized gas which emits light and heat. The size of the grapes, it turns out, are the key to this kitchen science quirk. The results were published this week in the journal Proceedings of the National Academy of Sciences.

Physicist Aaron Slepkov, an author on that study, tells us how grapes are able to harness the energy of these home kitchen waves and what this can tell us about the field of photonics.


Further Reading

  • Read the study in Proceedings of the National Academy of Sciences.
  • Learn about the psychology of wine in this Science Friday video.
  • While it might not be safe to create a hotspot in your microwave, there’s plenty of other science you can do at home! Try an activity from Science Friday Educate.

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Segment Guests

Aaron Slepkov

Aaron Slepkov is an associate professor of Physics & Astronomy at Trent University in Peterborough, Ontario.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Physicists work to answer some of the biggest questions out there, like what is dark matter? Can we unite relativity and quantum mechanics? And why do grapes produce a fireball when you microwave one? 

What, you haven’t heard of that mystery? Yes, if you microwave a grape it can explode in a violent, dangerous fireball in your microwave oven. And there are dozens of YouTube videos and scientists who have wondered about this fruitful question. 

Now a team of scientists, having experimented and destroyed several microwaves in the process, has an answer. And we do not recommend that you try this at home. Their results were published this week in the journal The Proceedings of the National Academy of Sciences. 

My next guest is one of the authors on that study. Aaron Slepkov is an associate professor of physics and astronomy at Trent University in Peterborough, Ontario. Welcome to Science Friday. 

AARON SLEPKOV: Thank you for having me, Ira. 

IRA FLATOW: You know, I had no idea that– so how many physicists have been thinking– especially you– about this question for, what, over 20 years? 

AARON SLEPKOV: Well, I’ve been thinking about it for over 20 years. I was an undergraduate at a small university in 1995. And I saw a website in 1994 that mentioned this. And we’ve been playing with it kind of ever since as a party trick. But we haven’t been doing research on it for very long– only about five years. 

IRA FLATOW: So your team decided to finally put this to the test. 

AARON SLEPKOV: I wouldn’t say we decided. I’d say we kind of fell into it. The project really started as a make-work project for some undergraduate volunteers to keep them away from my laser and my laser lab. And they made headway right away. And it’s blossomed to a project that involved over seven undergrads in the last six years, or five years. And now we take it pretty seriously. 

IRA FLATOW: And you destroyed a couple of microwaves in the process. A grape can do that. 

AARON SLEPKOV: Well, some were destroyed on purpose, and some were destroyed simply in the process of taking the measurements. 

A microwave oven doesn’t like to be run empty. It’s a cavity that builds up energy. And so usually you need some kind of load– a bowl of soup, or a steak. And so if you just run a couple of grapes in a microwave, it can hurt the magnetron in the microwave after a while. 

But we’ve run thousands and thousands of trials before a microwave– it doesn’t really break. It just becomes less powerful. And it moves into the food room. 

IRA FLATOW: So give us the answer. Why does the grape explode so violently? 

AARON SLEPKOV: Well, so it turns out that water is very special as an optical material at microwave frequencies. So microwaves which operate at 2.5 gigahertz, water has an index of refraction of 10, which is very, very high. And what that means is that microwaves can be trapped inside of a grape. 

And so a grape is just the right size. It turns out to be exactly one wavelength of microwave radiation long. And outside of the grape, the microwave is much, much longer. So it kind of gets trapped and sucked into the grape. 

So an isolated grape has these resonant modes inside of it where it’s hottest in the middle. And they don’t spark. But when you bring two of these resonant modes close together– two grapes close together– they kind of bond. There’s an energy hotspot– an electric field hotspot– right between them that can be many orders of magnitude higher than the electromagnetic energy that’s inside of the grapes. 

So they kind of form a bonding mode right between them at the point of contact. And that seems to have a high enough electric field to ionize potassium and sodium that’s in the grapes. 

IRA FLATOW: Amazing. I have a tweet from Darren, who says, OK, serious question. Would carrots do this, too? 

AARON SLEPKOV: Absolutely. The carrots would have to be sliced or balled up into something roughly the size of grapes. Any fruit that’s water-rich. 

Really, it’s the water. That’s one of the things that we discovered, that grapes were a bit of a red herring. We were able to do this with hydrogel beads, which is just the same stuff that’s in super absorbent diaper material. It grows to about the size of a grape, at which point it’s over 98% pure water. It’s got no skin. And just the water bottle alone is able to do that. Well, two, of them in contact are able to do this. So grapes are fine. Melon, olives, and blueberries. 

IRA FLATOW: Can people at home safely set this up and try it in their microwaves? 

AARON SLEPKOV: So I’ve been advised not to say yes, because everybody seems to say no. I have done it at home hundreds of times to no ill effect. 

IRA FLATOW: And tell us exactly what you did that did not produce a smashing success, so to speak. 

AARON SLEPKOV: Well, no, I’ve done it, and it works. If you’re going to try it at home, the best advice is to cut the grape in half, and leave a little bit of the skin bridge. Now that’s the traditional way of doing this experiment. And a lot of the scientists over the past few years believe that the skin bridge had something to do with the effect mechanistically. 

But really, I think what we discovered is the skin bridge is just a way to keep the two hemispheres close together so that the grapes could bond in the same way that two whole grapes do work. But if you just put two whole grapes in a microwave, they’re going to roll apart, and they’re going to separate. As soon as the grapes are more than a millimeter or two apart, they’ll no longer form this effect. So in all of our experiments, the grapes are in a little watch glass to bring them closer together. 

But if you want to do it at home, just cut a grape in half, leave the two hemispheres kind of bonded with a little bit of skin, and run the microwave for 10 seconds. It usually should spark within five to six seconds. If it doesn’t spark after 10 seconds, turn off the microwave. If it does spark, turn off the microwave after about a second or two of forming plasma. 

The biggest danger is that the plasma, it can grow in a microwave, because it’s resonant with the microwave radiation itself. And so it can leave the region of the grapes. Hot air rises. The hot plasma rises to the ceiling of the microwave. You don’t see it there. And it’s burrowing through the metal, and it’s doing bad things. But we’ve never had any explosions or anything. The key is to turn off the microwave right away. 

IRA FLATOW: Are there any follow up acts for you on this one? What do you do next? 

AARON SLEPKOV: Well, so Hamza Khattak, who is the undergraduate student who brought this past the finish line, he’s been doing some very interesting simulations trying to see if this is a dynamic process that’s running away towards resonance or running far from resonance. The point is that the water, being so interesting, changes its optical properties as it heats up. So the fact that locally, near the point of contact, it heats up differently than everywhere else could mean that this phenomena is self-tuning, or maybe detuning, depending on the size of a grape. So we’re doing some simulations. 

And we’re also, if you see some of the videos that are online, you’ll notice that often the grapes or the hydro gels, they bounce. They vibrate very fast. And so we’re investigating this mechanical motion. We feel that there’s some very interesting physics in the mechanical motion. 

IRA FLATOW: So it’s really just a pure research project at this moment. 

[CHUCKLES] 

AARON SLEPKOV: Yeah, it sure is. 

IRA FLATOW: Nothing wrong with that. Thank you very much for taking the time to be with us today, Dr. Slepkov. 

AARON SLEPKOV: I’m so pleased to be on the show. Thank you very much for having me. 

IRA FLATOW: You’re welcome. Aaron Slepkov, an associate professor of physics and astronomy at Trent University in Peterborough, Ontario.

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