Ontario professor studies the science behind spark in microwaved grapes

Aaron Slepkov and his buddies first tossed grapes into a microwave back in the mid-1990s.

Within seconds, the fruits spark like lightning.

He was hooked and broke out the trick at parties.

Now, the Trent University physics professor has published his examination of the phenomenon’s underlying mechanism in the Proceedings of the National Academy of Sciences of the United States of America. The study went viral.

“The project went from ‘WTF’ to ‘A-ha!’ in about five years,” said Slepkov. “It’s really discovery science, really curiosity based. Nobody starts a project and gets some undergrad to microwave grapes because they have designs on some big grant somewhere.”

Well into his physics education, Slepkov began thinking more seriously about the grapes experiment.

Later on he searched for more information on the phenomenon. There was no scientific literature, but he found the trick had gained popularity on YouTube with hundreds of videos and millions of views. YouTubers thought the grapes sparked due to conductivity on its surface, but there was nothing to back that up.

In 2013, a young, keen student pestered Slepkov to volunteer in his lab.

“He was young and untrained so I certainly wasn’t letting him near my expensive lasers,” Slepkov  said. “He was so persistent, I said for the cost of a microwave oven of $100, put him in the lab and have him microwave some grapes and find out what other conditions it works — other fruits, other shapes, that kind of thing.”

That summer, they discovered the grapes didn’t need to be cut.

“There have to be a pair of spheres or hemispheres — a single grape doesn’t spark, it heats up,” he said.

“Then we put two grapes on a concave watch glass so they couldn’t roll away, then they spark all the time.”

So, he thought, if you don’t need the “skin bridge” connecting the grapes, does he even need the skin, or the grapes?

“What if a grape isn’t a grape?” he thought. “What if a grape to a microwave was just a ball of water?”

So they tested hydrogel beads, the absorptive material found in diapers.

“When we dipped them in salt water to make ions that ionize, they spark,” Slepkov said. “We realized it wasn’t grapes held together by a skin bridge, this is about balls of water.”

The team found that a microwave — with a wavelength of 12 centimetres in air — is the exact same size of a grape in water due to what’s known as water’s high index of refraction, Slepkov explained. That’s when something interesting happens, he said.

Microwaves accumulate and become trapped inside the grape, he said, and those light microwaves pack themselves in the centre and the grape begins to heat up. When a second grape comes in, or the other half of a cut grape, the microwaves concentrate at the sides near each other creating an intense electrical field.

“The microwave light senses the other grape and wants to move there,” Slepkov said.

The electric field becomes so high, he said, that it begins stripping electrons off sodium and potassium molecules, creating ions.

“Once you have an ion of sodium or potassium, then all hell breaks loose and the rest of microwave oven is feeding that spark, ionizing the air and turning it into ball lightning,” he said.

Slepkov gives a lot of credit to Hamza Khattak, the final undergraduate student to work on the project who also helped with imaging because of his burgeoning photography skills, along with Pablo Bianucci, a physics professor at Concordia University, both co-authors on the study.

The internet loved the team’s work, especially its accessible and lively writing.

One favourite is the opening line of the paper: “It is a truth universally acknowledged that a pair of grape hemispheres exposed to intense microwave radiation will spark, igniting a plasma.”

It is an homage to the opening line in Jane Austen’s “Pride and Prejudice.”

Up until a week before publication, the article was titled “Grape Balls of Fire,” but the journal balked at it.

“The paper was written with a lot of lighthearted humour,” Slepkov said. “We do try to take an attitude of work as play.”

He said one potential application is antenna design for cellphones or wireless routers.

“We’re hypothesizing maybe you can change antenna design because the grapes are acting as a concentrator for wireless radiation or cellphone radiation — effecting how we design antennas to help act as a signal booster,” he said.

“This was such a silly project I didn’t tell anyone for the first three years because I thought I’d be laughed at. We do serious research, but it turns out there is some serious science at work here. I have students funded to microwave fruit because our gut tells us the answer will be interesting. And it is.”