After demonstrations of the curious behaviour of cats and the downright baffling behaviour of boys on skateboards, some of the most common videos to be found on the World Wide Web feature people inserting all manner of objects into microwave ovens. The results often border on the catastrophic, but occasionally they prove to be thought provoking. That was certainly the case for Aaron Slepkov, who holds the Canada Research Chair in Physics of Biomaterials at Trent University, when he first saw an on-line demonstration of how a sliced grape creates sparks in a microwave.
Aaron Slepkov, Canada Research Chair in Physics of Biomaterials at Trent University. Photo credit: Trent University
“It’s the definition of a parlour trick,” he explains, pointing out that although such videos date back some 20 years, they are always presented in the same way. In each case a grape is sliced down the middle, with a strip of skin left in between; once the oven is turned on a spark will shoot across the top of the two halves.
Explanations of this phenomenon struck Slepkov as somewhat superficial and he wondered why there had never been any serious study of what was happening, nor even any variation in the technique for generating this interesting effect.
“I didn’t think they were wrong,” he acknowledges. “I didn’t even know enough to know what could be wrong with that explanation.”
He regarded the search for answers as a good way of giving undergraduate students a taste of discovery science, but it turned out to be a much more ambitious exercise. A series of experiments that should have lasted a summer turned into an undertaking that kept several undergraduate students busy for years. The work, co-authored by undergraduate student Hamza Khattak and Concordia University physicist Pablo Bianucci was recently published in the Proceedings of the National Academy of Sciences.[ https://www.pnas.org/content/116/10/4000]
“We have an attitude in the lab of work as play,” he explains. “If you frame it as structured play, that’s research.”
That play began with substituting other types of grape-sized fruit in the experiment, something he had not seen in any videos. The same spark flew across a wide range of candidates: blueberries, gooseberries, grape tomatoes, even olives. More importantly, the students found that bisecting them was unnecessary — if two intact grapes were simply butted up against one another, the reaction still took place.
Two grapes create a plasma arc in a microwave oven. Photo credit: PNAS
An even more important insight surrounded the popular perception that the skin of these fruits might somehow be responsible for conducting energy. Slepkov recalls the biggest breakthrough coming when they were able to initiate a spark between two hydrogel beads, which are simply large drops of water held together only by a dilute polymer scaffolding.
“That’s when we realized this was an optics project,” he says, adding that this now fell conveniently into his bailiwick as a self-styled “lightsmith. “Once we knew these balls were acting as optical resonators, we said we should take this more seriously.”
He ultimately concluded that the phenomenon was caused by refraction, specifically a significant mis-match between the refraction index of water in the grape and the equivalent index for the surrounding air. The wavelength of the “light” — in this case microwaves — changes in the grape until it is about the same diameter as the fruit itself. When that happens, the radiation begins to resonate within the grape, focusing in that location until a significant amount of energy is accumulated. The resulting field pulls electrons out of potassium and sodium in the grape, creating a spark in the space between the two grape halves, or between two grapes.
The fact that the same thing could be demonstrated with water told Slepkov that this was not a matter of specific organic compounds interacting with microwaves, but a more general response that might be harnessed in practical ways. The emerging field of nano-plasmonics, for example, has been exploring the electronic behaviour of dimers tuned to visible light. In principle such work could be extended to consider a range of dielectric materials that are active at the microwave scale.
“We now say we opened a new kind of sandbox where you could investigate new types of structures that could exist in the nano field if you only had a material that had the properties that water has in microwave regimes,” he observes.