Goodness gracious, grape balls of fire!
It started out as a cool party trick that ignited lots of online videos and science fair projects: microwaving a grape cut down the middle with just a small bridge of skin bridging the halves sparks a brief plume of fire-like plasma.
Plasma is a lesser-known fourth state of matter first described in the 1920s. Plasmas are ionized gases that are electrically neutral on the whole, but made up of many positively charged ions and free electrons. They are very common in stars, lightning, and the Earth’s upper atmosphere. But aside from fluorescent light bulbs and plasma displays, they aren’t very common in our homes.
So how does microwaving a cut grape produce plasma?
That was a question that puzzled physicist Aaron Slepkov for nearly 20 years when he first learned about it as an undergraduate student. Over the years, he repeated the fascinating experiment many times. By 2013, Slepkov was a professor at Trent University supervising his own undergraduate students, and he asked them to uncover as much as they could about the mysterious phenomenon.
They’ve microwaved thousands of grapes and destroyed a dozen microwave ovens. But they also learned enough to publish an academic paper on the topic in Proceedings of the National Academy of Sciences (PNAS).
Until now, most explanations have focussed on the skin linking the two grape halves acting as a conductive bridge. On the surface this seemed like a good initial hypothesis, as the skin bridge is where the spark starts. The thought was that the bridge acts as a wet and ion-rich antenna because of the conductive nature of the grape itself.
However, the team quickly realized that two whole grapes could still produce plasma as long as they were in physical contact, suggesting that the skin bridge was acting as a tether for the two halves. Placing a pair of grapes into a curved watchglass was enough to maintain gentle contact by gravity.
In fact, the skin itself isn’t even necessary, because hydrogel beads soaked in water and salt that have no skin at all will still also spark, as long as they’re about the size of a grape.
In a demonstration that a skin bridge is not necessary, two whole grapes in a watchglass also creates plasma when microwaved. Credit: Khattak et al, https://doi.org/10.1073/pnas.1818350116 (CC BY-NC-ND).
Slepkov thinks of it as an optics experiment more than anything else. Microwaves are just light at a wavelength the human eye can’t see, and the grapes act as a lens that concentrate that light down to the point where the two halves meet.
Water concentrates microwave radiation uniquely well, and having two grapes or grape halves right next to each other lets that light hop between them. As a result, the light becomes so intense at the point of contact that it has enough energy to rip an electron off a sodium or potassium ion in the grape.
This is the first stage, forming a small ball of plasma that then triggers a cascade that ionizes the surrounding air, creating a bigger ball of plasma. Other salty and small spherical foods, like quail eggs and grape tomatoes, can trigger the same effect.
Beyond satisfying curiosity, the fundamentals of this discovery could have broad implications for other optical applications, like trapping and concentrating light for nanoscale microscopy.
For readers tempted to try this at home, Slepkov recommends limiting your experiment to 10-second bursts, and warns that it could impact how well your microwave works.