Plasma forms because the fruit is the right size to trap the devices’ electromagnetic waves
Slepkov Biophotonics Lab/Trent Univ.
Here’s a recipe for homemade plasma: Cut a grape in half, leaving the two sections connected at one end by the grape’s thin skin. Heat the fruit in a microwave for a few seconds. Then, boom: From the grape erupts a small plasma fireball — a hot mixture of electrons and electrically charged atoms, or ions.
This trick has been floating around the internet for decades, and previous explanations of the effect have focused on the importance of the connecting skin. But two whole grapes bumped up against one another do the same thing, as do similarly sized waterlogged beads called hydrogels, researchers report in the March 5 Proceedings of the National Academy of Sciences.
The team determined that the grapes act as resonators for the microwave radiation, much like a flute resonates with sound waves. A single grape is just the right size that the electromagnetic waves get trapped within the fruit, bouncing back and forth. Using thermal imaging, the researchers showed that a hot spot appears in the grape’s center, as a result of the trapped radiation. But if two grapes sit next to each other, that hot spot forms where the grapes touch, and salts within the grape skin are ionized and released, producing a plasma flare.
The effect makes for a spectacular kitchen fireworks display, but one you may not want to try at home — it could damage your microwave.
LIT UP Grapes produce a burst of plasma when microwaved. Scientists have now determined that this process occurs because the grapes trap radiation inside.
H.K. Khattak, P. Bianucci and A.D. Slepkov. Linking plasma formation in grapes to microwave resonances of aqueous dimers. Proceedings of the National Academy of Sciences. Published online February 19, 2019. doi:10.1073/pnas.1818350116.
E. Conover. Sound waves can make bubbles in levitated drops of liquid. Science News. Vol. 194, October 13, 2018, p. 14.
L. Hamers. Microwaved, hard-boiled eggs can explode. But the bang isn’t the worst part. Science News Online, December 7, 2017.