You’ve probably seen the videos of a grape — cut almost totally in half — in a microwave creates a plasma. A recent physics paper studies the phenomenon with a lot of high-tech gear and now the actual mechanism is known. [Veritasium] interviews the scientists and explains the grape plasma phenomenon in plain language. You can see the video below or read the paper directly.
Turns out the grape is about 1/10 of the microwave frequency and the refractive index of the grape at microwave frequencies might be as much as ten. A whole grape can get all the microwaves trapped inside, but two grapes — or two halves — that touch create fields strong enough to ionize the air.
The results include observations made from high-speed cameras, spectroscopy, and more. The plasma is mostly from potassium and sodium, apparently, The size of the grape matters, but it isn’t super critical because of the water content of the grape. Turns out it doesn’t even have to be a grape — hydrogel beads work, too.
The team also built models of the grape system to understand what was happening inside. The water content of the grape is high enough that the model just assumes the grapes are spheres of water.