Wednesday, February 20, 2019

Speaking of Science: Grape balls of fire

Speaking of Science
Talk nerdy to us

Plasma forms between a pair of hydrogel beads irradiated in a household microwave oven. (Hamza K. Khattak)

"It is a truth universally acknowledged that a pair of grape hemispheres exposed to intense microwave radiation will spark, igniting a plasma." So begins a paper in the Proceedings of the National Academy of Sciences, describing the phenomena of microwaved grapes. Grape halves in contact — or two grapes touching — nuked on high will burst into plasma, to the delight of dorm-room experimenters and YouTube scientists.

The authors of the new paper, using various fruits and hydrogel water beads, offered the first formal description of what's going on within these sparking snacks. The three physicists, at Canada's University of Trent and Concordia University, took thermal images of irradiated water beads and simulated the sparking fruits on a computer. (They also got quail eggs and other food the size of grapes to spark.)

If grapes aren't touching, temperatures and energy densities spike only in their centers, the physicists discovered. In other words, the grapes heat up from within, despite the external source of the microwave energy. That's because the grapes are essentially "spheres of water," as the researchers write. Energetic waves, with the right frequencies, moving through water spheres can resonate, similar to the way that pushing a child on a swing at the right intervals results in higher and higher arcs. And kitchen microwaves emit energy at the right frequencies to resonate within a grape.

If the grapes are in contact, a hot spot forms at the connection, which amplifies energy even more. That spot gets hot enough to spew out an ion gas. These ions, potassium and sodium, freed from the grapes, ignite. And, presto, plasma.

We're not saying you should try this — only that, as the study authors noted, "observing a piece of fruit burst into flames in a microwave oven is exciting and memorable."


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