Why do grapes produce plasma in the microwave?

For years, the Internet has embraced a mysterious and yet universal truth: a grape, cut in two and placed in the microwave, will suddenly begin to spit out plasma.

The glowing view had mystified us for years, garnering millions of online views and a variety of expert opinions, none of which were based on formal research. Finally, a team of three physicists finally think they have understood everything.

The recently released paper is the first peer-reviewed study on the subject, and the results suggest that the most popular explanation advanced by chair experts is probably incorrect.

Over the years, several online sources have suggested that grapes create plasma because microwave energy charges electrolytes into their heavily water-laden interiors.

This, in turn, is thought to cause a flow of energy between the two slices of grapes, a force that runs through the skin like an electrical wire. When enough energy is collected, the whole thing produces a spark of plasma – an ionized gas that emits light – at the skin bridge connecting the two.

The authors of the new study have dug a hole in this explanation. It turns out that the bridge, one of the key components of the theory, is not at all necessary. As long as two whole grapes were not separated by more than three millimeters, the researchers recorded a similar plasma eruption.

(Khattak et al., PNAS, 2019)

Even the skin of the grapes, which would have carried all this energy, was not considered necessary. When the fruit was replaced with two skinless hydrogel beads, the researchers saw the same kind of sparkle burst.

In fact, this would happen with just about any spherical replacement, provided that it is sufficiently liquid. The team reproduced plasma sparks using gooseberries, large blackberries and even quail eggs.

"Watching an inflamed fruit in a microwave oven is exciting and memorable," write the authors.

"As a result, a lot of attention has been previously focused on the plasma itself rather than on the source of the spark."

In other words, like flies attracted by light, we were distracted by the effect instead of looking for the cause. By combining thermal imagery and computer simulations, this new research has refocused our attention on the underlying truth – a truth that our eyes can not see.

The results reveal that when two whole grapes touch each other in the microwave, it creates a bond that creates a "hotspot" of energy where they embrace each other. It is at this stage that the researchers recorded the highest temperatures and the most important energy distributions.

(Khattak et al., PNAS, 2019)

This means that the energy of the microwave does not accumulate inside the grape, but that it is directed from both grapes to the same place, creating an electromagnetic field between them. two.

The authors explain that this electromagnetic hot spot accumulates energy, but it supercharges nearby electrolytes, which causes the projection of a plasma jet into the air.

"The absorption of microwaves by water serves to suppress internal modes and to allow the formation of hot spots on a range of ball sizes," the authors conclude.

The Internet may finally have answers, but when a mystery is solved in science, it is often replaced by another. In this study, for example, the researchers noticed that both grapes were inexplicably and repeatedly bumping.

Understand why is the next challenge and each piece of the puzzle will better understand how the wavelengths of light work at the nanoscale.

This study was published in PNAS.