Revealed: why grapes sparkle when you microwave them

For over two decades, millions of curious science viewers have been captivated by online videos showing sparks of grapes and the creation of microwaved light shows.

Scientists and commentators have put forward theories as to why this is happening, but Canadian researchers have solved the mystery once and for all by reporting their findings in the paper. PNAS.

The party tour usually involves cutting a grape almost in half, leaving the skin intact on one side and crushing it in the microwave. After a few seconds, the radiation ignites a hot spot. The grapes produce sparks in the center and emit bright light or plasma.

A popular theory has proposed that the hotspot creating spark occurs because the skin drives the electrons back and forth.

Pablo Bianucci, co-author of Concordia University, said the grape halves act as a "dipole antenna" of a radio, converting the microwaves into an electric current flowing through the bridge of the skin. This current would eventually generate the plasma.

Bianucci and his colleagues, Hamza Khattak and Aaron Slepkov, have decided to use thermal imaging techniques and computer simulations to test this and other theories.

They discovered that the effect can occur even without a skin bridge.

"By studying the system more, we find that it's not the skin that matters, but the fact that the grapes are like balls of water," says Khattak.

Essentially, this "optical bulk" effect concerns the whole grape rather than its surface. And no electric current is needed to create the plasma.

"The plasma is created due to an amplification of the electromagnetic field between the grapes," says Bianucci. This is due to the interaction of "trapped" microwaves. Having two grapes, or two halves, is the key.

Continuing along this path, the researchers demonstrated that hot spots were also formed with the help of two skinless and skin-sized hydrogel beads made from water.

This revealed that the intact skin contained only the halves of grapes. When the two whole pearls were brought closer together in the microwave, the electromagnetic buildup caused the pearls to shock one against the other.

Researchers are studying this oscillating movement more. "We are currently studying the rebound behavior if you keep the balls in contact with gravitational potential," says Khattak.

Stephen Bosi, a lecturer in applied physics at the University of New England in Armidale, Australia, was one of the authors of the original dipole antenna theory. He does not want to give it up completely.

The idea rests on the assumption that symmetrical halves of grapes sound, much like an incoming radio signal.

"Resonance means that when a wave of a particular wavelength strikes an object of the right size, so that these waves fit in perfectly inside or around of the object, the waves oscillate very intensely, "he explains.

"It's like an organ pipe – only the sounds of the right wavelength (or pitch) can play loudly (or intensely) because they fit perfectly into the length of the organ pipe."

According to Bosi, the new document, in which he was not involved, suggests that resonance is still involved – but is much more subtle than we had previously guessed.

"They have shown using both experiments and a hundred-year-old theory of light scattering called" Mie's theory "that when two resonant grapes (or two halves of the divided grapes) come together, they get bogged down to create a resonance extremely intense in the gap between the two halves, "he says.

"The intensity of microwaves is high enough in space to cause the ionization of air molecules – that is, some of the electrons that gravitate around the atoms in the air molecules." tear off their parent atoms.

"Electrons (negative charges) are now able to move almost completely apart from positively charged parent atoms (now called ions). Any gas (such as air) where this occurs is called "plasma". "

One of the results of this process is that the plasma is shining.

Canadian researchers are intrigued by the fact that water can make microwave wavelengths very small, Bianucci says, but it does not have the same effect on visible light.

Khattak explains that the way a material affects light depends on the wavelength of light, which is why you get rainbows.

"If we could find a material that would reduce the wavelength of light, as water would, at the microwave wavelength, we could reduce all that to get to very small points of light, "he explains.

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