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Microwave effect : physicists find an explanation for plasma flashes from wine grapes

no one knows how many people have made the Experiment. How many in the past years, before the switched microwave anxiously waiting for two wine is a bright plasma lightning grape halves of the shoots. This Trick has been around for more than 20 years, on the Internet there are countless Videos of them. Previously, researchers had found, however, no physical explanation for the phenomenon. The physicist Aaron Slepkov of the Trent University in Canada Peterborough, and his colleagues, this is now possible. Particularly important for the round shape, as well as the water content of the grapes, as the researchers in the journal "PNAS" letter seem to.

The grape skin as the ignition?

Usually, the Trick with two grape halves is shown, connected in the middle by a bridge from the grapes skin. After a few seconds in a commercial microwave, a flash of plasma lightning shoots out of the connector. Plasmas particle mixtures, in which, in addition to neutral atoms and charged particles such as ions or electrons, so free charge carriers.

The most important property of plasmas is their electrical conductivity. In the previous explanation of the grapes, attempt, try, the researchers write, tend to have the connecting bridge as well as the open moist surfaces of the halved fruit played a role in the formation of the plasma.

This theory, however, could not confirm the scientists: "We note that none of these components for the formation of the plasma is essential."

Hotspots in mini format

to demonstrate This, the researchers in an accompanying Video, in which you put two grapes in the microwave, the touch at a point. Also in this case, a Plasma is created after a few seconds, although there is no skin bridge. In order to show that the effect is grapes, nothing to do with the specific nature of wine – to do about the exact composition and shape, repeat the scientists trying once again with Hydrogel balls, which are almost a hundred percent made of water. Again, there was a Plasma bolt.

Plasma is formed between two Hydrogel balls, which are heated in the microwave.Photo: Hamza K. Khattak

This stand – though admittedly impressive – are not in the focus of researchers, but rather its origin. In the case of the two half-grapes about the "ignition" took place not in, but underneath the skin bridge. In order to localize them exactly, they had to find out the Points at which the grape accumulates in front of the Plasma bolt, especially a lot of heat. For Slepkov and his colleagues recorded the electrical fields, both in the grape as well as in their direct environment an extremely challenging task, since the size of these "Hotspots" is located below the wavelength of micro waves. In other words: they are so tiny that they are almost not to see.

With thermal imaging cameras and computer simulations, the researchers were able to locate the Hotspots yet. It was shown that the Hotspot is located at a distance from each other lying grapes in the middle of the fruit. Reducing the distance between them, moving the Hotspots in the direction of the other grape. The grapes, touch forms at the contact of a Hotspot, which is small, but much hotter than before, the Hotspots in the center of each fruit. This also explains that in the experiments of the researchers, not the skin bridge was crucial, but that the two bodies touched.

Necessary for the flash: an aqueous ball

According to the findings of researchers, the so-called Mie scattering, a scattering of electromagnetic waves in round objects, for the Hotspots. The grapes act as the water-balls, at a frequency of the micro wave of 2.4 GHz, especially in vibration. These resonances add up, and in the point of contact of the grapes, the electric field of the microwave radiation is concentrated to such an extent that in the grape peel existing sodium and potassium is partially ionized and ignited. The result of the plasma flash in the microwave.

An important role in the dielectric conductivity of the water, which indicates how permeable to water and to electric fields. In order to show that the aqueous contents of the body for the formation of the plasma flash is essential and not the surface, oven the researchers two touching quail eggs in a microwave. It is a Hotspot showed up at the point of contact of the eggs. This disappeared when the scientists emptied the eggs through a small hole. You filled it with water again, was the Hotspot back.

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Alexander Mäder

The findings could help Slepkov and his Team, to understand the scattering phenomena in the microwave range better. Specifically, they call for the development of new passive wireless antenna, and microwave imaging with very high resolution. It was so worth it in several ways, the experiments with wine grapes and plasma flashes still make a thorough to – from a purely scientific Motivation, of course. (with dpa)

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