How can I make grapes explode

All you need for the sparks storm in your own microwave is a bunch of grapes. In the popular do-it-yourself experiment, the grape is halved so that a fine skin bridge connects the two halves. After a few seconds in the microwave, you can marvel at miniature fireworks made of flashing plasma.

This effect has been known for around twenty years and has been documented many times in videos. The physical mechanisms behind the sparkling fruit have been unclear so far. Canadian researchers working with Aaron Slepkov from Trent University in Peterborough have now solved this puzzle.

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Your in the trade magazine PNAS According to a study, the round shape of the grapes and the properties of the water are decisive. Earlier explanations for this phenomenon have always emphasized the role of the skin and the open, wet surface in the formation of the plasma, the researchers write. "However, we find that none of these components are essential for the formation of the plasma."

The term plasma describes a particle mixture in which, in addition to neutral atoms, there are also charged particles such as ions or electrons, i.e. free charge carriers. For example, plasma can be generated by heating gases. A closer look at the development of the plasma flash shows, the researchers write in their article, that the ignition takes place below the skin bridge between the two halves of the grape.

First of all, the researchers discovered that whole grapes and balls made of hydrogel, which consists mainly of water, create plasma flashes when they are irradiated with microwaves. It is crucial that there is contact between the grapes or balls.

The experiment also works with quail eggs

Slepkov and colleagues used a thermal imaging camera and computer simulation to investigate the distribution of heat in grapes before the plasma flash. It was found that a "hotspot" forms between the grapes.

According to the researchers' findings, the so-called Mie scattering, a scattering of electromagnetic waves in round objects, is responsible for the hotspots. The dielectric constant of water also plays an important role, as it indicates how permeable water is to electrical fields.

To show that the content, not the surface, is what counts, the researchers placed two touching quail eggs in a microwave oven. There was a hotspot at the point of contact. This hotspot did not exist if the eggs had been emptied through a small hole. When the scientists filled the eggshells with water, a hotspot was created again.

Spectral analysis of the plasma flash indicates sodium and potassium, chemical elements that are present in the grape skin. Because the electric field of the microwave radiation is strongly concentrated at the point of contact, these atoms are partially ionized. The sodium and potassium ions are in resonance with the wavelength of the microwaves and can also ionize the surrounding air, creating a plasma.