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Why is a plasma produced when you put a grape in the microwave?

Credit: : PNAS / Université de Trent

When grapes are put into a microwave oven, a plasma is produced. While many researchers have attempted to provide an explanation, Canadian scientists have only recently come up with a valid answer to this mystery.

Explanation called into question

There are many videos showing this type of experiment – like the one visible at the end of the article.  Together these videos make up millions of views on YouTube. Each time, a grape is cut in half and is placed in a microwave oven you get an astonishing result: a light occurs in a kind of spark. Until recently there was no satisfactory explanation for this luminous plasma.

The most generally accepted hypothesis concerns the presence of a mini-bridge of skin formed when the grape is cut.  This bridge is rich in electrolytes. However it is also believed that the exposed aqueous surfaces encourages the conduct of an electric current.  Researchers at the University of Trent (Canada), were not convinced by this explanation, and tried to uncover this secret in a study published in PNAS on February 19, 2019.

Credits : PNAS/Université de Trent

Link to video

Mystery solved

Neither the skin of the grapes nor the moist surface seems to be essential in this respect,” said Hamza Khattak, lead author of the study.

This statement was made after a multi-step experiment.  Researchers aimed to place two whole grapes in a microwave oven against each other and then 3 mm apart. The same test was repeated by replacing the grapes with hydrogel beads almost entirely made of water. In each case, a plasma was formed. This experiment allowed the researchers to reveal the emission spectrum of the plasma. According to researchers, this is the result of potassium and sodium ionization at the point of contact. However, these ions are found in large quantities in the grape’s skin.

Grapes behave like spheres of water in which the resonant cavities responsible for the production of plasma are formed under the effect of waves,” Hamza Khattak.

Researchers modeled the energy and thermal distribution at the center of the grapes. This allows us to understand that the energy densities and temperature were higher in the case of a single grape. For two grapes placed side by side, energy densities and temperature were higher at the point of contact.

Sources : Ars TechnicaFutura Sciences

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