An internet parlour trick involves slicing a grape almost in half and throwing it in a microwave, igniting a plasma to create a fiery show.
Plasmas are formed when a gas is heated and ionised, releasing electrons. Previous explanations for the grape trick suggested that the little piece of skin left holding the two grape halves worked as an electron conductor to get things going – but it turns out that is wrong.
Aaron Slepkova at Trent University in Ontario, Canada and his colleagues decided to study the phenomenon using thermal imaging and computer simulations. They found that the grapes were actually trapping the microwaves inside them, which led to them heating up.
The team used thermal imaging to look at whole grapes sitting separately, and found the energy was focused in hot spots at their centres. This suggests microwaves were being trapped inside a ‘cavity’ in the grape.
“If you take two of these grapes and bring them close to each other, what happens is you don’t have an independent system anymore, but you do have a system where those two – we call them cavities – talk to each other,” says Thomas Volz of Macquarie University, Sydney, who was not involved the research.
Volz says that when the grapes are touching, they can exchange energy. These hot spots begin to build up between the two grapes and the microwaves interfere with each other to form a much hotter and more energy-dense spot than they would form independently.
Placing them next to each other focuses the energy into a very small spot, which is enough to ionise sodium and potassium within the grapes and ignite the plasma.
This find could have applications beyond grapes, such as in the design of an omnidirectional microwave antennae. Understanding how to focus the energy of the incoming waves could help detect weaker signals.
Watching what microwaves do in structures such as this could also help researchers understand what light, which is much more difficult to observe, might do by analogy, Volz says.
Journal reference: PNAS, DOI: 10.1073/pnas.1818350116
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