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A conflagration to come. (Credit: Kaiskynet Studio / Shutterstock)
Consider the humble grape. Small, spheroid, skin pleasantly taut, leaving a luster of softness on the tongue. Not a fruit against which you will need to defend yourself.
Put a gently touching pair in the microwave, however, and the harmless fruit turns into a literal firecracker. In just a few seconds, the microwaved grapes will start to spark as if they were electrified and, in some cases, even produce a plasma flash bright enough for the microwaves to glow from within. (Another method is to cut the grapes in half, leaving a strip of skin to connect the hemispheres, both of which produce the same effect.)
It has been a popular stuntman on Youtube for years, but the physics behind the grape explosion was elusive. This is obviously not the same process that produces sparks from metals, and oddly enough, the same thing does not happen for other foods – say peas. Nor has it helped that legitimate scientific research on the phenomenon has been legitimate.
But our long wait seems to be over. Three Canadian researchers think they know what turns grapes into fruity fringing fruit in a microwave oven.
Anger of grapes
The answer is based on the complexity of the behavior of electromagnetic waves when passing through various substances. It all boils down to the fact that grapes are essentially spheres filled with water and that their diameters correspond to the wavelengths of microwaves. These conditions combine to create a sort of trap for them. The incoming microwaves get stuck in the grapes, where the water helps to concentrate their energy, and they start to heat from the inside.
Only the resulting "hot spot" will not be able to accumulate enough energy to trigger a combustion. But when you combine two grapes, says Pablo Bianucci, co-author of the Concordia University study in Montreal, trapped microwaves can switch between them, creating a powerful electromagnetic field between the grapes. If left in the microwaves long enough (just a few seconds), the heat will be intense enough to turn the grape skin ions into plasma, creating an incandescent display.
Recipe for pyrotechnics
It's not just grapes. The researchers were able to replicate the phenomenon in hydrogel balls soaked in salt water and even quail eggs. It is not a question of showing, but of proving that it is not only the geometry that is responsible for the spark and the explosion. Researchers first thawed the eggs that touched each other like grapes and confirmed the existence of a hot spot. They then emptied the eggs of their contents and put them together in the microwave as before – but this time, nothing happened. It was the water inside the eggs, in addition to their shape, which made them hot.
The search appears in the Proceedings of the National Academy of Sciences.
The researchers discovered that grapes (or quail eggs) did not even need to be touching. They performed tests with the grapes wrapped in a comfortable paper container and separated by 15 layers of paper. Even separated, the grapes could heat up quickly.
Two grapes in the experiment. (Credit: Khattak et al.)
Researchers have given scientists in the chair (or microwave oven, as the case may be) something to think about when they cause kitchen fires, but they say their work also has practical applications. The work could, in the future, be used in the field of nanophotonics or be used to create new wireless antennas and lead to better imaging technologies by microwaves.
For the rest of us, the researchers also revealed that they were producing the same flamboyant effect not only with grapes, but also with tomatoes and large blueberries. In fact, any object rich in water the size of a grape would work, they say.
Needless to say, be safe.
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