Researchers at Trent University, Canada, have taken time away from their usual work on nonlinear optical microscopy to explain one of the great scientific mysteries of the modern home – why grapes produce plasma bursts when zapped with a microwave oven.
Here's some background for those unfamiliar with this viral activity. For reasons we'd rather not think about, someone once cut a grape almost (but not entirely) in half and put it in a microwave oven, creating sparks that in turn induced plasma – the state of matter seen in the Sun. People performing this wonder have become predictably popular on Youtube, with variations including alternative fruits. Others have demonstrated you can greatly improve the plasma production by partially drying the grape or putting it inside an upturned plastic container.
Fair warning: Some people have destroyed their ovens this way, so replicate at your own risk.
Fun as it is to maximize your plasma output, Dr Aaron Slepkov noted that popular-science presentations online provide explanations, but none of these have actually been verified through formal, let alone peer-reviewed, research.
Slepkov reported in Proceedings of the National Academy of Sciences that the most common assumption, which attributes the effect to surface conductivity in the grape skin, is probably wrong. As others have shown before, Slepkov confirms that pieces of similarly sized fruits give similar results, and even hydrogel water beads can work. The effect can be replicated without the grape peel or even the surface wetness that has been claimed to provide the necessary electrical conductivity. Quail eggs work when intact, but not if hollowed out.
Nor is the skin bridge essential – uncut grapes can produce the same effect provided the grapes don't roll apart, something Slepkov avoided using a concave glass.
The real story, Slepkov concludes, is suitably sized fruits. These “form resonant cavities that concentrate electromagnetic fields to extreme subwavelength regions.” He added: “The formation of plasma is due to electromagnetic hotspots.” These arise because resonances in the two halves of the grape cooperatively interact.
A single spherical object will become very hot, but it takes two hotspots in contact to cause the sparks that strip atoms of their electrons to make plasma. When the hotspots in each half of the grape are close enough and hot enough, sparks ionize sodium and potassium ions in the fruit, producing a plasma.
Crucial to the process is that the water, which makes up most of the grape, has a large index of refraction and low absorptivity for microwaves.
The work may even prove useful, as fruit in microwaves could provide an easier study model for phenomena previously only observed at nanoscales.