The plasma physics of grapes in the microwave

Ever wondered why sparks fly when you microwave two closely spaced grapes? While this simple way of generating a spectacular plasma has intrigued the general public, there has also been a long-running debate about exactly what physics is involved in the process. Dr Kwo Ray Chu and his team at National Taiwan University have recently unraveled the physics behind it.

Plasmas are often visually spectacular: glowing showers of sparks that seem to dance and change with time. You might have seen plasmas in flames or during welding. They are also common in stars like the sun, where the high heat and pressure forms a plasma that emits heat and light.

Plasma can also be somewhat destructive. You might have been warned never to put metal objects in a microwave. This is because the microwave can induce a concentrated electric field at the edges of the object which can arc or lead to ionisation of the surrounding air – which is one way of forming a plasma.

Another rather impressive approach for making plasma in a microwave involves using grapes. If you put two grapes closely together in a microwave oven, then something unexpected happens when you start to heat the grapes. Rather than simply exploding as tomatoes are prone to do, the grapes spit out fantastic fiery jets of plasma that arise from the heating and ionisation of gases and seemingly hop between the two grapes (Figure 1).

This has been a household phenomenon of broad interest and there have been an abundance of theories looking to explain exactly how and why plasmas form in the connected grape system. Two of them are discussed here.

The grape dimer system in a microwave oven

A dimer is a system made up of two subunits. In the case of the microwave grape, the subunits in the overall dimer are the individual grapes. Each grape can be thought of as an aqueous, or watery, sphere.

A microwave oven works by forming standing electromagnetic waves through the reflection of the microwave radiation on the oven walls. Normally, you would use this radiation to cause the water molecules in food to vibrate, producing the heat that can be used to cook food. In the case of the grape dimer system, however, the grapes create a fantastic light show first. A casual microwave oven user finds it a curious phenomenon, while many scientists are interested in the workings behind it.

Research reported in the Proceedings of US National Academy of Sciences (PNAS)

In 2019, a team of researchers in Canada came up with the first scientific explanation of this ‘grape arcing’ phenomenon. They suggested that the grapes would act as resonant cavities – essentially like a device that would reflect the microwaves within the structure of the grapes and amplify certain frequencies of the microwaves. This amplification could lead to microwave-excited resonances forming a very strong electromagnetic hotspot in the region between the two grapes, hence the sparks (Khattak et al, 2019). Their paper has generated worldwide interest and was reported by Nature, Science, and over 70 news agencies (pnas.altmetric.com/details/55788275/news). It was also chosen by PNAS as one of the Top 10 Stories of 2019 (pnas.org/page/topten2019).

The thermal imaging results of their experiments did indeed show a local hotspot in the region between the grapes. By putting thermal paper between the grapes – which changes colour when exposed to heat – Khattak’s team could see ‘etching’ from the focusing of the microwave radiation by the grapes. However, the resolution of the etching seemed to be much, much higher than expected. There is a limit to how tightly focused electromagnetic radiation can be, and that focusing limit is dependent on the wavelength of the radiation. The small size of the etched spots suggested the electromagnetic radiation was being focused to a size much smaller than this limit, which is known as the diffraction limit.

Explanation by Chu, et al.

Due to the violation of the diffraction limit, Chu and his team wondered if the hotspot causing the etching might not be an electromagnetic hotspot as reported. Instead, they asked themselves if this was purely electrical in nature, meaning that these kinds of hotspots could be generated by an electrical hotspot rather than an electromagnetic one. The reason is as follows.

When any non-conductive material is exposed to an electromagnetic field, the electrons in the molecules are slightly displaced by the external electric field to form polarisation charges. These polarisation charges are of opposite signs on opposite sides of the grape gap and can reinforce each other (Figure 2) to build up a large enough field to cause sparks and discharges.

Why is the hotspot electrical in nature?

Chu’s team presented evidence in the form of two experiments and a numerical simulation to prove that the grape sparks are due to an electrical hotspot (Lin et al, 2021). First, they used two hydrogel spheres (7 mm in radius) to model the grape dimer system (hydrogel is a type of material that can retain large amounts of water) and heated the dimer system in a 27 MHz capacitor.

At such a low frequency, it is not possible to excite electromagnetic resonances inside the dimer. However, arcing still occurs in the dimer gap when the gap distance is sufficiently small (Figure 3). This indicates that sparks can indeed form without electromagnetic resonances in the spheres.

Second, following the demonstration of the electrical nature of the sparks, one would still ask whether sparks can also be triggered by amplification of the microwave radiation due to the formation of resonances in the spheres. The resonances would mean certain frequencies of microwaves would be reflected in the structure that could then interfere with other reflected waves, making them more intense. So, Chu’s team numerically simulated interactions of a 2.45-GHz wave with the same dimer system. Results indicate that the water spheres are now in strong electromagnetic resonances. However, the gap region has only an enhanced electric field, but with a negligible magnetic field (Figure 4). This shows that an electrical hotspot, rather than the widely publicised electromagnetic hotspot, explains the curious grape sparks in a household microwave oven.

Finally, Chu’s team performed a definitive experiment at 2.45 GHz to verify the electrical nature of the hotspot, based on the fundamental difference between electrical and electromagnetic hotspots. The former would result in an attractive force between the two spheres due to the opposite polarisation charges on opposite sides of the gap (Figure 2), while the latter would result in a repulsive force due to the radiation pressure of an electromagnetic hotspot. Figure 5 and the corresponding video clearly show that the two spheres move toward each other.

The attractive force between dielectric objects/particles examined by Chu’s team holds great promise for understanding how more complex multi-particle systems work under the influence of electromagnetic radiation.

Dr Ethan Siegel presented a 16-page article on this long-standing puzzle and concludes: ‘At last, a high-precision experiment has pinned down why, and it’s simply classical electromagnetism at work, not a complicated resonance’ (Siegel, 2021).