Proc. Natl Acad. Sci. USA 116, 4000–4005 (2019)

Take a grape. Cut it in half, but leave a thin strip of skin connecting the two halves. Put it in a household microwave and turn it on full blast. Observe the ignition of a plasma.

figure a

Slepkov Biophotonics Lab, Trent University

This simple experiment has inspired many science-fair projects and has even generated its own dedicated category of YouTube videos, but the precise mechanism through which it comes about has never been fully understood. One plausible explanation is that the skin acts as a short dipole antenna and that the conducting ion-rich skin ‘bridge’ plays an important role — but so far this had remained untested.

Using a combination of thermal imaging experiments and finite-element simulations, Hamza Khattak and colleagues have uncovered an altogether more surprising explanation: the grapes form resonant cavities at their centres that concentrate microwaves to much smaller wavelengths, which leads to the creation of the plasma. The effect was known in nanoscale metallic objects, but it now looks set to become relevant to a range of different macroscopic objects at microwave frequencies.