Author: Aniruddha Sharma
Introduction
Do you know that the plasma state of matter can be perceived in our homes? All we need is a grape and a microwave oven.
Firstly, we need to slice the grape such that the thin isthmus is still holding the two halves together. Then we need to place it in the microwave and irradiate the grape hemispheres in it. In this way, we can create plasma.
[Grape plasma. Courtesy: PNAS. https://doi.org/10.1073/pnas.1818350116]
We don’t necessarily need grapes; we can also use hydrogel water beads to get the same results.
Until now, a general explanation was that the skin acted as a short dipole antenna and that the conducting ion-rich skin “bridge" played an important role. But till the recent past, there was not a correct explanation known until a recently published paper.
The Grape Plasma Phenomenon
When the grape is placed in the microwave oven, the key thing is to observe how considerable the wavelengths are inside the grape.
The wavelength of the microwave inside the microwave oven is 12 cm, whereas it is approximately 1.2 cm inside the grape. Therefore, it can be observed that the wavelength of the microwave is 10 times lesser in the grape compared to its wavelength in the air.
When we place the grape in the microwave oven and pass the microwave through it, the waves get trapped inside the grape. This is because the grape has a huge size and high refractive index.
Total Internal Reflection
When the microwave gets trapped inside the grape, it starts bouncing inside the grape. The microwave, unable to escape through the walls of the grape, then starts to resonate modes inside the grape. This means they oscillate inside the grape and generate the maximum electromagnetic field at its center.
[Courtesy: https://doi.org/10.1073/pnas.1818350116]
We expect the grape to heat from outside, but in reality, it heats from inside, as we can see above.
Intersection of two grapes
When we intersect the grapes and pass the microwave through it, the strongest oscillating electromagnetic field forms at the intersection point of the grapes.
We can catch sight of sparks at the intersection point of the grapes, i.e. the area with the strongest electromagnetic field. This is because the strong electric field ionizes the air, thus creating sparks. These sparks lead to the formation of plasma. The ions which are generated get more energy from the microwaves.
[Courtesy: https://doi.org/10.1073/pnas.1818350116]
Applications
This phenomenon can be used in fabrication of semiconductor microchips and lithography techniques.