What is the science behind grape plasma?

http://www.youtube.com/watch?v=_ux8nSWmAz0

and what factors determine how much plasma is produced and how long the effect lasts?

 

My guess is that the grape acts as antenna for the microwaves. Because of the shape of that grape, a very high field is created at the tips. This field is high enough, that the air is ionised, leading to a plasma. Without the glass, the plasma quickly thermalizes witht the surrounding air. With the glass the plasma exists longer.

How much plasma is produced should be determined by the time the grape is able to act as antenna. The plasma will damage the grape until the effect doesn't work anymore. There isn#t much one can do about that.

For a longer effect, I'd try to use a thicker glass that is able to contain the plasma longer.

This is all pure speculation, though.

 

Does it work with purple grapes?

 

Don't know if it's relevant but most fruit can, in the right circumstances act like a battery. The way the grape is cut seems to be crucial, with the little strip connecting the two 'halfs'. Could a potential voltage develope accross the grape? would this matter?

Also, some fruit gives off ethylene gas, could this be what's ionising?

Sadly, I haven't any grapes or a 1kw microwave, so you're on your own from now on!

 

Yes, the microwave generates an electric current across the electrolyte-filled grape. It heats the skin up to a couple thousand degrees, which turns the surrounding air into plasma. A side effect is the production of ozone, which is the poisonous gas mentioned in the video.

 

Sometimes when I try to do it, the grape fails to produce the effect seen in the video but ozone is still produced (maybe its another kind of gas but it smells really awful).

Also, putting certain types of glasses over the grape tends to reduce the effect. Maybe its because the glass is absorbing/reflecting some of the microwave energy?

 

Man in vid says you need to have at least 1000 watt microwave

I thought glass was pretty transparent to microwaves.

If you can't produce the same effect every time without the glass, you can't really be sure what the different glasses are doing to your experiments, when you choose to included them.

 

Microwaves could have 3 effects.

1) Energy absorbed by grape. Heating it till the grape ignites, or rather vapours released from the warm grape ignite above it.

2) EM oscillations interfere constructively above grape due to grape being approx same size as incident waves. Magnitude of becomes great enough spontaneously create plasma. I really doubt this because grapes don't reflect EM on 1cm wavelength. Interference wouldn't happen

3) EM oscillation induces motion of electrons in grape (ie. current). The bridge of grape acts as high resistance wire... Might be dissipating a few hundred watts across the skin strip. It gets hot enough to create plasma.


My money is on number 3. although if that's true you should be able to pull off the same stunt with a cm^3 of peach jointed by a piece of skin.

 

99% sure it's no3. Just tried it in my microwave, no plasma but I burnt the hell out of the central grape strip whilst the rest of the grape was largely unaffected

 

Dare you try another kind of fruit?

I think grapes might work so well because they're so juicy!

 

Maybe it's the power of your microwave? Microwaves that use less energy might induce a current through the grape skin bridge but fail to heat up the skin to a high enough temperature to produce the effect.

I've also noticed that when you do it with old grapes where the skin has started to rot and change color, the effect can be produced multiple times without the skin breaking down.