The other night I was reheating a bowl of leftovers in the microwave. There was a fair amount of liquid in the bowl, so I covered the bowl with plastic wrap when I put it in the microwave. When I put the bowl in, the plastic wrap was pulled tight over the rim of the bowl.
When I microwaved the bowl, the plastic wrap ballooned upward a huge amount (that makes sense, since the water started boiling). What happened next makes no sense to me. After the microwave shut off, the plastic wrap got pulled into the bowl and seemed to "shrink-wrap" the contents. I took the bowl out of the microwave, started peeling off the plastic wrap, and as soon as I made even a small opening in it the plastic wrap immediately shot upward and a huge amount of steam started escaping from the hole.
I haven't the faintest idea why the plastic wrap so rapidly shrank down onto the food in the bowl when I shut the microwave off and why it then reinflated with a huge burst of steam as soon as I made a hole in the plastic wrap. I've talked this over with a bunch of people and we can't seem to come up with a universally good explanation that accounts for all the different steps.
What exactly is going on here?
Thanks!
Best Answer
My guess is :
1) the temperature increases, some water turns into gas, thus the pressure increases. During this step, the film isn't impermeable, and some of the vapor probably flows out.
2) when you stop heating, the temperature decreases rapidly, and since there were some leaks, the pressure inside becomes smaller.
3) When you move it and make a hole, the liquid was probably superheated and boils suddenly thus releasing large amount of vapor.
Everything is only qualitative but it seems quite consistent in my opinion.
Edit for clarification : superheated fluids are fluids whose temperature/pressure is beyond their stability range. For example, liquid water at 105°C and 1atm, or the hand heater (the liquid bags that becomes solid when you press the piece of metal inside releasing heat). Microwave ovens are well known for producing supercritical water. The wikipedia page will be much more accurate.