[Marc]

Marc asked the Naked Scientists:
   
I've been trying to figure out what's going on with this...

When I use a microwave heat up food in a bowl covered in cling flim (and forget to punch a hole in the cling film!) - I've noticed that the cling film will billow up as the food heats up; but as soon as the microwave stops and the food is taken out, the cling film is sucked down onto the food - almost as if it is vacuum packed.

As soon as the cling film is ripped open - clouds of steam come pouring out.

So 2 questions:

Firstly: What is causing the cling film to be sucked down on to the food - is it something to do with the food cooling or air pressure or is it some sort of valve effect that's being created??

Secondly: Why doesn't the steam which is still inside the bowl keep the cling film ballooning out?

Any answers or thoughts would be appreciated.  Thanks.

What do you think?

[graham.d]

The bulging during heating is the trapped air expanding and then the extra pressure from steam. It is likely that some of this escapes otherwise I would expect the film to burst - you may think its tightly sealed but, as a comparison, a sealed pressure cooker would fail catastophically if it were not for the pressure release valve. Because the contents have outgassed, when cooled and the air contracts the film is drawn in; even though the seal is not perfect it still restricts the flow quite a lot.

[Geezer]

Mrs G claims it's unhealthy to microwave under cling film. Not sure if there is any truth to this, but hot plastics do give off some interesting compounds.

I usually use an inverted plate or bowl to trap the heat.

[RD]

... It is likely that some of this escapes otherwise I would expect the film to burst ...

I suspect the hotter the clingfilm membrane is the more permeable it becomes, so is bit like a one-way valve for hot gases.

http://en.wikipedia.org/wiki/Permeation#Permeation_in_everyday_life

[doppler1]

I agree with the whole steam being created and expanding creating the bulge but steam also gives off its heat very easily and can also contract quickly so it could be that as soon as the heat source is removed then the pressure caused by the creation of steam is reversed. When you open the film, the thing you are seeing is condensate in the hot air and not really steam.