Balloons: where does the air escape from?

[jovan]

Having to make explosion sound effects, I bought a bag of party balloons. I had a few left over and passed them around to some friends and we started fooling around with them.

Everything was gay and fine until someone asked: "Hey! I wonder where the air escapes from?"

"Pray tell?" I answered.

"Well, you know how a balloon will dwindle after a few days, what I'm wondering is: does the air escape through the knot or through the skin of the balloon?"

"The knot!" I said.

"Nonsense, fool!" Objected a companion. "The air escapes through the pores of the balloon's skin!"

I laughed contemptuously, which had the unfortunate consequence of spawning a brawl.

Now that I've recovered from having been furiously pummelled with heavy air-filled party accessories, I look back soberly upon this unfortunate altercation and am not quite so sure anymore about my initial affirmation.

And so, I come humbly to you, denizens of the dope, and ask for the answer to this most nagging of questions:

Does the air escape a balloon through the knot or the skin?

[Podkayne]

Gasses escape from a balloon by diffusing through the rubber.

Here's an experiment (http://www.flinnsci.com/homepage/bio/smellyballoons.html) that you can use to prove it to yourself.

The idea is to put something fragrant inside the balloon (perfume, vanilla extract, whatever). Make sure you use a straw or something so that the smelly stuff ends up inside the balloon, not all in the neck. Then blow up the balloon and wait for the fragrance to escape. Does it smell stronger at the knot?

[Mort Furd]

Both, but mostly through the skin.

[Skammer]

And that's why those mylar balloons tend to last a whole lot longer than your ordinary rubber party balloon. The skin is much less pourous.

[Lute Skywatcher]

I believe Newton's Apple (http://www.tpt.org/newtons/) explained this, along with why balloons filled with helium don't last as long as ones filled with CO2. Not only does gas diffuse through the rubber, helium atoms are smaller than CO2 molecules which allows the helium to diffuse quicker.

[bbeaty]

I remember an electron micrograph of latex rubber in some old textbook. Rubber looks like a pile of spagetti smashed flat, with lots of gaps between the strands.

Hmmm. I wonder what would happen if you poured some liquid helium cooled to 1K onto a stretched sheet of thin rubber. Superfluid He can get through very small holes. Would it go right through as if the rubber wasn't there?