Sunday, January 19, 2003

Bubbles 101

I woke up at the crack of dawn this morning when it was very quiet. I set some water to boiling and went into the other room. I knew I'd be able to hear it boil, but it seemed really loud.

That's no surprise to those of us who have worked at steam power plants. Steam is noisy stuff. Why?

When you hear a hiss, that's actually very high frequency sound, beyond shrill. OK, but why do we get that from the pot?

As you boil water, tiny bubbles form at the heated surface. Because of the slightly increased pressure at the bottom of the pan, the water must be heated a bit above the local atmospheric boiling point (212°F or 100°C is close enough for most of us).

Why do the bubbles form? That's not trivial - you might notice that below a certain size they don't seem to exist (likewise for drops of water - they'll only get so small by themselves). It's not your eyesight - smaller ones just don't exist long enough to see. As it happens, a number of small-scale forces such as surface tensionthat must be balanced to form a bubble, and they conspire to make it impossible to form stable bubbles below a certain size.

After formation, the bubbles try to rise through the bulk of the water. As they do the pressure against them decreases, making the bubble enlarge. But this increase in size increases the surface area. The increase in surface area increases the rate of heat loss from the bubble, and thus the rate of cooling of the vapor inside. Eventually the bubble becomes unsustainable and collapses, which generates noise and tremendous highly localized pressures.

A given bubble doesn't make much noise, but then there are thousands of them. The bubbles don't all go off at once, but they collapse at a very high frequency. This means that the corresponding noises are high frequency, which the ear perceives as a hiss.

Of course the bubble's heat is going to the bulk of the liquid above, and eventually the bulk is at about the boiling temperature. Then the bubbles can survive long enough to get to the surface and they get bigger as they rise. Now they're noisier when they pop at the surface, but the pops don't happen as often, so the character of the noise changes.

Why is it that when little bubbles collide they become a big bubble? Anytime you see something happening spontaneously it's because the final state has less energy or greater entropy than the initial state. This isn't the place to drag you through thermodynamics, but we can say that the bigger bubbles have less surface area relative to their volume than little ones do, and less surface area means less energy tied up in surface tension, so nature favors the bigger bubbles. The same logic applies to drops of water - they don't get smaller unless you force them to.

Had enough of bubbles yet? I haven't, and this is my blog, so you'll just have to suffer.

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