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Does this actually work? If so, why?

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Has anyone tried this with non-circular shaped pots? Does it still work? –  Geremia 2 hours ago

2 Answers 2

up vote 13 down vote accepted

It sometimes works and sometimes doesn't.

Making a foam costs energy because it increases the area of the air/water interface, so foams are thermodynamically unstable. The reason a foam develops is because surface active molecules are present, and these adsorb at the air/water interface. The surfactant stabilises the air/water interface because the rate it desorbs from the interface is slow i.e. they create a kinetic barrier to collapse of the bubble walls (NB it's a kinetic barrier: the foam is still thermodynamically unstable even when a surfactant is present).

An obvious example of a surface active agent creating a foam is bubble bath, which typically contains the surfactant sodium lauryl ether sulphate. SLES is a synthetic compound, but many foods contain natural surfactants like lipids and proteins. This is why boiling pasta creates a foam - if you tried boiling distilled water you would find that it does not foam.

So what then is the role of the spoon. Well it does two things:

Firstly remember that the foam is thermodynamically unstable, so if you can puncture the bubble wall the bubble will collapse just like a burst balloon. Typically foam bubbles can be punctured by touching them with something hydrophobic. Automatic washing powders include silicone oil (typically dispersed onto silica) to burst the foam and stop your automatic washing machine from foaming at the mouth. Lab demonstrations usually demonstrate the effect using PTFE powder. The mechanism of the bubble bursting is simply that the water contact angle on a hydrophobic surface is very large so the water retreats from the contact.

So if the surface of your spoon is hydrophobic it will burst any bubbles that touch it. However if whatever you're boiling has a powerful surfactant the surfactant will simply adsorb onto the wooden spoon and render it hydrophilic, and the bubbles will stop bursting. So how well the spoon bursts bubbles depends on the spoon and on what you're boiling.

The second mechanism is a lot simpler. In the foam from boiling water the bubbles are filled with steam, not air, and if the bubble touches anything that is below 100C the steam will immediately condense and this bursts the bubble. You can see this effect by blowing on the boiling foam. Assuming your breath is colder than 100C you'll see the foam retreat.

So the wooden spoon will burst bubbles simply by being cold (or at least colder than 100C). But of course the spoon is above a pan of boiling water so it will eventually heat up and stop bursting the bubbles. Once again the spoon may or may not have a big effect depending on how fast it heats up. Allegedly metal spoons don't work because they heat up too fast, though I must admit I've never tested this myself.

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It pops the bubbles. Any sort spoon (long enough) would work as well.

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I agree that the mechanical popping of bubbles as they form against the spoon should be dominant, so the type of spoon should not matter. By having the spoon in the middle, the foam will be collapsing inward, instead of over the sides. A metal grid would be very effective in containing foam, imo. –  anna v Jun 30 '12 at 10:57

protected by Qmechanic Mar 6 '13 at 23:14

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