Peter Blair, of South Otago, asks :-

Why do bubbles in fizzy drink, or boiling water, stream from a small number of places?

John Campbell, a physicist at the University of Canterbury, responded.

Because a bubble needs something to nucleate on, such as a sharp point.

The pressure inside a bubble depends inversely on the diameter of the bubble. The smaller the bubble the higher the pressure.

For example, if you are careful enough to have a soap bubble of different size at the ends of a blocked straw, when you unblock the straw the smaller bubble will vanish and the bigger one grow bigger.

Once a bubble has formed it is possible for more gas dissolved in the liquid to come out of solution and the bubble to grow. The same principal applies to growing crystals of sugar. Once one has nucleated it grows rather than new small crystals appearing.

For a bubble to first appear in the middle of a liquid where it didn't exist before then the bubble would be tiny, say initially one atom only. The pressure inside would be so enormous that the amount of energy needed to initiate this is impossibly large.

If the solution is very clean it is more energetically favourable for a larger bubble to form at a defect, eg a sharp edge. If you look carefully at where bubbles are streaming from in a glass of fizzy drink or boiling water you will see a small defect such as a sharp piece of glass, a rough edge, a scratch on the pan, or a human hair.

You can observe this by sprinkling sugar or salt into a glass of fizzy drink. After opening, the fizzy drink contains more carbon dioxide than it can carry in solution but bubbles cannot nucleate inside the liquid, only where there are rough places in the container. The sprinkled sugar or salt provides many nucleation centres so the liquid immediately fizzes up.

For the same reason air is clearest after rain. Water vapour cannot form a water droplet without something to nucleate on, eg a dust particle. Water drop formation thus cleanses the air. Very clean air, such as used in a Wilson Cloud Chamber, requires ions produced by cosmic rays to form the nucleation centres.