Origin of bubbles explained

Alan Edwards, of Belleknowes, asked:

The tiny bubbles in a flute of bubbly white wine appear to originate where the glass curves, particularly at the pronounced curve at the bottom of the glass. What is the mechanism behind this?

John Campbell, a retired physicist at the University of Canterbury, responded:

Bubbles form at the sharp points of defects in the glass because it is energetically not possible to form droplets of infinitely small diameter from nothing.

The pressure inside a gas bubble is inversely proportional to its diameter, so the energy inside one of infinitely small diameter is so high as not to be possible. This applies to the formation of any small bubble, or water droplet, or single crystal.

We are used to some of the effects of the formation of water droplets, so let us start there. Air contains water vapour, which is gaseous water molecules. The higher the temperature the more water vapour (humidity) is contained, provided there is liquid water around, so vaporisation can occur.

When the air cools it can no longer support so much water vapour, so it gets rid of it by condensing it to water droplets. But that cannot happen in extremely pure air, so it becomes supercooled.

With the atmosphere, if there is an impurity present, usually a dust particle, the vapour can condense onto it and a drop builds up. The dust particle acts as a condensing surface and the water droplet grows to the size that falls, i.e.: it rains. That is why visibility is best after rain. The dust in the atmosphere is removed by the raindrops, so the atmosphere is much clearer.

This is also the principle of the Wilson cloud chamber, whereby superclean air contains more water vapour than is normally possible. When an ionising particle enters the chamber, the large number of ionised air molecules it leaves in its wake act as impurities, so the vapour can condense out as a droplet on each of these impurities and a cloudlike trail shows the existence of an ionising ray.

With fizzy wine, the yeast produces CO2 gas, which, when the wine is bottled, is the gas above the wine and so is under pressure and hence dissolved in the wine. When the bottle is opened, the pressure is released and the wine needs to release some gas from the now supersaturated liquid. Tiny bubbles can only form where there is a small sharp point.

If you look carefully at bubbles being produced in a recently opened bottle, or in the glass, you will see them streaming from a few select points, i.e.: where there is a sharp point.

If you have grown single crystals of copper sulphate, or sugar, or salt, you know that if you dissolve as much of the material in water as it can hold, it thereby forms a saturated solution. As the water very slowly evaporates, it wants to precipitate the excess as a crystal, which cannot form from nothing if the solution and container are free of defects. But if one crystal is present, it begins to grow. A larger crystal will grow at the expense of smaller crystals and so one can get a large single crystal.

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