Supersaturation
When air or water holds more vapour or gas than it normally can; the excess condenses or bubbles out as soon as it has somewhere to start.
Air can hold only so much water vapour at a given temperature, and a liquid only so much dissolved gas. Push past that limit without anything happening and you have a supersaturated state, unstable and waiting for a trigger.
The trigger is usually something to start on. Water vapour finds it hard to form a droplet from nothing, but condenses readily onto tiny specks of dust, salt or smoke, called condensation nuclei; cloud droplets form this way. Electrically charged atoms, ions, work too.
That is how a cloud chamber reveals the invisible. Inside a sealed box the air is supersaturated with water or alcohol vapour. A fast charged particle, from a radioactive source or a cosmic ray, knocks electrons off gas molecules along its path, and droplets condense on the trail of ions, leaving a line of mist that lasts a few seconds. C. T. R. Wilson was inspired by the clouds and optical effects he saw on the summit of Ben Nevis in 1894, perfected his chamber in 1911 and shared the 1927 Nobel Prize in Physics for it. In 1932 Carl Anderson used one to discover the positron.
Snow crystals also grow from supersaturated air, and the amount of excess vapour, together with the temperature, decides whether they become plates, needles, columns or branching stars. Liquids behave in the same way: an opened fizzy drink is supersaturated with carbon dioxide, and bubbles form on scratches and specks in the glass.
Coming up in future stories
- A Mountaintop Rainbow Built the Particle Detector 1894 · Ben Nevis, Scotland
- The Emperor Who Sent Envoys to Learn Sugar 640s · Magadha, India and Tang China
- The Man Who Grew the First Snowflake on a Rabbit Hair 1936 · Hokkaido University, Sapporo, Japan