Why does my water freeze in this pattern like dandelion fluff (pictured above)?
Alex McDowell,
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London, UK
As the water freezes, it first forms a crust of ice at the surface and at the interface with the container, and dissolved gases migrate into the part that remains liquid. In the liquid part, the solubility of gases rises as it cools, but the liquid still becomes supersaturated and bubbles form.
Gases are insoluble in ice, hence all the remaining dissolved gases precipitate out as the core finally freezes. This is why the ice cube is transparent in places where it first freezes, with a layer containing some bubbles and a core that contains many bubbles.
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David Muir,
Edinburgh, UK
A glass of water in a freezer starts freezing from the outside and this works its way to the centre. Liquid water can hold more dissolved air than ice can, so, as water freezes, air and other impurities are driven into the liquid water.
The first ice to form next to the glass is very pure with no bubbles, so it is transparent. As the ice-water interface moves inwards, the concentration of air in the water increases. When the concentration reaches supersaturation, bubbles form between the ice-water interface and microscopic solid pollutants (dust), which act as nucleation points. These bubbles are captured by the advancing interface, which continues to carry the solid nucleating particles on its surface. This results in rows of bubbles converging towards the centre of the glass.
The number of bubbles increases and their size decreases with faster ice formation, so the cloudiness at the ice’s centre is due to many tiny bubbles being present, indicating that the centre froze much more quickly than the periphery.
For readers who want to know more, an was published by Norikazu Maeno at Hokkaido University, Japan, in 1967.
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