Bubble wrap
During the recent cold spell I stopped to admire the ice in a central fountain in a courtyard in Washington DC (see photo, top left). I noticed that the ice contained bubbles trapped in successive layers. I have seen this before but I was surprised to see that the bubbles were in a clear three-dimensional layout (see photo top right). First, the bubbles were arranged vertically as if they were a vertical string of beads with each bead a different size, suspended one below the other. Secondly, each string was arranged in a grid pattern which matched exactly the grid pattern of the darker grout lines of the blue-tiled fountain bottom. The bubbles appear to have been captured as the ice grew thicker. Where did the bubbles come from and how were they captured in the ice? I don鈥檛 know whether it is significant, but the fountain is built above a closed underground garage.
鈥� This question and the one that follows concern two very different phenomena. In the case of the frozen water in the tiled fountain, gases that readily dissolve in water are eventually forced out of solution during the freezing process. But why do they end up in this particular pattern? When a mass of water freezes, the outer layers tend to be clear, while bubbles concentrate in the centre, or whichever part freezes last 鈥� you can see this in the ice cubes in a household freezer.
In the fountain the ice will form on the surface and grow downwards. As it does so, the water below becomes supersaturated with gas. Bubbles will then start to form, but they will always nucleate best on sharp corners or rough surfaces. So because the smooth tiles are separated by roughened lines of grout, the bubbles form mainly on the grout then rise up towards the ice ceiling.
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As the process continues, more air comes out of solution under a descending ceiling of ice and the bubbles are trapped to form curtains above the grout lines.
David Stevenson
Newbury, Berkshire, UK
Wrapped bubble
Amazingly, when I got home from Washington DC I found another strange bubble phenomenon (see photos, bottom left and right). This narrow spiral air bubble was trapped in a frozen puddle in Weardale, County Durham. Nearby puddles of about the same size had typically a dozen or so irregular sausage-shaped bubbles, usually wider than this spiral but displaying a similar concentric nature to varying degrees. The one photographed was the most striking, however. How and why did it form?
鈥� In this case, dissolved gas is not the issue. The 鈥渂ubbles鈥� are the result of the puddles sitting on porous ground.
As the puddle begins to freeze, the water is still draining away under the ice. If the ice cover is thin enough the unsupported surface sags. A meniscus can form between the ice cover and the water below, and this then freezes to form a bridge to the ground.
The freezing tends to draw water up from the ground through these menisci because they conduct heat better than the air of the bubbles in between. The result of this process is usually a series of large, flat, frozen airspaces, but in this case there has been an unusually long progression of a freezing meniscus that has followed the line of contact with the receding water under the concave ice cover.
The rate of freezing and the rate of drainage of the water must have been just right for this spectacular, spiral figure to form.
David Stevenson
Newbury, Berkshire, UK
This week鈥檚 questions
Wrap up well
Recently I was flying at 12,000 metres and 800 kilometres per hour through air at a temperature of 鈭�50 掳C. The wind chill should have been horrendous, but luckily I was in an airliner. However, the cabin walls were only 10 centimetres thick. What insulation is used, and can I have some for my sea-level home?
Philip Welsby
Edinburgh, UK
Rubber horror
Why do rubber bands spontaneously melt? Often I find an ageing one on my desk that has turned into a sticky mess. After a few more months, the sticky mass solidifies and becomes brittle. Why?
Stuart Arnold
Munich, Germany