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| Soap Bubbles ~ Imaged by astrophotographer Göran Strand (web,blog). Each bubble reflects the surrounding scene twice, once upright and another inverted. All are suffused by shimmering colours. ©G�ran�Strand, shown with permission |
| The contraction force of surface tension increases the pressure inside the bubble. The smaller the bubble the greater the excess pressure. How then can bubbles ever form when a liquid boils? The pressure inside a zero radius bubble would be infinite! The difference in excess pressure with size show when two bubble intersect. More or less equal sized bubbles - as in G�ran's image have a near flat common surface. A small-large bubble union has a common surface bulging into the larger of the bubbles. |
| Foams get really complicated. Each surface pulls to minimise its energy. Calculating the structure of a foam that has the lowest energy is no trivial matter. Lord Kelvin in 1887 tackled the problem and considered a foam of equal volume cells. He derived an ideal low energy structure of packed 14-sided polyhedra, tetrakaidecahedra. There the matter stood until 1994 when Denis Weaire and Robert Phelan at Trinity College, Dublin derived a less regular structure of 12 and 14 sided polyhedra that had an energy 0.3% less than that of a Kelvin foam. Is a Weaire‑Phelan foam the least energy? We do not know. For a really fascinating account of foam physics try this video. |
Interference - The colours come from overlap and interference between light waves reflected from the closely spaced surfaces of the bubble skin. The skin is only a few wavelengths of light or less in thickness. In a particular direction, rays of one colour might be in-phase and bright while other colour waves are less in phase and dimmer. The interference colours shift and change with viewing direction and as the bubble skin evaporates and thins. |
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| Small bubbles are spherical because their skins "act" like sheets of elastic that try to reach a minimum energy configuration where the stretching is least - a sphere. Surface tension is the true agent, the unbalanced attraction at the surface between soap or water molecules. |
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Reflections - The two reflections arise from the bubble back and front sides. |
