7th November 2003 at 00:00
Surface tension lends itself to tricks where molecular behaviour is responsible for visually impressive results. One example to try with any year, for fun or serious investigation, is the rocket boat. Take an equilateral triangle of aluminium foil about 3cm each side and cut out a 0.5cm square halfway along one side. Place the foil on the surface of some clean water in a tray, with the point opposite the cut-out square facing a stretch of clear water. Carefully drop one drop of washing-up liquid on to the boat-water boundary of the innermost edge of the square. The boat zooms across the water as the detergent molecules spread across the surface - they can only get out from the square through the back end of the boat, and so they jet out, forcing the boat forward.

Everything needs to be clean, so that the surface tension of the water can hold the boat and then be broken down by the soap.

Another experiment, which looks particularly impressive if colours are added, is to fill a glass up to the brim with water and then keep dropping water very carefully in so that you can see the curved surface of the water higher than the glass, held there by surface tension. When one drop of washing-up liquid is added it reduces the surface tension and the water streams over the edge in a mini waterfall.

Who did early work on surface tension? A letter to Nature published on March 12 1891 (www.physics. ucla.educwparticlespockelspockels.html) reveals that Agnes Pockels (1862-1935), a young German woman caring for relatives and isolated from the scientists of the time, experimented at home before sending her work to Lord Rayleigh, president of the Royal Society 1905-008, who won the Nobel prize for his work on the density of gases. Discuss whether great progress in science could happen in this way now. Why haven't we heard of Pockels? Find out about other women who did significant work in the wings of science, for example, Caroline Herschel.

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