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William Illiam Morris (the wallpaper designer, rather than the carmaker) suggested that nothing should have a place that is not known to be useful or believed to be beautiful. Opals, though, might be both. A group of researchers from the University of Southampton, in England, and the German Plastics Institute in Darmstadt, led by Jeremy Baumberg, have discovered how to create a plastic with the gemstone's iridescent properties. Their invention could be used to make a sparkling substitute for paint, banknotes that are hard to counterfeit and chemical sensors that can act as visible sell-by dates.
Opals get their milky sheen and rainbow sparkle from the way light is scattered by the tiny crystals that form them. These crystals are stacked in what is known as a face-centred cubic structure. This means that the constituent atoms are arranged in a lattice of cubes, with one extra atom sitting at the centre of each cube's six faces. Light entering this lattice gets bounced around in ways that generate colour by reinforcing the peaks of some wavelengths and cancelling out those of others.
For many years researchers have been trying to develop a synthetic material with the same light-scattering properties as an opal, by etching patterns into various materials. That approach has failed. Instead, Dr Baumberg has built his opalescent material from scratch. He and his team grew tiny polystyrene spheres until they were some 200 nanometres across, before hardening them with a blast of heat. They then coated the spheres with a sticky polymer before heating them again. As the mixture was baked, the spheres moved naturally into a face-centred cubic structure.
The result is a flexible film of crystals with opalescent properties that can be used to coat malleable surfaces, producing attractive iridescent hues. The size of the spheres can be tailored to scatter particular wavelengths of light—a useful property for security applications in which it is important that materials can be identified precisely. Moreover, when the film is warped, the spaces between the crystals change—and the colours produced change with them. These two properties make opalescent film an obvious material for currency. Banknotes containing it would produce distinctive colours when stretched, unlike counterfeits made from other materials.
To use the film to detect food spoilage, Dr Baumberg proposes adding a sprinkle of carbon particles even smaller than the polystyrene spheres. These would nestle in the spaces between the spheres and cause the material to scatter light from even more angles, making it yet more iridescent. This arrangement could be “tuned” to react to specific toxic chemicals. Food packaging made from such a material would thus change colour as the rot set in.
Such packaging need not be expensive. The polymer spheres and carbon particles arrange themselves spontaneously into the correct crystal structure when encouraged by a little heat, so manufacturing opalescent film should be easy. Indeed Merck, a German chemical company that was a partner in the research, has already produced rolls of the stuff a metre wide and 100 metres long. Perfect for wallpaper.
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