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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