READY for a brighter tomorrow? Colours produced by a material鈥檚 microstructure rather than its chemistry are promising to replace conventional paints and pigments paving the way for vividly coloured aircraft that never need repainting and much harder to forge banknotes.
It鈥檚 all part of a trend that began with the launch of 鈥渋nterference colours鈥� for cars in the mid-1990s. These iridescent paints make the vehicles appear different colours from different angles, creating a dramatic effect as they drive by.
The hues of most paints stem from chemistry. The pigment absorbs certain colours of light and reflects the rest, and it is the reflected colours that we see.
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By contrast, iridescent paints work through optical interference. This phenomenon creates rainbows on an oil-covered puddle and the striking colours on a butterfly鈥檚 wings. When white light hits an oily puddle, it reflects partially off the top surface, but also off the water at the base of the thin oil layer. Because one set of reflected waves travel slightly further, they fall out of phase with the other. How far out of phase depends on the wavelength, or colour, of the light 鈥� and the angle of reflection. Colours that are completely out of phase cancel out, while others reinforce and appear brighter.
One of the latest interference techniques emerged last year when Mool Gupta of Old Dominion University in Norfolk, Virginia, coloured a piece of aluminium vivid blue without using paint at all (Optics Letters, vol 28, p 2342). The blue was created by the structure engraved in the surface of the metal: a tessellating pattern of hexagons, each engraved with fine parallel lines using an electron beam.
Gupta speculates that his 鈥済ratings鈥� could be impressed onto large areas of aluminium using technology akin to that used to press CDs. Then the effect could be applied not only to small objects such as cameras, but to massive machines, including aircraft. Using patterns instead of paint would eliminate the weight of the pigment and the need for expensive repainting.
So far his technique works only for colours with short wavelengths, but Gupta is trying to create other types of surface that make reds and yellows shine out. Despite this limitation, simply creating a vivid, constant colour is an important step, says Chris Lawrence of UK-based research company Qinetiq. Until now, most colours created this way have varied strongly with small changes in viewing angle.
But the attention-grabbing colour changes already have uses. Lawrence has discussed with companies how diffraction gratings could be used to create eye-catching packaging. The technique is already in use in gift-wrap, on which fine lines embossed during manufacture use interference to create shimmering colour.
He also wants new interference materials to replace security holograms, which were added to credit cards not only because they were difficult to make but also because people noticed when they were missing or badly made. But now, says Lawrence, 鈥減eople are so used to holograms that forgers can get away with sticking on a piece of screwed-up tinfoil鈥�. He hopes interference colours can recapture the public鈥檚 attention.
Right now, some interference products are already making headway. These paints contain tiny flakes that are engineered to create colour in the same way as an oil-covered puddle (see Diagram). The first was launched in 1996 by Flex Products of Santa Rosa, California. Called ChromaFlair, the flakes are made of reflective material sandwiched between thin layers of glass, and create different colours from different angles. This paint is available on certain exclusive cars. For example, Ford will this year release a limited edition of its Cobra Mustang in a colour called 鈥淢ystichrome鈥�, which changes colour from topaz to blue, through green and to black.
鈥淭he biggest market is automotive,鈥� says Alison Gilchrist, a colour chemist at the University of Leeds, UK. 鈥淏ut they are coming more and more into everyday products.鈥� Some mobile phones come in a shade of ChromaFlair, for example. And the market is growing fast: US business analysts at The Freedonia Group expect demand for specialty pigments to double by 2012. And now that competing products from the German companies BASF and Merck are pushing prices down, the chameleon-like colours are set to appear in everything from packaging to nail polish. Look out for them.
