Purer than the driven snow

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Today’s issue of Science contains a paper by Exeter University physicist Peter Vukusic and his colleagues, describing how the fine structure of a beetle’s scales produces one of the purest, most brilliant whites found in nature.

Here’s the abstract:

The colored appearances of animals are controlled by pigmentation, highly periodic ultrastructure, or a combination of both. Whiteness, however, is less common and is generated by neither of these, because it requires scattering processes appropriate for all visible wavelengths. We report whiteness resulting from a three-dimensional photonic solid in the scales of Cyphochilus spp. beetles. Their scales are characterized by their exceptional whiteness, their perceived brightness, and their optical brilliance, but they are only 5 micrometers thick. This thickness is at least two orders of magnitude thinner than common synthetic systems designed for equivalent-quality whiteness.

Vukusic’s team believes the beetle, which is native to south-east Asia, evolved such brilliant whiteness to camouflage itself amongst the white fungi which are abundant in its habitat. The colour of an object or surface is determined by the wavelength of reflected light, and ordered structures tend to reflect light of a specific wavelength. White light, however, is a combination of visible light of different wavelengths.

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Vukusic and his colleagues show that the beetle’s scales consist of completely random three-dimensional structures (shown in the scanning electron micrograph above), and therefore simultaneously scatter light of all wavelengths to produce the brilliant whiteness. They think industry will be interested in the findings, which could enable the whiteness of paper, plastics and paints to be greatly enhanced.

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