Beetle beauty captured in silicon

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Nov 20, 2010

Researchers in Canada have created a new material that mimics the brilliant iridescent colours seen in beetle shells. As the eye-catching effect can be switched off with the simple addition of water, the researchers believe their new material could lead to applications including "smart windows".

Structural colours, such as those on beetle shells and butterfly wings, differ from traditional pigments because the colour results from the interaction of light with periodic structures on the surface of the material. In certain biological materials, including the shells of scarab beetles, these exoskeletons take on a twisted or "chiral" structure, which causes reflected light to emerge circularly polarized.

Kevin Shopsowitz, working with colleagues at the University of British Columbia and FPInnovations, has now succeeded in mimicking this effect in a silica film. The breakthrough occurred with a certain degree of serendipity as the researchers were working with their industrial partner to develop forms of porous silica that could be used to store gases such as hydrogen. They were using nanocrystalline cellulose (NCC) as a template in silicon, which was then burned away to leave gaps within the silica.

But when Shopsowitz had forged the material, he discovered that is appeared to be iridescent. Analyzing the material with polarized optical microscopy (POM) revealed that the surface of the silica film had taken on a fingerprint-like texture during evaporation, with its associated spiralling pattern. Further analysis using transmission electron microscopy (TEM) confirmed that the individual nanocrystalline cellulose rods had organised into a "chiral nematic" structure.

"The eureka moment occurred when Kevin [Shopsowitz] discovered that the materials were iridescent," Mark MacLachlan, one of the researchers at the University of Columbia, told physicsworld.com. "Although NCC by itself forms iridescent films, we never thought it could be retained in the silica material."

Silica is usually a colourless material but modifying the surface in this way caused these films to reflect light at specific wavelengths. The researchers demonstrated that by changing the conditions of the synthesis, they could control how tightly wound the helix is (the pitch) and hence the wavelength of light that is reflected. In this way, they produced films that were a range of different colours.

What is more, Shopsowitz's team show that the iridescence can be turned off by the simple addition of water, before returning again when the material is dried out. They claim that this ability to switch between iridescent and colourless films, combined with the ability to control the pitch of the spirals, could be used to develop smart windows that respond to environmental conditions.

"It's fascinating research inspired by bio-mimetics," says Nicholas Roberts, a biologist at the University of Bristol, who specializes in neurobiology and sensory systems in nature. Roberts notes that liquid crystal chiral structures have been known for over 100 years and the similarities between cholesteric liquid crystals and beetle cuticles where noticed in the 1920s. "However, cholesteric liquid crystals are ordered fluids and the innovation here is to get the same self assembled structure be locked into something solid," he says.

The evolutionary significance of this ability of beetles is still not fully established. Writing in an article for the print edition of Physics World in August, zoologist David Pye of the University of London, UK, believes that – in the case of scarab beetles – it could be a tactic for improving communication within the species. It is widely accepted that these beetles take on bright colours to camouflage themselves within their forest environment: green for leaft backgrounds and metallic colours to imitate dappled sunlight. But if the eyes of these beetles have evolved to see polarized light, this would provide a system for these creatures to break the camouflage while remaining hidden.

This latest research is described in a paper in this week's Nature.

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