The butterfly effect

The coloured brilliance of butterfly wings is the inspiration for research into patterned films designed to more effectively protect bank notes and other documents from counterfeiters.

A Cambridge University physics department team led by Prof Ullrich Steiner plans to accurately mimic the complicated light effects produced by butterfly wings, which change colour and intensity depending on the angle from which they are viewed.

The project aims to produce thin, brightly-coloured films that would be impossible to forge.

Steiner said: ‘Our motivation is biomimetic. In nature, organisms such as butterflies produce brilliant colours — far more brilliant than is possible using just pigments. The appearance changes when looked at from different angles because the surface’s microstructure reacts with light in different ways.’

Steiner said butterflies’ wings use a complicated interplay of different physical effects to create a brightly-patterned appearance. The wings use different multilayered periodic patterns ranging from a micron to a few hundred nanometres in length. Often other optical elements, such as tiny micro-prisms, are used to scatter the light in a particular way.

Steiner’s team aims to copy each of the optical mechanisms found in nature. He said the benefits of the proposed patterned films would make them a good alternative to the holograms used to protect banknotes or credit cards.

‘The effect of a hologram on a credit card is very weak and you have to look closely to check its authenticity,’ said Steiner. ‘Anything you have to check closely is not that much use. You could probably get away with a cheap copy of a hologram.’

The project will build on the group’s previous work developing surfaces with specific properties. This has involved using electric fields to destabilise polymer films, producing nanoscale periodic surface patterns. Different layers of material produce new films with certain light-emitting characteristics.

De La Rue, the world’s largest security printers, has expressed interest in the technology and is involved in consulting on the project.

The research is one of a number of biomimetic projects Steiner and his team have undertaken in recent years. One of these, funded by a German research council, produced an anti-reflective coating filled with small pores, based on the properties of a moth’s eye.

Anti-reflective coatings allow more light to pass through glass. Steiner said the automotive industry has shown great interest in such coatings’ possible application to dashboard displays. Dashboards are currently designed with deeply mounted instruments to reduce the effects of light on display screens.

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