Polymer opals could help to detect counterfeit notes

Researchers at the University of Cambridge have taken inspiration from natural gem stone opals to develop a flexible polymeric material that changes colour when twisted or stretched.

Instead of through pigments, the so called polymer opals get their colour from their internal structure alone, resulting in pure colour which does not run or fade.

As such, the researchers believe they could be used to replace the toxic dyes used in the textile industry, as well as in bank notes to detect fraud.

The polymer opals are constructed of spherical nanoparticles bonded to a rubber-like outer shell. When the nanoparticles are bent around a curve, they are pushed into the correct position to make structural colour possible.

The shell material then forms an elastic matrix and the hard spheres become ordered into a durable, impact-resistant photonic crystal.

"Unlike natural opals, which appear multi-coloured as a result of silica spheres not settling in identical layers, the polymer opals consist of one preferred layer structure and so have a uniform colour," said Professor Jeremy Baumberg of the Nanophotonics Group at the University's Cavendish Laboratory, who is leading the development of the material.

Like natural opals, the internal structure of the polymer opals causes diffraction of light, resulting in strong structural colour. The exact colour of the material is determined by the size of the spheres.

In addition, because the material has a rubbery consistency, when it is twisted and stretched the spacing between spheres changes, changing the colour of the material. When stretched, the material shifts into the blue range of the spectrum, and when compressed, the colour shifts towards red.

The researchers say the polymer opals can be produced in an industrially scalable way and laminated simply onto any appropriate substrate, including fabric, for applications such as security, brand protection and clothing.

They are now actively seeking a partner to take the process to the next stage.

Laura Hopperton

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