Chameleon-like 'opal' can take on any colour
A new material could give a chameleon a run for its money - it can rapidly change colour to match that of any in the visible spectrum.
The synthetic material can be likened to an opal, a mineral that owes its variety of colours to its layered structure: regions with a high refractive index, in which light travels slowly, are interleaved with regions with a low refractive index. Light waves with a wavelength – or colour – similar to that of the space between layers are scattered in a way that gives opal its iridescent sheen.
The chameleon-like "opal" developed by British and Canadian chemists has a similar layered structure. But their material goes one better than nature. It can rapidly shrink or swell to change the distance between its layered regions, changing the colour of light that it scatters (see video above).
The starting point for the new material is a stack of silica marbles, each 270 nanometres across, on a flat electrode. A polymer is added on top to encase the spheres and to hold them in place. Next, the spheres are dissolved with acid to leave behind a regular pattern of air pockets inside the polymer. Finally, these pockets are filled with a liquid electrolyte and the structure is sealed.
The result behaves just like an opal. The polymer and electrolyte have different refractive indexes, and their repeating pattern scatters only blue photons to make the material an iridescent blue. But when a voltage is applied, the material becomes red, flitting across every other colour in the visible spectrum along the way.
"The polymer is crucial to the whole thing," says Ian Manners at the University of Bristol in the UK, and a member of the research team. "It contains iron atoms which can exist in two oxidative states."
When a voltage passes through the flat electrode, it draws out electrons from the polymer and oxidises the iron. That leaves the polymer positively charged and so negative ions from the electrolyte flood in. The oxidised iron's chemistry helps the polymer absorb the liquid, and the structure swells.
The pores shrink as the liquid inside them moves into the polymer. As a result the structure now scatters photons of a different wavelength and so has a different colour.
Good on paper
"The more you oxidise the system the more it swells," says Manners. Increasing the voltage slightly leads to more iron being oxidised, more swelling, and a greater shift towards red.
"We can currently get full spectrum tuning - blue all the way to red - in a little under 1 second," says Andre Arsenault, a member of the team and Manners's former PhD student.
Arsenault is chief technology officer at Opalux, a company he founded with fellow chemists from the University of Toronto. "Given the current switching speeds, an ideal first product may be something like full-colour electronic paper," he says. Although a pulse of voltage is needed to shift the colour, maintaining it in a given state requires no energy at all.