domingo, 22 de febrero de 2009

Chameleon-like ‘opal’ can take on any colour

A new material created by research scientists can rapidly shrink or swell to change the colour of light that it scatters.
British and Canadian chemists have developed a chameleon-like “opal” that can rapidly take any colour.
For creating it, the researchers first stacked up silica marbles on a flat electrode, each 270nm across. They then added a polymer on top to encase the sphere and hold them in place, and then dissolve the spheres with acid to leave behind a regular pattern of air pockets inside the polymer.
Finally, the team fill these pockets with a liquid electrolyte, and seal the structure.
The researchers say have revealed that the polymer and electrolyte have different refractive indexes, and their repeating pattern scatters only blue photons to make the material an iridescent blue.
However, according to them, 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. It contains iron atoms which can exist in two oxidative states,” New Scientist magazine quoted Ian Manners at the University of Bristol in the UK, and a member of the research team, as saying.
The researchers say that the flat electrode draws out electrons from the polymer and oxidises the iron when a voltage passes through it, leaving the polymer positively charged and thereby causing negative ions from the electrolyte to flood in.
They say that 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. Consequently, the structure scatters photons of a different wavelength, and thus has a different colour.
“The more you oxidise the system the more it swells,” Manner says.
According to the research team, 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.
“Given the current switching speeds, an ideal first product may be something like full-colour electronic paper,” he adds.
Although a pulse of voltage is needed to shift the colour, maintaining it in a given state requires no energy at all.
An article describing the research project has been published in the journal Angewandte Chemie International. (ANI)

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