The increasing strength of a magnetic field has been used to change the colour of a solution of iron oxide from red (top left) through orange and green to violet (bottom right).

Credit: Yadong Yin

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ADELAIDE: Chemists have created a liquid that can be made to change to any colour of the rainbow by applying a magnetic field to it. It has the potential to make inexpensive colour display screens.

The liquid is a solution of magnetic nanoparticles of iron oxide, which can be persuaded to change colour by tuning the strength of a magnetic field applied to them. The colour change is rapid and fully reversible.

“By reflecting light, these crystals – also called photonic crystals – show brilliant colours,” said chemist Yadong Yin who led the research at the University of California in Riverside, USA. “Ours is the first report of a photonic crystal that is fully tunable in the visible range of the electromagnetic spectrum, from violet light to red light.”

When suspended in water, the specially designed iron oxide nanoparticles cluster and self-assemble into crystals, according to the paper published by the researchers in the latest edition of the German chemistry journal Angewandte Chemie.

Rewritable electronic paper

Nanoparticles are particles less than one millionth of a meter wide. By comparison, a sheet of paper is about 100,000 nanometres thick. If a magnetic field is applied, the nanoparticles change their arrangement. Different arrangements and spacing of the crystals change the wavelengths of light that the crystals reflect, so we see different colours of light.

High quality, colour display screens could be made using millions of small pixels containing the photonic crystals, according to Yin. Each pixel could be made into a different colour by applying a magnetic field, potentially to form large-scale display boards such as posters and billboards. These displays would reflect light, unlike LCD screens, which are lit from behind.

“What should make the technology commercially attractive is that iron oxide is cheap, non-toxic and available in plenty,” said Yin. Other potential applications include ink that can change colour magnetically, and rewritable electronic paper, he said.

“This is an elegant method that allows researchers in the field to assemble photonic crystals and control their spacing by using a magnetic field,” commented Orlin Velev, who studies nanoparticles at the North Carolina State University in Raleigh. “A simple magnet can be used to change the colour of a suspension through the whole visible spectra. This has potential to result in usable precursors for various photonic devices.”