A material developed by researchers at the University of California, Riverside can take on any color of the rainbow, simply by the scientists changing the distance between the material and a magnet. It could be used in sensors or, encapsulated in microcapsules, in rewritable posters or other large color displays.
The researchers made the material using a high-temperature method to synthesize nanoscale, crystalline particles of magnetite, a form of iron oxide. Each particle was made about 10 nanometers in diameter because, as they get much larger than this, magnetite particles become permanent magnets, and therefore would cluster together and fall out of solution. The 10-nanometer particles group together to form uniformly sized spherical clusters, each about 120 nanometers across; in tests, these clusters have stayed suspended in solution for months.
By coating these clusters with an electrically charged surfactant, the researchers cause the clusters to repel each other. When researchers use a magnet to counteract the repellent forces, the clusters rearrange and move closer together, changing the color of the light they reflect. The stronger the magnetic field, the closer the particles, with the color changing from the red end of the spectrum toward the blue, opposite end, as the magnet gets closer to the material. Moving the magnet away allows the electrostatic charge to force the particles apart again, returning the system to its original condition.
“The beauty of this system is that it is so simple,” says Orlin Velev, a chemistry and biomolecular-engineering professor at North Carolina State University. “It can be used over large areas because it’s very inexpensive and very easy to make.” The work is published in the early online edition of the journal Angewandte Chemie.
A number of other researchers have developed color-changing materials, some of which are also controlled with magnetic forces; others use electrical or mechanical forces. The Riverside researchers, led by Yadong Yin, a professor of chemistry, however, are able to pack far more magnetic material per spherical building block that was previously possible. Sanford Asher, a professor of chemistry and materials science at the University of Pittsburgh who has encapsulated magnetite particles in polymer spheres, says that the new approach increases the amount of magnetic material by fivefold.
As a result, the new materials can be tuned to a larger number of colors than previously made materials. Indeed, North Carolina State’s Velev, who works on materials that change color in response to electronic signals, says he knows of no other material capable of taking on such a wide range of colors.