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.
The Riverside researchers found that processing the materials at high temperatures ensured that the 10-nanometer particles formed with a crystalline atomic structure. It also caused the particles to group together to form similarly sized clusters. In contrast, more commonly used room-temperature synthesis results in particles that form irregular agglomerations. The uniformity of the clusters and the crystallinity of the particles seem to improve the magnetic response of the materials, Yin says, although he and his colleagues are still looking into the underlying mechanisms involved.
The materials can switch colors at a rate of twice a second, which is still too slow for use in TVs and computer monitors. Yin hopes to increase switching speeds still more by using smaller amounts of material, perhaps in microscopic capsules. Such small amounts will make it easier to present a uniform magnetic field to the entire sample, potentially aiding the rearrangement of the clusters. Also, such microcapsules could be arranged to form pixels in a display, as is done now with E-Ink, a type of electronic paper used in some electronic book readers and a cell phones. (See "A Good Read.")
But even with faster speeds, Yin doesn't expect the materials to replace current computer-monitor technology. Rather, he has his sights set on larger-scale applications that would take advantage of the low cost of the materials. Examples could include posters that can be rewritten but don't have to change as fast as displays of video.
One significant drawback of the current materials is that they would need a constant power supply to preserve the magnetic field and hold the microcapsules at a set color. Yin's next step is to develop a version of the materials that remains stable after their color is changed--that is, until they're switched to a new color. If this is possible, then a poster could be printed with something like the read-write head on a hard drive, Yin says. It would preserve the image until it's rewritten with another pass of the print head, using no power in between.
"At this stage it's fun to play with," Velev says. "Maybe at later stages it could be used for some decorative purpose, such as paint that changes color, or some new types of labels or display boards. Right now it's a beautiful piece of research."
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