Hasta La Gadget!

Research into color-changing nanoparticles could pave the way for a new kind of display technology. A breakthrough promises tiny molecules that can change color in response to an external magnetic field that can be used to create outdoor displays and posters.

“We have developed a new way to induce color change in materials that can be fabricated on a large scale and is pretty close to commercialization,” says Yadong Yin, an assistant professor of chemistry at University of California, Riverside, who led the study that included contributions from South Korean scientists.

The technique centers on polymer beads, called magnetochromatic microspheres, which are dispersed in a liquid such as water, alcohol or hexane.

Inside the beads are magnetic iron oxide nanostructures. Changing the orientation of the nanostructures with an external magnetic field helps produce the change in color of the beads.

The process is similar to the way electrophoretic displays, more commonly known as electronic ink, work. The two systems share common properties such as being bistable (stable in two distinct states), being readable in direct sunlight and consuming very little power.

To fabricate the polymer beads or microspheres, researchers mixed magnetic iron oxide particles into a resin. The resin solution was then dispersed in either mineral oil or silicon oil, which transformed the resin into spherical droplets in the oil. An external magnetic field organizes the iron oxide particles into periodically ordered chains that display a reflective color if viewed along the direction of the magnetic field.

“For instance, in a vertical field, the particle chains stand straight so that their diffraction is turned ‘on’ and and corresponding color can be observed from
the top,” say the researchers in their study.  When the field is switched horizontally, the microspheres are forced to rotate 90 degrees to lay down the particle chains so that the diffraction is turned off. The microspheres then
show the native brown color of iron oxide.  Depending on the direction of the external magnetic field there can also be intermediate stages.

As the final step, the liquid system which holds the particles is exposed to ultraviolet radiation to polymerize the resin droplets and make them into solid microspheres. This allows for switching between two states. The solid state allows for the color information to be frozen and retained for long times without the  need for additional power.

Yin did not explain exactly how many colors can be obtained from the display but said the system can handle a reasonably wide range, though switching to colors at the opposite ends of the spectrum could be a challenge.

The researchers published the result of their study in the latest issue of the Journal of the American Chemical Society.

Yin sees applications such large outdoor displays that can be expensive to do with LCDs or other display technologies. “If you want a huge LCD display outside the house it can be uneconomical,” he says. “We can do it for much cheaper with this new technology.”

The displays are reflective, so they can offer high visibility even in strong sunshine, says Yin. The new material also can be used to make environmentally friendly pigments for paints and cosmetics.

Here’s a quick video that shows the rotation of the microspheres in a vertically changing external magnetic field. The color is switched between on (blue) and off states.

Photo: Colorful microspheres/University of California, Riverside