June 18, 2009: Researchers at the U. of California-Riverside and Seoul National University have fabricated microscopic polymer beads that change color according to changes in magnetic fields, touting their possible use in reusable signs, magnetically activated security features, and environmentally friendly pigments.
The “magnetochromatic microspheres” are said to be compatible with dispersion media including water, alcohol, hexane, and even polymer solutions, able to retain magnetically tunable colors in various chemical environments. They have “excellent structural stability” with the color change not affecting their structure or intrinsic properties, according to UC-Riverside’s Yadong Yin, assistant professor of chemistry.
Their work appeared in the June 15 online issue of the Journal of the American Chemical Society.
In the lab, researchers mixed magnetic iron oxide particles into a resin (a liquid that solidifies on exposure to UV-curable resin), and dispersed the resin in oil where it transformed into spherical droplets. An external magnetic field was applied to organize the iron oxide particles into periodically orderedstructures , which display a reflective color if viewed along the direction of the magnetic field. Exposing the liquid system to UV radiation polymerized the resin droplets to make them solid microspheres. Changing the orientation of the array “switched” the colors on and off via interference effects, rather than pigmentation (think the color schemes seen in some birds, butterflies, and beetles). The color states are also bistable, which is required for rewritable displays.
“Conventional methods to produce tunable structural color rely on changing the periodicity of the array or the refractive index of the materials — changes that are difficult to achieve or involve slow processes,” said Yin in a statement. In the UC-R/Seoul method, the color is tuned by changing the relative orientation of the microspheres’ periodic arrays through stimulation of external magnetic fields — which, he added, “has the additional benefits of instant action, contactless control, and easy integration into electronic devices already in the market.”
The researchers next plan to further explore specific applications for the magnetochromatic microspheres. Candidates include rewritable energy-saving display units such as papers and posters; future work will target development of a similar material for chemical and biological sensors.
Optical microscopy images of magnetochromatic microspheres with different diffraction colors switched “on” by using external magnetic fields. (Image credit: Yin lab, UC Riverside.)