Magnetochromatic polymer beads to pave way for large size inexpensive outdoor displays

Microscopic polymer beads that change color instantly and reversibly when external magnetic fields acting upon the microspheres change orientation, have been fabricated by a research team at the University of California, Riverside (UCR). This research work promises a novel kind of display technology for large sized rewritable outdoor displays and posters which will be inexpensive compared to its present day counterparts. The beads or �magnetochromatic microspheres� have excellent structural stability. They are also highly compatible with various types of dispersion media such as water, alcohol, hexane and even polymer solutions, allowing them to retain magnetically tunable colors in a variety of chemical environments. As opposed to the traditional methods, the instant and reversible color change in polymer microspheres takes place without any change in their structural and intrinsic properties. The research work provides a new mechanism for inducing color change in materials. As a result, bistable photonic materials with switchable colors can be designed on a large scale. The mechanism involves dispersing these magnetochromatic microspheres in a liquid like water, alcohol and hexane or polymer solution. These microspheres contain iron oxide nanostructures which rotate owing to the external magnetic field acting on it, thereby rendering an instant color change in the polymer beads. In their lab experiments, the researchers embedded arrays of spatially ordered magnetic iron oxide nanostructures within each polymer microsphere, enabling its colors to be switched on and off simply by changing the microsphere�s orientation � or more precisely the orientation of the array. Furthermore, the new system has the advantage of producing bistable color states, required for making rewritable displays.
Applications of the new material include display type units such as rewritable or reusable signage, posters, electronic papers and labels, and other magnetically activated security features. The new material also can be used to make environmentally friendly pigments for paints and cosmetics, as well as ink materials for color printing. �The new technology has a great potential for a wide range of photonic applications because the on/off switching of the diffraction color by the rotating photonic sphere is fast, greatly simplifying the pixel structures,� said Seoul National University�s Kwon, whose lab collaborated on the research. Yin explained that the color observed in the new materials is �structural color� because it is caused by interference effects rather than pigments. The conventional methods for producing tunable color changes depend on tough-to-achieve and slow process techniques like changing the periodicity of the array or the refractive index of the materials. In our method, the color is tuned by changing the relative orientation of the periodic arrays in the microspheres by conveniently using external fields. The use of magnetic fields as external stimuli has the additional benefits of instant action, contact less control and easy integration into electronic devices already in the market.
To fabricate the microspheres, the researchers first mixed magnetic iron oxide particles into a resin, which is initially in liquid phase but later turns solid on exposure to ultraviolet curable resin. They then dispersed the resin solution in oil (mineral oil or silicon oil), whereupon the resin transformed into spherical droplets in the oil. Next, the researchers applied an external magnetic field to organize the iron oxide particles into periodically ordered structures. These structures display a reflective color if viewed along the direction of the magnetic field. Finally, the research team exposed the liquid system to ultraviolet radiation to polymerize the resin droplets and make them solid microspheres.