The wellspring of new display concepts still runs strong. This Display Daily article will describe an intriguing new display technology just reported in the scientific literature.

Art Berman
Insight Media Consultant

The research was undertaken by a team of scientists headed by Yadong Yin, an Assistant Professor of Chemistry at University of California, Riverside. Other team members were from Seoul National University.

The new display is based on so-called magnetochromatic microspheres. The first step in the creation of the color switchable material is to prepare the microspheres. These are fabricated by mixing magnetic iron oxide particles into a UV curable resin. The resin solution is then dispersed in either mineral or silicon oil. The mixing process transforms the resin solution into spherical droplets.

An external magnetic field is then used to organize the iron oxide particles within the microspheres into periodically ordered chains. These chains will display a reflective color when viewed along the direction of the magnetic field. This is structural color, color developed by the interference of light rather than by pigments.

The color reflected the microspheres will change in response to a change in the direction of the external magnetic field. At this point in the fabrication of the material, the change in color is due to the reorientation of the ordered chains within the microspheres.

As the final step in the fabrication process, the material is exposed to ultraviolet light. This polymerizes the resin solution thus converting the droplets into solid microspheres. The microspheres are claimed to have excellent structural stability.

When a field is applied vertically, that is perpendicular to the plane of the film, the chains orient perpendicularly. This can be called the "on" state and the diffraction color can be observed from above. This condition is illustrated in the left hand portion of the accompanying micro photograph. The short blue wavelength microspheres have smaller average diameters than the longer wavelength red microspheres. When the field is applied parallel to the plane of the film, that is horizontally, the microspheres are forced to rotate 90 degrees and the diffraction is turned "off." The microspheres then show the brown color typical of iron oxide. This condition is illustrated in the right hand portion of the accompanying micro photograph. Depending on the direction of the external magnetic field, intermediate stages are also possible.

The solid state condition of the microspheres allows the color to be retained for an extended period of time without the need for application of an external field. That is, the resulting display is bistable and thus, consumes little power.

Since the display is reflective, it can be high visibility even in direct sunlight.

Another reason that the researchers believe that the new technology has great potential in a wide range of photonic applications is that the on and off switching times are reported as "fast."

A summary of the scientific article just published by the team is available on-line at URL: http://pubs.acs.org/doi/suppl/10.1021/ja903626h/suppl_file/ja903626h_si_001.pdf.

The article illustrates the device in a test apparatus in which switching is accomplished using the physical movement of permanent magnets. Clearly means of implementing a switchable magnetic field in a practical display device needs further development.

A short video illustrating the response of the optical material has been posted on YouTube at URL: www.youtube.com/watch?v=LKsgzk5quZU.

Potential applications for the new material are suggested in the article and include rewritable signage, posters, papers and labels, as well as magnetically activated security features. Yin also sees the potential for inexpensive, large outdoor displays. The research team reports that the material is almost ready for commercialization.