Magnetically responsive liquid crystals enable novel display applications

June 30, 2014 // By Paul Buckley
Scientiists at the University of California, Riverside have constructed liquid crystals with optical properties that can be instantly and reversibly controlled by an external magnetic field and paving the way for novel display applications.

The crystals opens up the possibilities of applications relying on the instantaneous and contactless nature of magnetic manipulation - such as signage, posters, writing tablets, and billboards.

Commercially available liquid crystals, used in modern electronic displays, are composed of rod-like or plate-like molecules. When an electric field is applied, the molecules rotate and align themselves along the field direction, resulting in a rapid tuning of transmitted light.

“The liquid crystals we developed are essentially a liquid dispersion, a simple aqueous dispersion of magnetic nanorods,” said Yadong Yin, an associate professor of chemistry, who led the research project. “We use magnetic nanorods in place of the commercial nonmagnetic rod-like molecules. Optically these magnetic rods work in a similar way to commercial rod-like molecules, with the added advantage of being able to respond rapidly to external magnetic fields.”

On application of a magnetic field the nanorods spontaneously rotate and realign themselves parallel to the field direction, and influence the transmittance of polarized light.

The magnetically actuated liquid crystals developed by the Yin Lab have several novel advantages. First, they can be operated remotely by an external magnetic field, with no electrodes needed. (Electrical switching of commercial liquid crystals requires transparent electrodes which are expensive to make.) Second, the nanorods are much larger than the molecules used in commercial liquid crystals. As a result, their orientation can be conveniently fixed by solidifying the dispersing matrix.

Further, the magnetic nanorods can be used to produce thin-film liquid crystals, the orientation of which can be fixed entirely or in just selected areas by combining magnetic alignment and lithographic processes. This allows patterns of different polarizations to be created as well as control over the transmittance of polarized light in select areas.

“Such a thin film does not display visual information under normal light, but shows high contrast patterns under polarized light, making it immediately very useful for anti-counterfeit applications,” Yin said. “This is not possible