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Liquid Crystal Goes From Transparent To Mirrorlike With A Simple Jolt

Photonics: A tunable mirror reflects different parts of the spectrum with an applied voltage

by Neil Savage
October 9, 2014

: Photo of a liquid-crystal mirror switching from transparent to reflective.
Credit: ACS Photonics
A stack of two cholesteric liquid crystals allows a sign with the word “mirror” to show through when no voltage is applied (left). But with an applied voltage of 80 V, it reflects the eyes of a cartoon frog (right).

A new liquid-crystal device reflects different colors of light with increasing applied voltages, transitioning from mostly transparent to almost mirrorlike (ACS Photonics 2014, DOI: 10.1021/ph500259h). The tunable mirror might lead to smart windows that can block heat in the summer and let it pass through in the winter, or provide a way to switch a tablet screen between an indoor and outdoor setting, its developers say.

Credit: ACS Photonics
A stacked liquid crystal goes from transparent to reflective and back again as an applied voltage increases and decreases.

Timothy J. White of the Air Force Research Laboratory, in Ohio, and colleagues made the mirror out of materials called cholesteric liquid crystals (CLCs). These liquid crystals are made of molecules that arrange themselves into a helical structure. The range of wavelengths the CLCs reflect depends in part on the separation between molecules in the crystal. Usually CLCs reflect a narrow wavelength range of about 50 to 100 nm. White’s team used applied voltages to broaden this range to about 600 nm, wider than the visible spectrum, meaning the liquid crystals reflected enough wavelengths that they became mirrors.

The team prepared the CLCs with commercially available components used for making liquid crystals. They combined a liquid-crystal diacrylate monomer, dopants that make crystals with a right- or left-handed twist, and a mixture of compounds that serve as a host to keep the liquid-crystal molecules in the helical structure. In a typical liquid crystal with a different type of host, applying an electric current would cause the molecules to shift position. The researchers then added a small amount of a photoinitiator to the mixture and polymerized the material with ultraviolet light. The temperature at which they polymerized the CLCs dictated the center point of the wavelengths reflected.

Because a CLC will only reflect light with a polarization that matches the handedness of its helices, the researchers made a right- and left-handed crystal and then stacked them together to reflect all of the incoming light.

Photos of a liquid-crystal mirror reflecting different colors.
Credit: ACS Photonics
: A tunable liquid-crystal mirror reflects an increasingly broader range of colors as applied voltage rises from 0 V (top left) to 15, 30, 60, 90, and 110 V (bottom right). The black bar below each image indicates the part of the spectrum reflected.

As they applied an increasing voltage to the device, the reflected wavelengths broadened symmetrically around the center wavelength, altering the returned color. The researchers hypothesize that turning up the voltage physically stretches the polymer network, widening the gap between the molecules in the helical structure. To go from almost transparent to mirrorlike took an 80-V jump. The switching speed is very slow compared to standard liquid crystals—about 25 seconds to turn the reflection on and 10 seconds to switch it off. Applying an extra 15- or 30-V burst for a second or so at the beginning can speed those times to about two seconds, fast enough to change the properties of a smart window or to switch a tablet screen from transmissive to reflective. There may be ways, though, to speed up the material’s transition, which would be necessary for displays such as computer or TV screens, White says.

D. J. Broer, a chemist at Eindhoven University of Technology, in the Netherlands, who also works on CLCs, thinks the liquid crystals could be “a useful new approach for smart windows.” But he worries that using two stacked liquid crystals might prove expensive for buildings.


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