Flipping flakes change color

By Chhavi Sachdev, Technology Research News

Someday, we will all have computer screens we can fold up and tuck into our pockets. Researchers at the University of Rochester are trying to make that reality a colorful one.

Though color electronic paper won’t be available any time soon, it is becoming more feasible thanks to the researchers’ work with polymer cholesteric liquid crystal (pCLC), which could be used in electronic ink and a variety of other colorful applications.

Objects absorb certain wavelengths of light and reflect the balance, which we see as color. Depending on their composition and angle, the liquid crystal particles reflect only a specific color, said Kenneth L. Marshall, a research engineer at the Laboratory for Laser Energetics at the University of Rochester. The effect is like looking at an iridescent insect from different angles, he said.

The flakes could have a wide range of applications -- in smart windows to conserve energy or protect privacy, as a camouflage or decorative coating, and embedded in documents or objects for security, tagging and identification, said Marshall.

Each particle of polymer cholesteric liquid crystal is about 40 microns across, or about half the diameter of a human hair. The uniformly aligned flakes are suspended in silicone oil. Light striking these flakes is reflected back selectively, making them appear shiny. When an electric field is applied, the flakes rotate, changing angle and alignment, so that “the color appears very muted grey or even black,” Marshall said.

Coatings of the flakes could allow large objects like helicopters to change color. “If the coating contained flakes with different colors and the flakes of each color type were treated to respond differently to electric fields, one could switch different colors on and off in different areas,” Marshall said. “One could change the surface from one of high reflectivity, say green, to one of low reflectivity, for example... flat black.”

Cholesteric liquid crystals reflect different colors at different angles because their molecules form tiny spiral structures. The flakes are made of many of these spirals, and each spiral has a slightly different orientation than its neighbors, said Marshall. When light hits these structures, it splits into two circularly polarized, or spiraling, beams; one beam twists in the same direction as the spiral, and the other twists in the opposite direction.

The color of the particle comes from the reflection of the light component that corkscrews in the same direction as the spiral. If a spiral of the cholesteric liquid crystal material is the same size as the wavelength of green light -- about 543 nanometers -- then only green light will be reflected, said Marshall.

The physical and optical properties of these flakes make them well-suited for use in colored ink, said Marshall. They continue working despite large changes in temperature and they can withstand force that might otherwise cause misalignment, he said.

The flakes switch in 80 to 500 milliseconds, depending on their size and shape and the type of electric field used. This rate is comparable to that of conventional, or nematic, liquid crystal displays. The trick to forming letters, numbers and pictures is getting the flakes to orient consistently and in unison.

Both the type of electric field and the strength of the current affect the rotation of the flakes. With an alternating electrical current, the flakes rotate a full 90 degrees. With a direct current, the angular rotation is only about 5 to 10 degrees. A rotation of 5 to 10 degrees is enough to change a flake from reflecting a color to not reflecting any color, he said. “The particle charging is nearly instantaneous” and it changes back when the electric field is removed, Marshall said.

The general area of technologies for electronic paper “is an exciting one that could have enormous impact on consumer electronics,” said John A. Rogers, director of condensed matter physics researchat Lucent Technologies’ Bell Labs.The flakes are one of three or four emerging technologies thatcould be suitable for the ink part of a flexible pocket-display, he said. The other crucial piece is making the circuitry that would control the flexible screen, he added.

It is too early to say the liquid crystal flake technology is better than the rest, Rogers said. It is at “a very early stage of development, and it is not clear… that it offers compelling advantages" over the other approaches, he said.

The flakes could be very useful encapsulated in a solid film, said Sigurd Wagner, a professor of electrical engineering at Princeton University. “In the overall scheme of developing electronic books, the display technology is just one of many technical problems that must be solved. [This work is] one of many that will be needed,” Wagner said.

The researchers are working with the flakes “in very small quantities in order to understand the fundamental physics and switching mechanism,” said Marshall. Their aim is to more closely control the flakes' motion. “We have some measure of control over the angular flip, but we don't have complete control yet,” Marshall said. Part of the control problem is that the flakes are irregular in shape.

In theory, the different colors could be addressed individually or in combinations depending on voltage or frequency of the electric field, Marshall said.

They will also focus on encapsulating the flakes and host fluid in a flexible polymer binder in order to make plastic coatings with the color-changing property, Marshall said.

A full-color electronic paper prototype could be ready in two or three years, he said. It will take about five years to produce practical devices using the flakes, he said.

Marshall’s research colleagues were Tanya Z. Kosc, Stephen D. Jacobs, and Brett Klehn at the University of Rochester. They presented the research in the Novel Optical Materials and Applications Meeting at Cetraro, Italy, held between May 20 - 27, 2001. The research was funded by Reveo, Inc., and the Center for Electronic Imaging Systems at the University of Rochester.

Timeline:   5 years
Funding:  Corporate; University
TRN Categories:  Materials Science and Engineering
Story Type:   News
Related Elements:  Technical paper, "Polymer Cholesteric Liquid Crystal Flakes for Display and other Electro-Optic Applications," presented at the Novel Optical Materials and Applications Meeting at Cetraro, Italy, May 20 - 27, 2001.


November 14, 2001

Page One

Crossed nanowires compute

Disappearing links shape networks

Stored light altered

Flipping flakes change color

Evolution optimizes satellite orbits


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