‘Electronic Paper’ promises more colourful displays

Researchers at the University of Rochester are racing to develop a technology that would not only make flexible, paper-like video displays a reality, but could make them in full colour.

The technology being developed at the University of Rochester is based on polymer cholesteric liquid crystal (pCLC) particles, known also as ‘flakes,’ which are dispersed in a liquid host medium.

These flakes in many respects resemble the metallic particles or ‘glitter’ that are used as pigments for automobile body finishes and decorative applications, and come in a variety of colours spanning the visible and near-infrared spectrum.

Unlike those more conventional particles, the apparent colour of pCLC flakes can be made to change or completely disappear as they rotate in an electric field. This rotation, or ‘switching,’ effect is the underlying principle for using pCLC flakes as the active element in image displays and other applications.

The flakes do not need the backlight used in typical computer screens because they reflect light the way a piece of paper does, thus a display using these flakes would use less electricity and could be easily viewed anywhere that a regular paper page can be read.

There are endless possibilities for surfaces that could be coated with ‘switchable’ pCLC flakes to produce continuously changing banners in a store window or rewriteable paper that also accepts computer downloads. Some ideas include camouflage for vehicles that changes with the terrain, switchable solar reflectors, or filters for instruments used in fibre optic applications and telecommunications.

The flake technology has some unique advantages when compared with other display technologies. For example, pCLC flakes are highly resistant to temperature variations, allowing them to be used in a much wider climate range than conventional liquid crystal displays.

Because only a very small amount of flake motion is required to produce a relatively large effect, the response time of pCLC flakes can be competitive with the standard liquid crystal displays found in today’s laptop computers, palmtop computers and other competing electronic paper technologies.

Several different electronic paper technologies are also under development in various labs around the world, and some are close to commercialization. Many can offer gray scale displays, but all have difficulties producing colour.

This is where pCLC flake technology has a distinct advantage given the plethora of colourful flakes available, claim the Rochester researchers. Additionally, whereas typical electronic paper technologies use absorption to produce colour and reflect light by scattering, the colour produced by pCLC flakes is based entirely on reflection and is inherently polarized. This unique capability of pCLC flakes is due to their liquid crystalline properties, and greatly broadens the scope of applications of the technology to other areas beyond information and image displays.

‘The ability to actively manipulate polarized light by means of an electric field is extremely useful for a large number of applications in optical technology, including switchable and tunable colour filters, optical switches for fibre optics or telecommunications, and switchable micropolarizers, in addition to information displays,’ says research engineer Kenneth L. Marshall, who heads the team developing the technology at the Laboratory for Laser Energetics at the University of Rochester.

‘The ability to produce this electrically-switchable polarization sensitivity in a material that can be conformally coated on flat or curved surfaces is one of the most unique and exciting aspects of this technology.’ Marshall sees other applications such as ‘smart windows’ that could change colour, reflect sunlight, or become completely opaque at will, environmentally robust switchable ‘paints,’ and even as ‘patterned’ particles for storage of encoded and encrypted information and document security. Other, more ‘off-the-wall’ application concepts could even include living room wallpaper that you can tune to different colours, or even to new patterns you download from the Internet.

But don’t expect to be finding these switchable pCLC flakes in products very soon. ‘There are a number of issues that need be solved first, like getting all of the flakes to move in the same direction at the same time,’ says Tanya Kosc, a doctoral candidate who presented the team’s work at an Optical Society of America’s annual meeting. ‘We don’t have complete control over flake motion yet. Sometimes a flake will flip completely over instead of stopping at the point in its rotation that we want it to.’

The size and shape of flakes largely influence how they move, but making uniform flakes is a difficult task. The initial method for producing flakes required melting the pCLC material and spreading it out at high temperature with a knife edge to form a thin layer or film. This motion also helps to align the pCLC molecules uniformly to produce the bright reflection of a given colour. The film is then fractured into tiny, randomly shaped flake-like particles by pouring liquid nitrogen over it.

The team has just created square, regularly sized flakes for the first time by moulding the material through the openings of a wire mesh. They’re working now on making the fabrication process more efficient and on measuring the behaviour of the new flakes in an electric field to understand the best ways to manipulate them.

‘When we finally are able to make them behave uniformly,’ says Kosc, ‘we’ll be able to think more about applications in actual devices.’

Understanding exactly how and why an electric field causes each flake to reorient is crucial to creating a system that can be used reliably for products that can camouflage a vehicle, store encoded information, or bring the latest news in full colour to a folded flexible film in your pocket.