A cholesteric liquid crystal is a type of liquid crystal

Liquid crystal

Liquid crystals are a state of matter that have properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of LC phases, which can be...

 with a helical structure and which is therefore chiral

Chirality (chemistry)

A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....

. Cholesteric liquid crystals are also known as chiral nematic liquid crystals. They organize in layers with no positional ordering within layers, but a director axis which varies with layers. The variation of the director axis tends to be periodic in nature. The period of this variation (the distance over which a full rotation of 360° is completed) is known as the pitch, p. The pitch varies with temperature and it can also be affected by the boundary conditions when the chiral nematic liquid crystal is sandwiched between two substrate planes.

Examples of compounds known to form cholesteric phases are hydroxypropyl cellulose

Hydroxypropyl cellulose

Hydroxypropyl cellulose is a derivative of cellulose with both water solubility and organic solubility. It is used as a topical ophthalmic protectant and lubricant.-Chemistry:...

 and cholesteryl benzoate

Cholesteryl benzoate

Cholesteryl benzoate, also called 5-cholesten-3-yl benzoate, is an organic chemical, an ester of cholesterol and benzoic acid. It is a liquid crystal material forming cholesteric liquid crystals with helical structure....

.

Some companies, such as Chiral Photonics

Chiral Photonics

Chiral Photonics, Inc. is a photonics company based in Pine Brook, New Jersey, founded in 1999. The company is developing a new class of optical devices based on twisting glass optical fibers. These in-fiber devices aim to displace discrete optical elements such as lasers, filters and sensors...

, have begun to explore CLCs as the basis for photonic devices.

The Industrial Technology Research Institute has developed a rewritable ePaper called i2R, which requires heat, and no electricity, to store imagery on a flexible cholesteric liquid crystal panel. ITRI

A US company, Kent Displays, has developed "no power" Liquid Crystal Displays

Liquid crystal display

A liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....

 using Polymer Stabilized Cholesteric Liquid Crystals: these are known as ChLCD screens. A drawback of ChLCD screens is their slow refresh rate, especially at low temperatures. Kent has recently demonstrated the use of a ChLCD to cover the entire surface of a mobile phone

Mobile phone

A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...

, allowing it to change colours, and keep that colour even when power is cut off.

Cholesteric liquid crystals are similar to nematic crystals with long axes lying parallel to each other in a suitable plane. The can be said as a twisted nematic structure, with a helical structure with a definite pitch between 200 nm to 20,000 nm.