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The image shows a rotation of the director about 180° in a cholesteric phase. The corresponding distance is the half-pitch, p/2.

A cholesteric liquid crystal is a type of liquid crystal with a helical structure and which is therefore chiral. Cholesteric liquid crystals are also known as chiral nematic liquid crystals. They organise 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 is varying 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 and cholesteryl benzoate.

Some companies, such as Chiral Photonics, have begun to explore CLCs as the basis for photonic devices.

A US company, Kent Displays, has developed "no power" Liquid Crystal Displays 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, allowing it to change colours, and keep that colour even when power is cut off.[1]

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.[2]

References

  1. ^ Tetsuo Nozawa. "[SID] Entire Surface of Handset becomes LCD Display". Nikkei Tech-On. http://techon.nikkeibp.co.jp/english/NEWS_EN/20090609/171529. Retrieved 2009-06-10.  
  2. ^ Television and video Engineering 2nd edition, Tata McGraw Hill, A M Dhake ISBN 0074601059
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