Unit 1- Class 6- Liquid Crystals (Application of liquid crystals)

Applications of liquid crystals

(i) DISPLAYS
All information displays utilizes the ability to control light, in order to function. By controlling what parts of a display are bright and what parts are dark, information is passed to the user. Regardless of the complexity of the display, the basic working principle remains the control of light from small area of the display. This can be done in two ways- active displays and passive displays

Active display
• Each area is equipped with the ability to generate light
• Different areas are made to produce light by hitting only these areas with the electron beam
E.g.: cathode ray tube (CRT), light emitting diode (LED)

Passive display
• Does not generate light by itself
• Controls the amount of light that passes through
• Utilizes ambient light
• Do not consume electrical power in order to generate light
• Ideal for battery operated equipment
E.g.: liquid crystal display (LCD)

In LCDs in order to enhance the difference in the brightness between dark (turned off) and bright (turned on) areas. Dichoric dyes are used to give desired coloures to the display with a good contrast



(ii) THERMOGRAPHY
Liquid crystals can be used to measure temperature utilizing the selective reflection property of chiral nematic liquid crystals when a light is incident on a cholesteric mesophase parallel to the axis of rotation of the helix, radiation of very small wavelength range corresponding .
gets divided into two beams one of the beam is reflected and the other is transmitted. If the wavelength range is in the visible region, the reflection give rise to light colours. The and hence the colour of the reflected light depends on the temperature of the mesophase. Temperature dependence of the colour reflected by liquid crystals has been utilized for application in thermography.



(iii) DETECTION OF AIR POLLUTANTS.
It has been observed that the colour of liquid crystals changes in the presence of impurities. When impurities diffuse in to the cholesteric liquid crystal film, the pitch is altered and hence the colour changes. This property is used in the detection of contaminants in the atmosphere.



(iv) Solitary wave propagation
A high intensity laser beam injected in a liquid crystal can produce a local reorientation of the director molecules. In this way the light produces its own wave-guide and the laser beam will not diffract but stays confined in a narrow beam. The soliton application can lead to an addressable liquid crystal wave-guide to switch light between several optical fibers.