Cholesteric Liquid Crystals IAS Target

Cholesteric Liquid Crystals

07 Jun 2020

Category : Science and Technology

Topic: Cholesteric Liquid Crystals

Liquid crystals (LCs) are a state of matter which has properties between those of conventional liquids and those of solid crystals. For instance, a liquid crystal may flow like a liquid, but its molecules may be oriented in a crystal-like way. Within a domain, however, the molecules are well ordered. LC materials may not always be in a liquid-crystal state of matter (just as water may turn into ice or water vapor). Liquid crystals can be divided into thermotropic, lyotropic and metallotropic phases.
  • Thermotropic and lyotropic liquid crystals consist mostly of organic molecules, although a few minerals are also known. Thermotropic LCs exhibit a phase transition into the liquid-crystal phase as temperature is changed.
  • Lyotropic LCs exhibit phase transitions as a function of both temperature and concentration of the liquid-crystal molecules in a solvent (typically water).
  • Metallotropic LCs are composed of both organic and inorganic molecules; their liquid-crystal transition depends not only on temperature and concentration, but also on the inorganic-organic composition ratio.
Cholesteric liquid crystals are omnipresent in living matter under both in vivo and in vitro conditions and address the major types of molecules essential to life. In the animal and plant kingdoms, the cholesteric structure is a recurring design, suggesting a convergent evolution to an optimised left-handed helix.
Scientists synthesised a series of novel photo-sensitive cholesteric liquid crystals at room temperature for widest thermal range which can be used to make optical storage devices such as optically rewritable boards, advertising boards and so on. Most of the modern display devices such as computers, mobiles, TV screens, and so on are made up of liquid crystals. Cholesteric liquid crystals are special kind of materials which have the property of reflecting the light of wavelength equal to its pitch length, and this pitch length is temperature-sensitive.
Cholesteric liquid crystals are generally used as thermal sensors. If such cholesteric liquid crystals are made photo-sensitive, then these materials can be used for optical storage devices and other related applications. The devices made using these liquid crystals can be used in Syberia to Saudi Arabia where extreme temperatures were reported. These room-temperature liquid crystals can be used for creating optical storage devices, liquid crystal displays and so on.
The phenomena driving such optical storage device lies in tuning the molecules with light. Energetically more stable trans-state of azobenzene based photosensitive molecules turned to metastable cis configuration with the illumination of suitable UV light. Bringing them back to the original trans-state can be done using either by shining light of higher wavelength (~ 450nm) or by keeping it in a dark state. The later process is known as thermal back relaxation, where only room temperature is playing the role. The idea behind the phenomena is to increase such thermal back relaxation to retain the optically written images for a long time.
An optical storage device based on above phenomena was fabricated using one of these materials. The device has shown very high thermal back relaxation with good contrast between the illuminated region (dark state) and masked region (bright state). This process took almost 5 hours to relax back to its original configuration (i.e., trans-state). Such devices are extremely useful in creating rewritable advertisement boards where one can store the images for several hours and then can be rewritten or in some cases can be kept as permanent storage device. Then can also be used as optical rewritable boards for schools and colleges.
The roles and functions of biological cholesteric liquid crystals include maximisation of packing efficiency, morphogenesis, mechanical stability, optical information, radiation protection and evolution pressure. This invention will definitely help to bring down the cost of the devices with a very simple yet powerful photoisomerisation concept.