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Engineering LibreTexts

3.10: Summary

  • Page ID
    35365
    • Liquid crystals are characterised by their high orientational and low positional molecular order.
    • Molecules capable of forming liquid crystals are always anisotropic – typically they will be calamitic (rod-shaped).
    • There are three types of calamitic liquid crystal: nematic, smectic and chiral nematic. They are defined by their differing degrees of positional order.
    • The degree of orientational order of a liquid crystal can be quantified using the \[\text { order parameter } Q=\left(3\left\langle\cos ^{2} \theta\right\rangle-1\right) / 2\]orderparameterQ=(3cos2θ1)/2
    • Defects in liquid crystals are given the name disclinations. Each type of disclination is assigned a positive or negative number; the magnitude indicates its strength whilst the sign indicates which disclinations can cancel each other out.
    • Disclinations can be viewed directly by polarised light microscopy. For example, in a nematic they appear as schlieren brushes.
    • Liquid crystals also exhibit birefringence when viewed through crossed polars.
    • The most common modern commercial use of liquid crystals is in liquid crystal displays.

    Going further

    Books

    • Peter J. Collings & Michael Hird, Introduction to Liquid Crystals: Chemistry and Physics, Taylor & Francis, 1997.
    • Peter J. Collings, Liquid Crystals: Nature’s Delicate Phase of Matter, 2nd Edition, Princeton University Press, 2002.

    Websites

    • Liquid Crystals: a Simple View on a Complex Matter
      A presentation covering the different liquid crystalline mesophases and their appearance under polarised light microscopy.
    • PLC Virtual Textbook
      Contains an introduction to liquid crystals and their phase transitions, including virtual experiments.
    • The Basics About Liquid Crystals
      A tutorial created by the Liquid Crystal and Photonics Group of Ghent University, Belgium.
    • Liquid Crystal Disclinations Seen Through Cross-Polars
      A simulation of the movement of schlieren brushes for different types of disclination in a nematic liquid crystal.
    • Introduction to Anisotropy
      A DoITPoMS TLP describing the anisotropy found in liquid crystals and other materials in further detail.
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