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3.10: Summary

Last updated

Nov 26, 2020
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- 3.9: Commercial Uses
- 3.11: Questions

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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=(3⟨cos2θ⟩−1)/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.

Going further

Books

Websites

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