Liquid Crystals
AimsBefore you startIntroductionOrder and disorder – molecular orientationOrder and disorder – molecular positionDefectsOptical properties – birefringence in nematicsOptical properties – birefringence in chiral nematicsOptical properties – observing defectsObserving phase transitionsCommercial usesSummaryQuestionsGoing furtherTLP creditsTLP contentsShow all contentViewing and downloading resourcesAbout the TLPsTerms of useFeedbackCredits Print this page
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Summary
- 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 \({\rm{order}}\;{\rm{parameter}}\;Q = {{(3\left\langle{{\cos }^2}\theta \right\rangle - 1)}} \;/\;{2}\)
- 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 viewedthrough crossed polars.
- The most common modern commercial use of liquid crystals is in liquid crystal displays.