LIQUID CRYSTALS


 P
hysics textbooks normally identify, as the three phases of matter, the solid, liquid and vapour phases. Liquid Crystals are a different state of matter which exhibit a degree of order between that of a fluid and a solid. For this reason they are sometimes known as mesomorphic phases, and the materials which can form such phases are called mesogenic. The molecules in mesogenic materials usually take a rod-like form.

 A normal fluid is isotropic, that is, its properties are independent of direction. The simplest liquid crystalline order is orientational, in that the molecules in the phase are primarily oriented in one special direction, known as the director. This anisotropic fluid is the nematic liquid crystal phase. A liquid crystal closer to a solid is the smectic phase, in which the molecules are arranged in layers, but within the layers the molecules have no fixed positions. Usually, a liquid becomes more ordered as it is cooled down (thermotropic materials), but some materials form liquid crystals in solution, in which case the order increases as the concentration of water is reduced (lyotropic materials).

 Liquid crystals have attracted interest for many reasons, some theoretical and some more practical. For mathematicians and theoretical physicists, liquid crystals are a natural laboratory for broken symmetries and the practical application of pure mathematical disciplines such as topology and group theory to physical problems. Under the microscope liquid crystals present the most wonderful coloured patterns which provided an enormous intellectual challenge before they were understood. For the engineer, cheap and compact liquid crystal technology provides the most promising replacement to the bulky cathode ray tube long used in computers and TVs. For the biologist, liquid crystal-like materials form the building blocks for much of the soft tissue out of which living cells and aggregations of cells are constructed. Liquid crystal science is truly interdisciplinary.

 In Southampton, research in liquid crystals is carried out in the School  of Mathematics, as well as in the schools of  Physics, Engineering Science and Chemistry . Within the School of Mathematics, the principal investigators are Dr Giampaolo D’Alessandro and  Professor Tim Sluckin.

 Recent research projects in the mathematics faculty have sometimes involved the molecular theory of fluids, sometimes the continuum theory of liquid crystals due to F.C. Frank and F.M. Leslie, and sometimes an intermediate scale. This last type of theory was pioneered by Lev Landau and applied to liquid crystals by Pierre-Gilles de Gennes, who won the 1991 Nobel prize in physics for his work. Yet other projects have involved the dielectric and optical theories of liquid crystal properties.

In these pictures we see some pictures of different liquid crystals under the microscope. In larger samples nematic liquid crystals are cloudy, but when they are cooled into the isotropic phase, they go clear at a phase transition known as the clearing point.
 

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Of particular interest in Southampton have been the properties of liquid crystals at surfaces (important in the design of liquid crstalline display devices) and the properties of defects in liquid crystals (for instance, see the point where the pale pincers seem to hold the black background above; this is the characterisitc signature of the nematic phase). In the core of a defect, the ususal anisotropic liquid crystalline properties no longer hold, and one is forced to use relatively advanced mathematical methods to tease out the local behaviour.
 

Here is a (schematic) picture of some liquid crystalline molecules. They are : (a) the cyano-biphenyl series, invented by Professor G.W. Gray FRS, which were the first liquid crystals to have a nematic phase in a temperature regime useful for applications; (b) para-azoxyanisole, the classic liquid crystal used in studies going back to the end of the 19th century; and (c) a spherocylinder; this is a  cylinder capped with hemispherical ends , and is a common idealisation used in mathematical studies.

 


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To view a computer simulation of liquid crystal behaviour, click here.  The movie, made by Dr. Martin Bates (now of York Chemistry Department),  shows a fluid of ellipsoidal molecules as it heats from a solid crystal, through a smectic (layered) state, then through the orientationally ordered nematic phases, until it reaches an isotropic fluid phase.

 

These are pictures of  liquid crystal in  a test tube Description: X:\hairdryer1.gifDescription: X:\hairdryer2.jpg

To view the whole movie of me heating a liquid crystal with the hair dryer beyond its clearing point, click here

To read an undergraduate level account of the liquid crystal phase, read the article
The liquid crystal phases: physics and technology, by Tim Sluckin,  in Contemporary Physics 41, 37-56 (2000).
For a zipped up beta-version of this article click  here.

The Liquid Crystal Group in Southampton has active international links, especially with groups in Italy, Slovenia, Romania, Japan and Ukraine.

Ph.D projects are available in Southampton in both theoretical and experimental aspects of liquid crystals.
 
 



Written by Tim Sluckin 11 October 2000

Updated 16 November 2012