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Phase transitions in liquid crystals

A liquid crystal is a substancl that flows like a liquid but still maintains some of the ordered structure characteristic of crystals. Materials that qualify as liquid crystals are of anisotropic shape (usually rod-shaped or disc-shaped), and are called mesogens. Their shape is of particular importance because these material undergo ordering phase transitions. Liquid crystals can be divided into two broad categories: thermotropic and lyotropic. Thermotropic liquid crystal phases are formed by pure mesogens. Here phase transitions are a function of temperature and heat is either consumed or generated during the transition. Lyotropic liquid crystal phases arise from mesogens in a solvent. Here concentration controls the phase transition. Some of the most prominent liquid crystal phases are the following:

Nematic Phase: In the nematic phase the mesogens all point in the same direction, essentially expressing the molecular anisotropy as a phase anisotropy. This preferred direction of orientation is simply called the director of the nematic state. Experimentally the director can be observed with polarization microscopy or birefringence experiments.

Nematic.jpg

Smectic Phase: In this phase the mesogens have orientational as well as positional order. This means that not only do they point in the same direction, but they are also orderly layered perpendicular to the director. In a smectic A phase the molecules are, on average, normal to the layers. In a smectic C phase, the director is tilted with respect to the layers.

Smectic.jpg


Chiral Nematic (Cholesteric) Phase: In a cholesteric phase the nematic molecules have chirality, i.e. they have a preferential twist with respect to one another. The cholesteric phase was named after the cholesterol molecule, where this structure was first observed!

Chiral nem.jpg


Chirac Smectic (Smectic C*) Phase: In the chiral smectic phase, the tilt direction of the mesogens rotates as one progresses through the layers.

Chiral smec.jpg


Perhaps the most popular application of liquid crystals is the LCD srceen , short for Liquid Crystal Display. In this application, the anisotropy of liquid crystal molecules is exploited, since it leads to a susceptibility to electric and magnetic fileds. A less obvious and somewhat surprising application of the liquid crystal doctrine is in the biological field. The phospholipid and water mixture that forms the cell membrane is thought to be a liquid crystalline phase. Moreover, numerous cellular protein components are rod-shaped and in in vitro studies undergo isotropic to nematic phase transitions at critical concentration.

Text adapted from:

Hampley, I.W., 'Introduction to soft matter', Willey & Sons, England (2007)