Research at CMMPEMaterialsIntroduction to liquid crystals (page 1 of 4)

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Jump directly to the following sections: Phases of matter - What is an LC?


1. Phases of matter

Conventionally, matter can exist in four different forms, or phases:

In solids, molecules are rigidly held together by intermolecular bonds, giving rise to materials that are firm and maintain their shape. Intermolecular interactions give rise to an often highly ordered crystalline form. In the general case of asymmetric molecules, the overall lattice structure is anisotropic (ie: appears different when viewed from different directions).

When a solid is heated, thermal energy causes many of the inter-molecular bonding to break down. Molecules are still in close proximity to each other, but are permitted to slowly flow over and around each other. Molecular orientation is therefore random and isotropic. The material exhibits fluidic properties, and is said to be in the liquid phase. Liquids (under gravity) will try to flow to cover the surface of the containing vessel, although this can be limited by surface tension effects.

As greater amounts of thermal energy are given to the system, the molecules travel at high speeds and are widely spaced. Intermolecular interactions therefore become negligible, resulting in no molecular ordering whatsoever and an isotropic gaseous phase. Gases expand to fill the volume of the vessel containing them.

At extremely high temperatures, materials can become ionised as electrons are disassociated from their respective nuclei. This state of matter is known as a plasma and consists of very low density free-floating positive and negative ions. Plasmas will therefore conduct electricity. They too will expand to fill the volume of the containing vessel, but can also be contained and manipulated by electromagnetic fields.

In addition to the four basic phases of matter, additional phases can also sometimes be observed in certain types of materials under appropriate conditions. Examples of these include supercritical fluids (fluids under extreme pressures, exhibiting gaseous mobilities with liquid densities), degenerate gases (where gas molecules are forced close together so that they almost resemble a solid), and Bose-Einstein condensates (an extremely low temperature state, where all of the particles (bosons only) within the material co-exist within the same quantum energy state in a coherent form of matter).

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2. What is a liquid crystal?

Liquid crystals are a phase of matter existing between the solid and liquid phases. We call such an intermediate state of matter a "mesophase". They are fluidic in nature, like a liquid, but also exhibit a degree of molecular order similar to (but weaker than) that observed in crystalline solids. Not all materials will exhibit liquid crystalline properties, but instead are limited to molecules that have a structural anisotropy, eg: rod-shaped molecules (calamatic liquid crystals), disc-shaped molecules (discotic liquid crystals) and board-like molecules (sanidic liquid crystals).

There are two distinct types of behaviour in liquid crystals, thermotropic and lyotropic. In the case of thermotropic liquid crystals a mesophase appears as a result of thermal effects. By either heating above the crystalline solid phase or cooling from the isotropic liquid phase a liquid crystal mesophase will appear. The temperature on heating at which the state of matter changes from that of a solid to that of a liquid crystal is called the melting point. For the liquid crystal-isotropic liquid transition the respective temperature is referred to as the clearing point. Lyotropic liquid crystals differ in that a mesophase is observed when the concentration of the solvent is just enough to disrupt the crystal order to promote fluidity but not enough so that all order is lost and the solution becomes isotropic.


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