Liquid Crystal Compounds / Materials



LIQUID CRYSTAL COMPOUNDS

Liquid-crystal-forming compounds are widespread and quite diverse. Soap can form a type of smectic known as a lamellar phase, also called neat soap. In this case it is important to recognize that soap molecules have a dual chemical nature. One end of the molecule (the hydrocarbon tail) is attracted to oil, while the other end (the polar head) attaches itself to water. When soap is placed in water, the hydrocarbon tails cluster together, while the polar heads adjoin the water. Small numbers of soap molecules form spherical or rod like micelles, which float freely in the water, while concentrated solutions create bilayers, which stack along some direction just like smectic layers. The slippery feeling caused by soap reflects the ease with which the layers slide across one another. The liquid described in this .passage is human blood. In its usual state within the human body, blood is an ordinary disordered isotropic fluid. The disc like shape of red blood cells, however, favours liquid crystallinity at certain concentrations and temperatures.

Many biological materials form liquid- crystals. Myelin, a fatty material extracted from nerve cells, was the first intensively st’udied liquid crystal. The tobacco mosaic virus, with its rod like shape, forms a itematie phase. In cholesterol the nematic phase is modified to a cholesteric phase characterized by continuous rotation of the direction of molecular alignment. An intrinsic twist of the cholesterol molecule, rather like the twist of the threads of a screw, causes this rotation. Since the molecular orientation rotates steadily, there is a characteristic distance after which the orientation repeats itself. This distance is frequently comparable to the wavelength of visible light, so brilliant colour effects result from the diffraction of light by these materials.

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