History of Liquid Crystals Goes from Cloudy to Clear
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History of Liquid Crystals Goes from Cloudy to Clear Liquid crystalline materials display a degree of structural order intermediate between that of crystals and that of isotropic liquids. When samples of liquid crystals were first examined in the mid1800s, they were not immediately recognized as a novel and exotic phase of matter. For instance, a chemist investigating the properties of natural fats noticed that one material, stearin, appeared to have two distinct melting points. The first transition was from solid to a cloudy liquid, and the second from cloudy to clear liquid. One possible explanation for such behavior could be the presence of an impurity that gives rise to a two-phase coexistence region in the phase diagram, so it was not immediately obvious that the cloudy liquid was anything remarkable. Another set of early observations in the mid- to late-1800s involved the use of polarized light. Crystals can alter light's polarization, and the effect changes dramatically when a crystal sample is rotated with respect to the light beam. Most liquids have no effect on polarization; liquids containing chiral molecules can alter light's polarization, but the effect shows no variation with rotation of the sample. Thus it was an important discovery that certain biological materials such as the outer covering of a nerve fiber mixed with water could produce changes in light's polarization, and that the effect varied with the angular orientation of the sample. This was perhaps the first clear evidence that such liquids are not isotropic but contain some kind of directional order. A few researchers also noted that some compounds synthesized from cholesterol briefly turned bright blue as they cooled, an effect that was not fully explained until nearly a century later. A German physicist, Otto Lehmann, was the first scientist to construct a microscope with a heating stage, and later added polarizers. Lehmann's goal was to observe the process of crystallization, and using his microscope he saw that some materials appeared to have an intermediate amorphous phase between crystal and liquid. In 1888 an Austrian botanist, Friedrich Reinitzer, found that an organic material closely related to cholesterol had two distinct melting points, and wondered what the intermediate cloudy liquid phase could be. He sent samples to Lehmann, who examined them in his microscope.
MRS BULLETIN/NOVEMBER 1996
Lehmann concluded that the liquid was a uniform fluid phase—not a two-phase mixture—and observed that it affected polarized light as a crystal would. Thus he coined the name "liquid crystal." Daniel Vorlander, a chemist, headed a group that went on to synthesize many liquid crystalline materials, including the first material known to have two distinct liquid crystalline phases. He realized that materials whose molecules were linear and elongated in shape were the most likely to display liquid crystalline phases. Within a few decades the major classes of liquid crystal phases were known. In a 1922 paper, Georges Freidel of France outlined t
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