Crystals Between Order and Disorder

In 1888, the botanist Friedrich Reinitzer [9.5] was studying cholesterol in plants. He had the idea of observing a cholesterol benzoate crystal through the microscope as it underwent melting. At 145.5°C, the crystal became cloudy but fluid. It then transformed into a transparent liquid, very like water, at 178.5°C. The natural inclination of any physicist would have been to doubt the purity of the sample. Indeed, this had been the conclusion of those scientists who had previously made the same observation. Reinitzer, however, trusted in the quality of his compound. He therefore introduced the idea that melting could take place in two stages, and in this way opened up a new area of research the study of liquid crystals, intermediate states between liquids and solids. 

Upon closer observation, we discover flexible and mobile lines, which we shall soon discuss. It was Georges Friedel who first understood the significance of these lines, at the beginning of the century. He invented the term nematic phase, from the Greek word urj/aa for thread . Cooling the sample further, we 

How should these observations be interpreted Let us compare the overall shapes of a small molecule such as methane and a molecule of 8GB (see Fig. 9.1). Whereas the tetrahedral form of methane is quite close to spherical, 8CB is more like a long rod. Such a shape opens the way to arrangements intermediate between crystal and liquid. It is not difficult to imagine a liquid made up of parallel rods with randomly distributed centres of mass (see Fig. 9.0b). This is the nematic phase. We could also imagine these rods organised in plane layers, within which their centres of mass are still distributed randomly as in a liquid. On average, the rods are perpendicular to the planes. The layers are themselves stacked up n a periodic way. This is the smectic phase (see Fig. 9.0d). 

To sum up, our two experiments have enabled us to identify three families of liquid crystals  

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