Liquid Crystals

In a liquid crystal, the individual molecules have restricted mobility. They are more ordered than in a true liquid but less ordered than in a crystalline solid; thus, they are in an intermediate state. Because of this intermediate state, the liquid crystalline state is often called a mesophase. In general, materials that exhibit liquid crystallinity have rigid rod- or disk-like shapes.

A liquid crystal contains many small regions, each with a million or so molecules, with a preferred orientation for the molecules. Thus, even though the molecules in a liquid are in constant motion, the molecules in a particular region are all aligned with each other. On a larger scale, many regions of preferred orientation exist, but these regions are randomly oriented with respect to one another.
Liquid crystals are found in two types of ordering: the nematic form and the smectic form. Figure 4.4 illustrates both types of ordering. Weak intermolecular forces produce nematic ordering, but stronger intermolecular forces produce smectic ordering. Stronger intermolecular forces also produce larger regions of order in the liquid.

Figure 4.4. Schematic comparison between (a) the nematic form and (b) the smectic form of a liquid crystalline material.
An example of a liquid crystal is 4’-octyl-4-cyanobiphenyl.

4'-Octyl-4-cyanobiphenyl

CNCH3CH2CH2CH2CH2CH2CH2CH2Liquid crystal at room temperature

At room temperature it adopts the smectic form. Figure 4.5 is a scanning tunneling microscope image clearly showing the smectic alignment of the molecules.
(NEED PERMISSION FOR FIGURE 1A FROM REF BELOW)
Figure 4.5. A scanning tunneling microscope image of the ordering of the molecules of 4’-octyl-4-cyanobiphenyl. (from Smith and Horber, et.al., Science, 7/7/89, p43)

Liquid crystals are widely used in electronic displays because an external force, such as an electric field, increases the size of the ordered regions. Liquid crystals with large ordered regions have different optical properties than liquid crystals with small ordered regions. Most significantly, the large ordered regions become visible. Some watches and computer displays have a sealed container of liquid crystals. The surface of the container has a pattern of transparent electrodes. Depending on which electrodes are powered, a pattern of dots, lines, or symbols appears comprising the information visible on the display.