The main application area of liquid crystals is in electro-optic devices. These are electrically controlled devices that modulate light in a desired way. We have already given a short description of the optical properties of liquid crystals, and to conclude this short tutorial we will now give a brief description of how liquid crystal molecules respond to electric fields.
Molecules, as opposed to ions, are neutral entities. This means they have no surplus electric charge. However, the charge distribution at different parts of the molecule may very well vary. If one end of the molecule has a slight surplus of positive charge, and the other a negative charge surplus, the molecule becomes a dipole.
Almost all liquid crystal molecules are dipoles, although the charge distribution normally is much more complicated than just described. A dipole in an electric field wants to turn such that the positive end points along the field and the negative one in the opposite direction. Therefore, one could be tempted to believe that this is the mechanism behind the electro-optic effects observed in liquid crystals. However, except in the very special cases of polar liquid crystals, which were introduced around 1975 by Robert B. Meyer and which since then are a very important field of liquid crystal research (see next page), this is not at all the case. In the majority of cases, in nematic and cholesteric materials, and in most smectic materials, the short range molecular organization is such that the local dipoles are everywhere compensating each other on the average, and this tendency to anti-parallel order is so strong that it is not at all affected by external electric fields. In other words, there is a "head-and-tail" symmetry in the distribution of the rodlike molecules. There are always as many molecules "head up" as "head down". The nematic liquid crystal therefore has quadrupolar order and not dipolar (polar) order, which is also the reason why we can change sign on the local director (n -> -n) without changing any macroscopic properties of the material.
The mechanism behind the fact that the local optic axis is affected by an external electric field is instead based on the anisotropic properties of the nematic; in this case the important point is that the dielectric constant is different in the direction along the director from the one in the direction perpendicular to it. Most often the dielectric constant is larger along n, and because the director turns in such a way that the maximum value of the dielectric constant lies along the direction of the field, the director, and hence the optic axis, orients along the field for such molecules.
In a solid the dipoles are too tightly bound to be easily reoriented by an electric field. In a normal liquid the thermal motion of the molecules normally overcomes the tendency for the dipoles to orient. In a polar liquid crystal, on the other hand, we find just the right combination of order and flexibility to make the dipoles follow the sign of the field. Because the dipoles have a fixed sterical relation to the director, the field will turn the optic axis. This can be used to control the way the optic axis points, or to unwind the helical structure of a chiral tilted smectic phase. Polar liquid crystals is the topic of the next chapter...