Discovery of LCs
     LCs Introduction
     History of LCDs
     TFT basics
   LCD Modes

  LC Related Links

  About Author

[Basics of liquid crystals]

The molecules composing solid generally possess both positional and orientational order, while those composing liquid do not, they can move freely in a random fashion. In addition to the solid and liquid phase, there are some condensed phases which show intermediate order, for example liquid crystal.

The simplest LC is the nematic phase, (the word nematic comes from the Greek νημα meaning thread), the molecules are rod-like with one axis much longer than the other two. This long axis tends to point along a certain direction, also called director, shown in figure 1.

Figure 1. Nematic liquid crystal with a preferred orientation.

Types of LC

The most common type is the calamitic LC, with rod-like molecules, as shown in fig. 1. There are other liquid crystal phases, which possess both positional order and orientational order. Smectic phase is a typical one, fig ?? shows the Smectic A and Smectic C phases, where a layered structure exists. Others like smectic B, E, F, G, H, I, J and K, have short-range positional order or bond orientational order within the plane of the layer; the letters denoting the chronological order of discovery. If the molecules are chiral or doped with chiral molecules, then a chiral nematic (smectic) phase is formed, figure 2 shows a cholesteric phase and a chiral smectic C phase.

Figure 2. Cholesteric and chiral smectic C liquid crystals

In addition to rod-like molecules, disc-like molecules also formed liquid crystals because of one axis is much shorter than the other two, they are called discotic LC. Figure 3 shows a discotic nematic phase and columnar phase.

Figure 3. Discotic liquid crystal phases.

All the LC mentioned above are temperature dependent, and therefore called thermtropic LC. In contrast, LC phase can also be formed when some molecules are dissolved into solutions, these LC's properties highly depends on the concentration of the components in the solutions, therefore they are called lyotropic LC. A typical lyotropic LC molecule has two parts, one is polar and  therefore hydrophilic ("like water"), and the other is nonpolar and therefore hydrophobic ("hate water"). Figure 3 shows two typical structures of lyotropic LC.

Figure 4. Structures formed by amphiphilic molecules in polar solvents.

Electro-magnetic properties of LC

The electric and magnetic susceptibilities (the susceptibility of a material or substance describes its response to an applied field) are different along the director and perpendicular to the director. The susceptibilities difference (anisotropy) determining the molecules' reorientation's final direction parallel or perpendicular to the applied field: if the susceptibilities along the director is larger than the one normal to the director, then the molecules will align along the applied field, vice versa.

Optical properties of LC

LC is a birefringent medium. Light polarized parallel to the director ¡°sees¡± one refractive index, which is called extraordinary refractive index. Light polarized perpendicular to the director ¡°sees¡± a different refractive index, called ordinary refractive index. Therefore, if light is linearly polarized parallel or perpendicular to the director, will experience no change to its polarization state. However if the incoming light's polarization is at an angle other than 0 or 90 to the director, then a phase retardation will exist at the exit, and the light becomes elliptically polarized. These optical properties are used in the LC displays.

Chiral nematic and smectic C phases are optically active, i.e. a linearly polarized light is rotated as it passes through such a medium.

Overall, the electro-optic properties described above (response to an applied electric field phase retardation that causes light transmission between crossed polarizers) are the basis for the LC displays.

Further Readings and References:

P. J. Collings and J. S. Patel, "Handbook of Liquid Crystal Research", Oxford University Press (1997).

Y. S. Chandrasekhar, "Liquid Crystals", Cambridge University Press (1992).

P. G. de Gennes and J. Prost, "The Physics of Liquid Crystals", Oxford University Press (1995).


Last update: April, 2006
Questions? Contact author.