5. Main types of Liquid Crystals



Nematic Liquid Crystals

The simplest mesophase is a nematic LC. In nematics, the long molecular axes are preferably oriented in one direction, defined as the director n, (Fig.9), and molecular dipoles are compensated, so in equilibrium this mesophase is electrically neutral. Both directions of director, +n and -n, are equivalent. The ordinary nematic structure shows an optically positive uniaxial behaviour with the optical axis parallel to the director n.

Figure 9. Structure of nematic liquid crystal:
vibration of molecules represents thermal fluctuations of the director n

Nematic LCs possess a relatively low viscosity; they can be deformed by even small external forces. Considering only weak distortions, the nematic phase formally acts as an elastic medium, and the deformations can be treated by the continuum theory of nematics.
An undeformed LC is one in which the director n points in the same direction throughout the LC. A deformed LC is one in which the director changes its direction from point to point. The deformations in LCs can be described in terms of three basic types of deformation: splay, twist and band (Fig.10).




Figure 10. Deformations in nematics:
(a) splay; (b) twist; (c) bend

Elastic constants corresponding to these deformations are K1, K2, and K3, respectively. Then the free energy density of distortions is


This equation is the basis for the theoretical treatment of defects and textures of nematic LCs. For a stable distortions, the solution of the above equation have to yield a relative minima in the free energy. Distortions that do not correspond to minima of Fd are unstable and should therefore not occur in stable textures.



Dielectric anisotropy of LCs