Thermodynamics for Liquid Crystals
In the preceding chapters, the polarization of light as an electromagnetic wave resulting from the interaction with crystal structures was described, with the light electric vector in an anisotropic medium analyzed as ordinary and extraordinary waves that travel at different speeds through the crystal, resulting in a phase lag that produces crystal birefringence. The mesophases of liquid crystals then were introduced as having a crystal-like fluid structure characterized by the different degrees of positional and orientational order of the different mesophases. That order created structural anisotropies that would produce different optical effects in light passing through much as crystals do, but owing to the fluid nature of liquid crystals, the birefringence was alterable by applying an electric field. An orientational order parameter was then devised to parameterize that fluid molecular order for use in theoretical studies. Finally, the liquid crystal as a material medium responsive to stresses was characterized by its viscosity and elasticity.
A theory of the behavior of liquid crystals at the very least must be able to physically and quantitatively describe the dielectric anisotropy that produces birefringence and the subsequent controllable interaction with light that forms the basis of liquid crystal displays. The theory also must address the solid, mesophase, and liquid phase transitions that are the essence of the liquid crystals themselves. ...