Nematic

From Freepedia

In nematic liquid crystals the centers of gravity of molecules have no long-range order. The molecules tend to be parallel to a common axis, labeled by a unit vector (director) <math>\hat{\mathbf{n}}</math>. Opposite orientations of the director are indistinquishible, even if the molecules are asymmetric. Hence, the nematic order parameter is a second-rank tensor

<math>Q_{\alpha\beta}=\frac{1}{2}\langle 3 n_\alpha n_\beta-\delta_{\alpha\beta}\rangle</math>

where

<math>n_\alpha</math> are the components of a unit vector along the major symmetry axis of the molecules

The transition from the isotropic liquid state to the nematic phase is a (often weak) first order phase transition. The word nematic comes from the Greek νημα = thread. It refers to thread-like topological defects observed in nematics. They are called 'disclinations'. Hedgehog is the other type of topological defects in nematics.

Nematics have fluidity similar to that of ordinary (isotropic) liquids but they can be easily aligned by an external magnetic or electric field. An aligned nematic has the optical properties of a uniaxial crystal and this makes them extremely useful in Liquid Crystal Displays (LCD).