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the linear defect (dislocation line) by the atomic planes of atoms in the
crystal lattice.
4.5. Liquid Crystals

Liquid crystals (LCs) are matter in a state that has properties between
those of conventional liquid and those of solid crystal. For instance, a
liquid crystal may flow like a liquid, but its molecules may be oriented in a
crystal-like way. There are many different types of liquid-crystal phases,

which can be distinguished by their different optical properties (such as
birefringence). When viewed under a microscope using a polarized light
source, different liquid crystal phases will appear to have distinct textures.

The contrasting areas in the textures correspond to domains where the
liquid-crystal molecules are oriented in different directions. Within a
domain, however, the molecules are well ordered. LC materials may not

always be in a liquid-crystal phase (just as water may turn into ice or
steam).
One of the most common LC phases is the nematic(fig.4.5.1a) . The

word nematic comes from
the Greek νημα (nema),
which means "thread".
This term originates from

the thread-like topological
defects observed in
nematics, which are

formally called
'disclinations'. Nematics
also exhibit so-called
a b c
hedgehog topological
Figure 4.5.1 defects. In a nematic

phase, the calamitic or rod-
shaped organic molecules have no positional order, but they self-align to

have long-range directional order with their long axes roughly parallel. [13]
Thus, the molecules are free to flow and their center of mass positions are
randomly distributed as in a liquid, but still maintain their long-range

directional order. Most nematics are uniaxial: they have one axis that is
longer and preferred, with the other two being equivalent (can be
approximated as cylinders or rods).

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