Lecture handout including QS - Department of Materials Science ...

BH12 Course B: Materials for Devices BH12
A sample of nematic LC will not necessarily have a single, uniform director across its whole volume,
but instead may be broken down into smaller, differently oriented regions, or domains.
Where differently oriented domains meet there is
a ‘glitch’, called a disclination. Observation of a
LC sample between crossed polars typically
shows a schlieren texture: bright regions of
nematic LC with uniform director, separated by
dark boundary regions where the director is
aligned with the polarizer or analyzer (or is
parallel to the light path), and hence the sample is
in extinction.
Further types of Liquid Crystals
There are several different classes of LC, based upon their overall tendency for molecular alignment.
Smectic: molecules organise into layers
⇒ orientational order, some positional order
Chiral nematic: helical twist
Smectic A
D parallel to
layer normal
Smectic C
↑
D
D
D
The molecules in a Chiral Nematic (also called cholesteric) have their long axes (and hence director)
in a plane (in the above, this is shown as the horizontal); the director rotates along the axis perpendicular
to this plane, tracing out a helix. For polarized light propagating along this axis, the plane of
polarization rotates with D. The pitch is the distance along the axis taken for a complete 360° rotation
of the director. It can vary over a very wide range, e.g. ~100 nm - 100 µm, depending upon the LC
molecules involved, the degree of polymerization, and the concentration in solution.