Chemistry Reference
In-Depth Information
2
P
2
^
n
2
d
V
Twist angle
^
n
1
P
1
1
Figure 1.19.
The twisted nematic cell. The twist angle
θ
=
π/
2 and the two polars
make an angle Ψ = 0 or
π/
2.
the voltage is off, the cell is transparent. When the voltage is on, the cell
becomes dark.
STN has a twist angle greater than 180 degrees and its electric-optical
response is much steeper so that they can be driven in the multiplexing
displays such as those found in the SVGA format color notebook computers.
1.6. DEFECTS IN LIQUID CRYSTALS
The local translational and orientational order of atoms or molecules in a
sample may be destroyed by singular points, lines or walls. The discon-
tinuities associated with the translational order are the dislocations while
the defects associated with the orientational order are the disclinations.
Another kind of defect, dispirations, are related to the singularities of the
chiral symmetry of a medium. The dislocations were observed long after the
research on them began. The dislocations in crystals have been extensively
studied because of the requirement in industry for high strength materials.
On the contrary, the first disclination in liquid crystals was observed as
early as when the liquid crystal was discovered in 1888, but the theoretical
treatment on disclinations was quite a recent endeavor.
There is only the orientational order in nematics so that only disclina-
tions may appear with no dislocations. In the other kinds of liquid crystals,
