Chemistry Reference
In-Depth Information
(a)
(b)
(c)
Figure 1.30. Typical finger-print textures in cholesteric liquid crystals: schematics
(upper) and images (lower). (From Bouligand & Kleman, 1970.)
The Cano wedge
The Cano wedge can be used to measure the cholesteric pitch. Two
substrates are wedged by a small angle 2
γ
. The cholesteric liquid
crystal between the substrates is homogeneously aligned. Under polarized
microscopy, the equally spaced disclinations appear at the middle of the
wedge. The separation of the lines 2
d
is associated with the pitch
P
P
=8
d
tan
γ.
(1.43)
This technique has been applied to measure the pitch of cholesteric liquid
crystals.
1.6.8. Textures
Under the polarized optical microscope, the liquid crystal films show color-
ful patterns,
i.e.
, the optical textures. Each liquid crystal phase shows its
typical texture which provides the means to identify the phase of the liquid
crystals. The typical textures are the Schlieren, threadlike, homeotropic,
homogeneous, marble, finger-print, focal-conic, Dupin cyclide, fan-shape,
sanded, mosaic, and so on. They are the integrations of many typical
defects mentioned above. Demus & Richter (1978) were the first to review
