15.1 Remarks on Nomenclature and Notations 275
Fig. 15.1Cartoon of the
orientation of molecules in
the nematic phase, as shown
in [74]
Fig. 15.2Schematic
double-twist configuration
When the local helical axis points in the radial directions perpendicular to another
fixed axis, the director configuration is referred to asdouble twist structure,cf.
Fig.15.2. There the short lines indicate the director, the thin horizontal lines mark
the directions of two of the helical axes which are orthogonal to the cylinder axis. The
molecular arrangement in a double-twist cylinder with a diameter determined such
that the twist from the center of the cylinder axis is about 45◦, can be more stable
than the single-twist configuration of an ordinary cholesteric state. In larger volumes
3D supra-molecular structures are spontaneously formed in theblue phases. These
structures contain defects where three orthogonal double-twist cylinders touch each
other. The 3D arrangement of the defects determines the symmetry of the blue phase.
The phases referred to as ‘BP1 and BP2 have cubic symmetry of fcc and bcc type.
Blue phases with icosahedral symmetry and with an irregular structure also exist.
Theblue phasesare found in chiral substances between the ‘ordinary’ cholesteric
and the isotropic liquid phase. The name comes from the blue shine observed in
cholesteryl benzoate, as first reported by Reinitzer in 1888. He sent this substance to
Lehmann who studied the spontaneous birefringence as function of the temperature.
Lehman noticed that the ‘blue phase’ is optically isotropic, in contradistinction to the
optically anisotropic cholesteric phase which Lehmann then called ‘liquid crystal’,
a name he had previously used for substances now calledsuperionic conductors.
Decades later it was recognized that there is not one blue phase but several types of
blue phases which occur in a very narrow temperature intervals. For over hundred