Textures finger print

Figure 1.30. Typical finger-print textures in cholesteric liquid crystals schematics (upper) and images (lower). (From Bouligand Kleman, 1970.)...

Under the polarized optical microscope, the liquid crystal films show colorful 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... 

PBLG solution shows the finger-print texture, seen in Figure 6.20. A pair of neighboring dark-bright stripes corresponds to a half period of the helix, P/2, which is about a few microns. It is a practical means of estimating the pitch of cholesteric liquid crystals. 

Figure 6.20. The finger-print texture of PBLG soiution (20% PBLG in dichioromethane, Mn = 40000). (From Ritter, 1986.)...

The alternatively black and white retardation lines are connected to the cholesteric pitch or more precisely to the anisotropy of polarizability of the macromolecules and to their orientation relative to li t propagation. Figure 2 illustrates how in the case of the same polymer, these finger print patterns are modified by dilution of the cholesteric polymer in a simple solvent. A quantitative measurement of the disappearance of the texture at the order-disorder transition temperature can be obtained by recording the light... 

Fig. 8.22 Microscopic finger-print texture of the cholesteric phase observed in geometry shown in the Inset, arrows indicate the direction of the incident light on the texture...

Locally, like nematics, cholesterics are uniaxial. On the macroscopic scale, due to averaging, the helical structure is also uniaxial, the optical axis coinciding with the helical axis which is always perpendicular to the local (nematic) optical axes. Under a microscope, cholesterics can show the focal-conic or finger-print textures. When the helical axis is perpendicular to the limiting glasses, the uniformly colored planar texture is observed. The color depends on the relative value of the pitch Pq with respect to a light wavelength. 

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