Fingerprint chiral nematics

Liquid crystalline (LC) solutions of cellulose derivatives form chiral nematic (cholesteric) phases. Chiral nematic phases are formed when optically active molecules are incorporated into the nematic state. A fingerprint texture is generally observed under crossed polarizers for chiral nematic liquid crystals when the axis of the helicoidal structure is perpendicular to the incident light (Fig. 2). 

Fig. 2 Fingerprint pattern for chiral nematic mesophase of 40% EC in m-cresol. (View this art in color at www.dekker.com.)...

The mesophase defect textures exhibited by 47 were typical of those normally found for a chiral nematic phase, except they were only revealed upon annealing, which is probably a function of the viscosity of the material. Thus the sample was annealed just below the clearing point. After 24 h, large areas of the preparation evolved to show fingerprint defects and the Grandjean plane... 

Fig. 56 a fingerprint texture for an uncovered droplet of compound 47, and b the Grandjean texture of the chiral nematic phase... 

Fig. 11.10 a POM image of the chiral nematic (N -LC) phase of (S)-Poly-4 in 10 wt % lyotropic LC solution in toluene showing a double-spiraled texture. Inset shows a fingerprint texture with a helical half pitch of 1.5 pm. b Schematic representation of the N -LC phase of (5)-PA17. Reprinted with permission from [18]. Copyright 2012, American Chemical Society... 

FIGURE 3.1 Chiral nematic LC induced by the addition of chiral dopant into nematic LC. Schlieren texture ( left) and fingerprint texture ( right) are observed for nematic and chiral nematic LCs, respectively, in polarized optical microscope. 

Plate 5 Fingerprint textnre of the chiral nematic (cholesteric) liq-nid crystal phase. 

After the discovery of the liquid crystal behavior of HPC in solution, a multitude of studies have reported the ordering phenomena of different cellulose derivatives, according to the examples given below [90]. Figure 14.6 presents the optical microscopy of an aqueous suspension of microcrystalline cellulose at a concentration of 16 wt%, where the nematic field and a chiral nematic phase typical to a fingerprint texture maybe observed 10 minutes and respectively 1 day after the preparation of the suspension [116]. 

The stripes arise by the influence of the boundary layer for the Nl type of chiral nematics, due to the fact that disk-like micelles prefer a flat orientation on many, if not all, solid surfaces adsorption layers of the surfactant form on the solid. Initially hydrophobic surfaces will be covered by a monolayer, the hydrophilic ones by a bilayer. Therefore no spontaneous formation of fingerprints occurs with N -type chiral nematics instead, Grandjean textures are observed. The periodicity of the fingerprints equals half of the pitch, if the sample is prepared correctly. Especially the sample thickness must be large compared to the pitch otherwise there will be helix unwinding by the action of the solid boundary. 

Figure 2. Chiral nematic arrangements in thin cells (a) planar or Grandjean, (b) fingerprint, and (c) focal conic.

There is a further geometry of practical interest for light incident on chiral nematic films, related to the pitch of the helix in which we consider light propagating in a direction normal to the helix axis, i.e., as in the fingerprint texture, but with a pitchp less than A. In this short pitch chiral nematic case, the chiral optical tensor is averaged in space and the macroscopic optic axis is col-linear with the helix axis [27]. The macro-... 

Figure 6. Photomicrographs of (a) planar, (b) focal conic, and (c) fingerprint textures in chiral nematics, and the schlieren texture demonstrating brushes in an apolar nematic (d) observations through crossed polarizers.

Figure 11. Photomicrographs of concentration gradients with (a) planar and (b) fingerprint textures in chiral nematics. In both plates the high concentration of chiral compound is at the top left with the achiral compound at the bottom right.

The N -LC is used as an asymmetric liquid reaction field and it is prepared by adding a small amount of a chiral dopant to a nematic liquid crystal (N-LC). The formation of the N -LC can be confirmed under the polarizing microscope (POM) by the change of the characteristic nematic Schlieren texture to a fingerprint-like or striated Schlieren texture of the chiral nematic phase. The distance between the fingerprint lines of the optical pattern is equivalent to half of the helical pitch of the N -LC phase. Therefore, the stronger twisting power of the dopant can be observed by the shorter helical pitch of the optical pattern under the POM (Fig. 9.33). 

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