Fingerprint texture

Figure 7.6 Fingerprint texture of lyotropic cholesteric double-headed arrow corresponds to pitch.

However, temperature dependence of the twist (the pitch was determined from fingerprint textures) follows Eq. (1) and in both cases gives positive... 

However, further analysis of the behavior of the system in LC cells cast doubt on this interpretation. First, while intuitively attractive, the idea that relaxation of the polarization by formation of a helielectric structure of the type shown in Figure 8.20 would lower the free energy of the system is not correct. Also, in a thermodynamic helical LC phase the pitch is extremely uniform. The stripes in a cholesteric fingerprint texture are, for example, uniform in spacing, while the stripes in the B2 texture seem quite nonuniform in comparison. Finally, the helical SmAPF hypothesis predicts that the helical stripe texture should have a smaller birefringence than the uniform texture. Examination of the optics of the system show that in fact the stripe texture has the higher birefringence. 

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

As or Ts of opposite signs occur in pairs to form dislocations and pincements (fig. 4.2.6). Such pairing can be observed directly in the fingerprint textures which are exhibited by cholesterics of large pitch when the helical axis is parallel to the plates (figs. 4.2.7 and 4.2.8). 

Fig. 4.2.8. Fingerprint textures of cholesterics showing pincements, (a) after Robinson, Ward and Beevers, and (Jb) after Bouligand. ...

Xj < 2A0Z3 and attractive for x > 2AoZ,. Thus, as in the case of smectic A (see fig. S.4.7), there can result a clustering of like edge disclinations to form a grain boundary . Such clustering is often observed in fingerprint textures (fig. 4.2.7). 

Figure 13.5 shows textures of the T-CN suspension with different cellulose concentration observed by POM. Samples were dropped on the slides. With increasing concentration the images become colorful and exhibit different textures, [a] and [b] show typical banded texture and schlieren texture, respectively. We could also see the typically fingerprint texture for N -LCs in [d], and the pitch is about 15 pm. However, it is more difficult to observe the fingerprint texture as other kinds of cellulose reported before. 

Figure 13.6 POM images of T-CNs (3 wt%) after sonicatlon. [a-f) show the formation process of the fingerprint texture.

An interesting optical behavior was also found in the red circle of [Fig. 13.6f]. It was a transformation of the fingerprint texture and the distance between the contiguous three lines was about 200 pm. That was probably because the interface was distorted by the flow of the anisotropic phase [27],... 

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... 

In Fig. 11.11, the chiral induction of (rac)-Poly-4 forming an N -LC phase is illustrated. The POM image of (rac)-Poly-4 shows a Schlieren texture characteristic of a lyotropic N-LC phase [52], prepared from a 10 wt % toluene solution (Fig. 11.11a). Upon addition of chiral dopant (S)-Dl at 10 wt %, the N-LC phase of (rac)-Poly-4 changes into an N -LC phase (Fig. 11.11b). The POM image of (S)-Dl/(rac)-Poly-4 reveals a fingerprint texture with a helical half-pitch of 2.0 pm. 

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. 

Fig. 4.30 A fingerprint texture of a cholesteric liquid crystal seen in a polarization microscope (the distance between stripes equals a half-pitch)...

Figure 6.1 Optical texture of the monomers (x200) (a) schlieren textures for PPUB upon heating to 104°C (b) droplet texture for PPUB upon heating to 158°C (c) nematic droplets for PPUB upon cooling to 156°C (d) schlieren textures for PPUB upon cooling to 91 °C (e) focal conic texture for UHAB upon heating to 121 °C and (f) fingerprint textures for UHAB upon heating to 152°C. Reproduced with permission from Ref. [39],...

Chiral dithienylcyclopentene compounds can also be used to induce the cholesteric-isotropic phase transition [36]. A cholesteric polygonal fingerprint texture was exhibited by 10 wt% of 2 as a mesogenic dopant in a conventional achiral nematic 5CB as shown in Figure 5.6a. The cholesteric phase to isotropic transition temperature for the doped 5CB was 42 C. With UV irradiation at 310 nm (30 mW cm ) for 30 s, the sample went into the isotropic phase (Fig. 5.6b) whereas upon visible fight irradiation at 670 nm a reverse process was reached within 30 min (Fig. 5.6c). 

ZLI-389 can efficiently induce a cholesteric phase with the characteristic fingerprint texture (Fig. 5.10). After UV irradiation at 300 nm for 50 s, the cholesteric phase disappeared and a characteristic schlieren texture of nematic phase was observed due to the dramatic HTP decrease of the dopant during photoisomerization. Visible light irradiation of the sample for 30 s resulted in the reappearance of the cholesteric fingerprint texture [43]. 

---