Textural fingerprints

Disordered carbons usually exhibit a multiscale organization (structure, microtexture, texture)4. Structurally, they are made of more or less distorted polyaromatic layers, nanometric in size. The spatial association or the layers, from the nanometric to the micrometric scales, gives rise to different microtextures (lamellar, porous, concentric, fibrous, etc.) forming the carbons skeleton4. The multiscale organization is the fingerprint of the kind of precursor and of the formation conditions (temperature, pressure, strains, time, etc.) met either in laboratory experiments or in Nature, and is directly related with numerous properties. 

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

Another technique widely used to measure the cholesteric pitch is based on the Bragg scattering of monochromatic light (obtained by a helium-neon laser) from fingerprint or planar textures of the cholesteric32 Its angular dependence is strictly related to the cholesteric 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. 

In summary, the nature of the hybrid therefore allows, to some extent, the control of both texture and morphology of the tin dioxide materials obtained after calcination. Nevertheless, treatment at 500 °C leads to similar BET surface area and mean crystallite size, which shows that annealing at very high temperature erases completely the fingerprint of the organic linker. 

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). 

Rey, A.D. Structural transformations and viscoelastic response of sheared fingerprint cholesteric textures. J. Non-Newtonian Fluid Mech. 1996, 64 (2-3), 207-227. 

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... 

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). 

Some enduse applications involve parts that are subject to dirt or fingerprints. The breakdown of the smooth surface into a texture shadowed surface partially masks the dirt. Unfortunately it also contributes in the long run to dirt trapping which may defeat its original purpose. We need look no further than our computer keyboard to illustrate this point... 

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. 

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