Oily streak

Fig. 14. A sample of a lamellar liquid crystal between crosses polarized in an optical microscope gives a pattern of "oily streaks" and Maltese crosses (a) while the Hquid crystal consisting of an array of cylinders shows the characteristic sectional pattern (b).

Hexagonal mesophases can be recognized by their typical fan-shape texture (Fig. 7a). Lamellar mesophases typically show oily streaks with inserted maltese crosses (Fig. 7b). The latter are due to defects, called confocal domains, that arise from a concentric rearrangement of plane layers. In some lamellar mesophases these defects prevail. Hence no oily streaks occur but maltese crosses are the dominant texture (Fig. 7c). 

The textures in homeotropic lamellar phases of lecithin are studied in lecithin-water phases by polarizing microscopy and in dried phases by electron microscopy. In the former, we observe the La phase (the chains are liquid, the polar heads disordered)—the texture displays classical FriedeVs oily streaks, which we interpret as clusters of parallel dislocations whose core is split in two disclinations of opposite sign, with a transversal instability of the confocal domain type. In the latter case, the nature of the lamellar phase is less understood. However, the elementary defects (negative staining) are quenched from the La phase they are dislocations or Grandjean terraces, where the same transversal instability can occur. We also observed dislocations with an extended core these defects seem typical of the phase in the electron microscope. 

Friedel (2) interpreted the transversal striations on oily streaks as small adjacent confocal domains that have a tendency to gather in lines. We already noted that such a situation exists in DADB (12) (but the lines are attached to the surface), and that c domains pin up on l lines moreover, oily streaks in cholesterics have clear confocal domains. However, the transversal striations on l or L lines are not compatible with c domains since we do not see there the typical Maltese cross on the contrary, the hyperbolic directions would be at a small angle to the sample plane if they exist. We do not reject FriedeFs explanation, but we must make it compatible with observations, particularly with the longitudinal striations. 

Blumstein and coworkers studied the cholesteric behavior of polyesters with azoxybenzene mesogenic units and the same chiral spacer, (+) 3-methyladipic acid. They could clearly observe oily streak textures, which are typical of low molecular weight cholesterics, for the following homopolymer and copolymers ... 

Cholesteric oily streak textures could again be clearly observed for these LC polymers containing chiral spacers. In general, the pitch of the helical packing increased in a regular manner not only with temperature, as judged by an iridescent color, but also with the amount of achiral component in the copolymers. 

A dispersion of spherulitic liquid crystalline particles in brine exists between 0.8 gm/dl NaCl (Figure 2(a), first sample on the left) and 1.2 gm/dl. As the salinity is increased to about 1.4 gm/dl NaCl, the amount of liquid crystals as well as the birefringence increase, and the texture observed using PLS is intermediate between those of the spherulite (S) and lamellar (L) structures. The aqueous solution is a homogeneous lamellar phase between 1.6 and 1.8 gm/dl NaCl. The surfactant molecules form bilayers with their polar heads toward the brine. Figure 3(a) shows the lamellar structure as observed by polarized microscopy at 1.6 gm/dl salt and without any polymer. The bands represent "oily streaks" in a planar background. 

A.—If it be a fittle shaked, oily streaks appear sliding here and there, just as spirit of wine distils down the head of the alembic in streaks like veins. 

The often observed textures for Sa are the focal-conic fan texture and the homeotropic texture. When the molecular orientation is homeotropic, the optic axis is perpendicular to the film surface so that the preparation appears black (pseudoisotropic) on a crossed polarizing microscope. As with the homeotropic nematics, the homeotropic Sa phase can be differentiated from the true isotropic phase by conoscopic observations. Further more, if the cover slip of the homeotropic Sa preparation is slightly moved, the orientation is disturbed resulting in oily streaks in form of bright bands. The homeotropic Sa phase may thus be distinguished from the homeotropic nematic phase. 

Figure 2. The optical microscopy pattern of a lamellar liquid crystal between crossed polarizers contains "oily streaks" and Maltese crosses.

Recently Ribeiro et al. [65] reported on the synthesis and characterization of a variety of tolanes that had optically active sulfinate groups. Some of these compounds, see 23, were found to possess a phase that exhibited oily-streak textures typical of chiral nematic phases and also defect pattern associated with columnar phases (the earlier photomicrograph Plate 6 for 14P1M7 is similar). 

The chiral nematic phases can show a planar Grandjean textnre, with oily streaks caused by defects, but they can also show strong reflection colors, depending on the pitch of the helical structure within the phase. 

Figure 3. Typical optical textures ofa lamellar phase (a) mosaic oily streaks, (b) Maltese crosses.

Fig. 5. Photomicrographs of sample 62 oily streaks at 250°C (left), and schlieren texture at 295 C (right). Cross polars, original magnification 250x.

For homopolyesters prepared from di-n-propyl-p-d erphenyl-i, U -carboxylate and branched aliphatic diols, the existence of S phases was easily established from the simple focal conic or fan textures (Fig. and the tendency to be homeotropic which indicates uniaxiality. Oily streaks also occur. They appear as bright bands or ribbons which start from air bubbles in the dark homeotropic regions. On the other hand, homopolymer ... 

Cholesteric phases are quite complex and are easily deformed by even small forces, causing a broad range of observable textures. Among the observed textures, the planar textures, known as oily streaks, are among the most commonly observed textures [1,12],... 

Very recently, chiral 1,4-diphenylbutadiene-based mesogens 16 have been found to show isothermal phase transition behavior from the SmA phase [56]. As shown in Figure 5.13, the thin film of 16 exhibited an SmA phase with a focal conic texture at 118 °C, and the photoirradiation with 360 nm light resulted in the disappearance of the focal conic texture accompanied with the appearance of an oily streak texture, indicating the phase transition from SmA to N (Fig. 5.13). Upon further irradiation, the complete conversion was observed. 

Figure 10.25 (a) Microphotograph of the oily streak in the cholesteric liquid crystal. The bright finger is the oily streak. The dark background is the planar texture, (b) Schematic diagram showing the structure of the oily streak on a cross section. 

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