Cholesteric oily streaks

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. 

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 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],... 

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. 

As already mentioned, the layered structure of cholesteric materials imposes certain limitations on the topological classification of defects based on ho-motopy groups a more general theory is still lacking. In this section we discuss macroscopic defects such as focal conic domains and oily streaks whose existence depends crucially on the layered character of ordering. 

In the regime L/p 1, the elastic theory considers the cholesteric medium as a system of equidistant (and thus parallel) layers and that the curvatrue distortions are predominant, (5.8). The description of defects such as edge dislocations, oily streaks, and focal conic domains in cholesterics is often based on the results obtained for simpler layered medium, namely, the smectic A phase. 

Figure 5.22. Network of oily streaks in a cholesteric planar texture. Cell thickness 15 pm, cholesteric pitch 0.48 pm. The oily streaks are (a) straight at zero voltage but (b) buckle when the voltage exceeds some threshold value. The short side of the field of view is 420 pm. The narrow streaks do not buckle while the wide streaks start to buckle at voltages higher than the medium width streaks.

Although the phenomenon is similar in appearance to the buckling of dislocations in Cano wedges [98], [99], it cannot be understood solely as a dielectric response [99] of the cholesteric. The behavior of the oily streaks can be explained only if one adds a spedlic surface anchoring term with an amplitude W K/pXo the standard energy density, comprised of the elastic terms (5.7) and the dielectric term---- (/ E). ... 

The anchoring term is caused by tilting the cholesteric layers at the boundaries. The tilt angle increases from 0 outside the streak to max = arctan at the vertical middle plane of the streak, see Figure 5.21(b) = lajh is the width of the streak renormalized by the cell thickness h. The anchoring penalty increases when 2a increases (since max increases) this increase can be avoided if the oily streak simply elongates, preserving max = const. 

FIGURE 2.11 Polarized microscopic image of a plataar-aligned cholesteric phase ( Grandjean" or "oily streak" texture). 

Fig. 1. Cholesteric texture with oily streaks of polyester HTH/DPG sample at 290 °C (original magnification 300 x).

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