Chevron layer structure zigzag defects

Fig. 5.1.14 Cl and C2 states, distinguished by the relationship between the direction of the chevron layer structure and the direction of the surface pretilt, as shown in (a) and (b). The tilting direction of the chevron layer structure is confirmed by the direction of the zigzag defects, as shown in (b). (c) Smectic layer models of the Cl and C2 states.

Chevron layer structure with zigzag defects). 

Fig. 6.1.4 Changes in texture induced by AC-field treatment, (a) Texture I. Initial virgin texture with chevron layer structure and zigzag defects, (b) Textures II. A rooftop texture is observed after the application of an AC field of medium strength, (c) Texture HI. A quasi-bookshelf layer structure with stripe domains is observed after the apphcation of a strong AC field.

One of the important aligning techniques, which allows us to avoid zigzag defects between two adjacent smectic layers bent in opposite directions (chevrons), remains the oblique evaporation of silicon monoxide [141-144] (Fig. 7.25). Zig-zag defects are avoided by the promotion [25] of only one possible bend or tilt of the smectic layers due to a specific oblique director orientation at the boundaries (Fig. 7.25). Samples with the antiparallel evaporation direction contain uniformly tilted layers. Those with parallel directions exhibit the chevron structure of tilted layers free of zig-zag defects. Reference [135] shows that in the case of parallel evaporation the variation of the evaporation angle results in different chevron bend angles. 

Figure 92. The shrinkage in smectic layer thickness due to the molecular tilt 0(T) in the SmC phase results in a folding instablity of the layer structure ( chevrons ). Even if the fold can he made to go everywhere in the same direction (in the figure to the right) to avoid invasive zigzag defect structures, the switching angle is now less than 2 9, which lowers brightness and contrast.

If zigzag defects already exisL they can be removed by an adequate AC electric field treatment which causes a change from the chevron structure to a slightly bent layer structure [24,25] which is termed "quasi-bookshelf." Alternatively, a quasi-bookshelf alignment has btwn obtained for spe dasses of FLC materials [26]. To summarize, there are at present four different optical structures which avoid the occuneooe of zigzag defects and are considered for applkation ... 

These electromechanic effects result in sound generation in the audible frequency range up to some kilohertz (8739). Higher harmonics may cause ultrasound (91). Therefore these effects lead to an electroacoustic response which can be exploited in applications such as speakers or headphones. The frequency characteristics and the achievable maximum vibration amplititde are two parameters that are decisive for the application of this material in such devices. As already mentioned, both depend very strongly on the quality of the alignment of the Sc FLC polymer, the layer structure (chevron, bookshelf, or other), the existence of zigzag defects in the sample, and the... 

---