Rubbing chevrons

The chemical composition of zeolites may be quite complicated, and they may possess a large variety of different structures. This has turned into a myriad of names for zeolites, where each material, natural or synthetic, claimed or supposed to be different in terms of structure, composition and/or properties has received a specific and arbitrary name. In addition to mineral names (such as faujasite, mordenite, natrolite, etc.) there is a vast variety of names for synthetic materials. Very frequently, synthetic zeolites are named with a trivial code related to the claimed inventor or owner of patent protected rights followed by a sequential number. For instance, ZSM-5, SSZ-24 and RUB-13 are zeolites discovered by researchers at Mobil, Chevron and Ruhr University Bochum, respectively. There are also zeolites with other names, such as zeolite Beta (despite its discovery by Mobil researchers), silicalite (a pure Si02 version of ZSM-5) and Theta-1 (discovered by BP researchers). 

The stripe-shaped texture parallel to the rubbing direction is observed due to the formation of periodic undulation of the layer structure (Fig. 6.3.19). In the case that a strong AC voltage is applied, the chevron structure is deformed into the bookshelf structure and the strain occurs. The strain is relaxed by the rotation of the layer direction [55] which forms the periodic layer undulation. 

Also, the optical state (transmission, color) is very often practically the same on both sides of a zigzag wall, as in Fig. 93b. Indeed, if the director lies parallel to the surface (pretilt o =0) at the outer boundaries, the chevron looks exactly the same whether the layers fold to the right or to the left. However, if the boundary condition demands a certain pretilt a 5 0, as in Fig. 105, the two chevron structures are no longer identical. The director distribution across the cell now depends on whether the director at the boundary tilts in the same direction relative to the surface as does the cone axis, or whether the tilt is in the opposite direction. In the first case we say that the chevron has a C1 structure, in the second a C2 structure (see also Fig. 106). We may say that the Cl structure is natural in the sense that if the rubbing direction (r) is the same at both surfaces, so that the pretilt a is symmetrically inwards, the smectic layer has a natural tendency (already in the SmA phase) to fold accordingly. However, if less evident at first sight, the C2 structure is certainly possible, as demonstrated in Figs. 105 and 106. 

Figure 111. Geometric features in the Canon screen of Fig. 87 showing rubbing directions, smectic layer direction, chevron direction, and director pretilt a, equal to the optic axis tilt out of the screen surface.

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