Surface pretilt

Note 3 Surface pretilt is the deviation angle of the director away from the surface. It is used to control the threshold voltage and affects viewing angles. 

The molecular orientational states of SSFLCs are classified by the optical viewing conditions and the relationship between the directions of bend of the layer structure and the surface pretilt angle. The molecular orientational models of the states have been considered and illustrated with regard to the experimental results, and useful information has been obtained from optical simulations using the models. The influence of the surface pretilt angle on the orientational and the optical properties of SSFLCs has been described. 

The molecular orientational states of the SSFLCs have been analyzed by polarizing microspectroscopy and optical simulation. The X-ray studies indicated that the chevron layer structure is determined by the bulk properties of the FLC, but the molecular orientation in the smectic layer is strongly influenced by the surface properties. The effect of surface pretilt angle on the molecular orientation and the optical properties of SSFLCs have been studied by the optical simulation based on the molecular orientational models. 

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.

It is known that memory angle depends on surface pretilt angle. The temperature dependences of the memory angles of the CIU and C2U states were measured for all cells, and are shown in Figs 5.1.17(a)-(c). The solid lines represent the... 

In Fig. 5.1.25, the director twist angle 4> is plotted as a function of the cell thickness direction Y at various surface pretilt angles, where represents the director twist angle at the chevron interface and is expressed as... 

Fig. 5.1.24 Relationships between the surface pretilt an e and the memory angle of the CIU state (O) and the C2U ( ) state at 25°C for (a) CS-1014, (b) SF-1212 and (c) SCE-8. Solid lines indicate simulation results by using the models.

Fig. 5.1.26 Calculated transmission spectra for various surface pretilt angles of (a) CIU and (b) C2U states.

On the other hand, the memory angle of the C2U state increases as the surface pretilt angle decreases. However, the memory angle of the C2U state seems from observations to be almost constant, because it depends only slightly on surface pretilt angle. 

These director profiles were calculated for CS-1014. The simulated memory angles of each director profile are shown in Fig. 5.1.25. In Pig. 5.1.26, the calculated transmission spectra for each director profile are shown. These figures indicate that memory angle and the transmission intensity in the bright state depend on the surface pretilt angle. 

Ferroelectric liquid crystal devices have suffered from a number of characteristic alignment defects which are absent from nematic liquid crystal devices. The most important of these, termed zig-zag defects (from their characteristic shape) result from the chevron structure (Fig. 23). The chevron can form with the layer tilt toward either of the directions perpendicular to the layer planes if there is also a surface pretilt, its direction and that of the layer tilt may... 

Figure 90. Surface-stabilized configuration with less than optimum efficiency, switchable between two symmetric states with low optical contrast. The surface pretilt angle has been chosen equal to the smectic tilt angle 0in this example. For a strong boundary condition with zero pretilt, a different extreme limiting condition with approaching zero at the boundary is also conceivable, without any essential difference in the performance of the cell.

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. 

It should be mentioned that Fraser [92] has carried out a theoretical investigation of the twisted nematic device that incorporates surface pretilt of the director and electric field effects. Much of the analysis is naturally extended from the ideas presented on pretilt in Section 3.4.2 and electric field effects in Section 3.5. 

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