Dielectrically Induced Texture Transitions

The texture transitions have been observed in smectics A many years ago [104], however, they are especially pronounced in materials with large dielectric anisotropy. A high-quality planar texture goes to a quasi-homeo-tropic optically transparent texture via intermediate structural defects (80CB, Ae = 8.2, d = 50 /xm, and voltage 30 V) [105]. Other substances were studied in [106]. A homeotropic texture can also be obtained by applying the field to the less-ordered focal-conic texture if Ae 0 [107] (erasing a defect structure produced either by the thermal action of a laser or by electrohydrodynamic instability [108]). 

The threshold voltage observed experimentally for a transition from a planar to a homeotropic texture depends on the layer thickness according to Uq oc (for the Frederiks transition the threshold voltage would be independent of thickness). The model accounting for the experimental data has been developed by Parodi [109]. 

Parodi [109] investigated five alternatives for the formation of the transition layers between the surface and the volume, by allowing for the possibility of the appearance of vacancies, by areas filled with the translationally disordered (nematic) phase, by separate cylinders of the smectic phase, etc. It is characteristic of all these models that the firee energy density is written in the form 

Only one of the alternatives studied by Parodi produces realistic values of the threshold field for a texture transition. The structure of the transition layers for this case is shown in Fig. 6.30(b). The periodic distribution of 

FIGURE 6.30. Homeotropic texture of a smectic A in an electric field Ae 0. (a) Initial geometry (b) Parodi texture transition [109] (c) wave-like instability [7], experimentally observed patterns corresponding to a Parodi transition [109] (d) from a homeotropic orientation and (e) from a planar orientation. 

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FIGURE 12 Mechanisms for inducing transitions between scattering (focal conic) and clear (homeotropic) textures in a smectic A device. The material is assumed to have positive dielectric anisotropy. 

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