Materials of Negative Dielectric Anisotropy

We have so far discussed only materials of negative dielectric anisotropy. Electrohydrodynamic distortions are observed even in weakly positive materials,but only when the initial orientation of the director is perpendicular to the applied field. Striations appear above a threshold voltage but vanish at still higher voltages and there is no dynamic scattering. The frequency dependence of the threshold voltage is shown in fig. 3.10.6. 

At low voltages (-1 V), the molecules are aligned normal to the electric field and therefore parallel to the glass plates, as expected for a material of negative dielectric anisotropy. In this regime we have a nematic single crystal. 

Case A planar alignment, Ca <0 0. This is the most studied, classical case, since the conductivity anisotropy of usual nematics (substances without a smectic phase) is typically positive. As for , there is a wide range of materials with negative dielectric anisotropy. 

The texture change or memory effect is observed in cholesteric materials with negative dielectric anisotropy [71]. The liquid crystal layer is homogeneously oriented by boundary forces to form the planar texture which is completely transparent if the band of selective light reflection is outside the visible spectrum. The substrates are covered with conducting films that are in contact with the liquid crystal. When a d.c. or low frequency field is applied, the sample is transformed to the so-called focal conic texture. In this texture, the liquid crystal is broken up into small domains which are randomly oriented and have diameters of a few microns. Since these domains are optically anisotropic, they act as scattering centers for visible light. Therefore the focal conic texture exhibits a milky white appearance. 

In EHC, for the usual case of driving with a pure ac field of frequency to = Unf, the eigenvector Uq of Eq. 30 inherits the additional periodic time dependence and the eigenvalue 1 becomes a Floquet coefficient. Then there is an additional discrete symmetry (z, t) (-Z, t + 1/(2/)) and each component of Uq has a definite parity. Generally the conductive mode (for even parity the out-of-plane component of director and vy, for odd parity the in-plane components of velocity) destabilizes first at low frequencies f. For materials with negative dielectric anisotropy, <, < 0, there exists a cut-offfrequency fr so that the dielectric mode with the other parity destabilizes first for f>fc, where oo for 0. The existence of these two regimes was first pointed out by Orsay s group [9, 10]. For frirther details see Refs. [14, 61]. 

The conduction regime as described by Figs. 2 to 4 occurs in suitably doped nematic liquid crystals of negative dielectric anisotropy. This can be seen qualitatively by studying the denominator of Eq. [32]. The first (viscous) term must be larger than the second (dielectric) term for the denominator to be positive and instability to occur. This restricts negative values of Ae to the range —2 to 0 for materials... 

For a nematic material with positive dielectric anisotropy, induced birefringence can also be observed. However, the liquid crystal must be in the uniform parallel orientation at zero volts.Above the threshold voltage, the director aligns itself parallel to the applied field. With crossed polarizer and analyzer, the voltage dependence of the light intensity is reversed from that described previously for a fluid of negative dielectric anisotropy. ... 

In nematic materials with negative dielectric anisotropy and electrical resistivity less than 1-2 X 10 ohm-cm, conduction-induced fluid flow occurs during the application of an applied voltage. The wide-angle forward-scattering phenomenon known as dynamic. scatter-ing 9,60 ig most important manifestation of the turbulence that... 

Optical storage effects in mixtures of nematic and cholesteric materials with negative dielectric anisotropy were first observed by Heil-meier and Goldmacher. They reported the following sequence of... 

In the case of those LC materials which exhibit negative dielectric anisotropy, cells can be constracted which align vertically and twist on applying a field, exactly the converse to the twisted nematic effect from positive anisotropic LCs. Cells of this type are of interest because they can form a superior black state. 

Scheme 4.11 Examples of typical super-fluorinated materials (SFM) used in the current generation of active matrix LCD. The liquid crystals 7-13 have positive dielectric anisotropy, compounds 14 and 15 have negative dielectric anisotropy. The approximate orientation of the molecular dipole moment for the two classes of material is indicated by arrows.

There are two types of electro-optic effects which occur with cholesteric materials. The materials which possess negative dielectric anisotropy exhibit the so-called reflective optical storage mode while positive materials undergo field-induced cholesteric-nematic phase changes. 

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