Alignment nematics

Theoretical analysis indicates that occurrence of such convective instabilities depends on anisotropy of electrical conductivity and dielectric properties in the initial aligned nematic material. That is, conductivity parallel to the direction of alignment must differ from conductivity perpendicular to this direction. Calculation of the stability condition requires knowledge not only of these anisotropic electrical properties but also of anisotropic elastic and viscous properties which oppose disruption of the alignment and flow. 

The concept of local perturbations of the director around nanoparticles, often linked to homeotropic anchoring to the nanoparticle surface, is a concept often brought forward in discussions of thermal, optical and electro-optic properties of nanoparticle-doped nematic liquid crystals, which adds a slightly different perspective to the invisibility of smaller particles in aligned nematics. This appears to be of particular relevance for particles coated with either hydrocarbon chains or pro-mesogenic as well as mesogenic units. 

The dielectric constants of an aligned nematic phase are dependent upon both the temperature and the frequency of the applied field at temperatures below the clearing point. The dielectric permitivity, j, measured parallel to all three axes above the clearing point in the isotropic liquid is the same. Therefore, the dielectric anisotropy of the same compound in the liquid state is zero, see Figure 2.10. The sign and magnitude of the dielectric constants and, therefore, the dielectric anisotropy are dependent upon the anisotropy of the induced molecular polarisability, Aa, as well as the anisotropy and direction of the resultant permanent molecular polarisation determined by permanent dipole moments. 

If the cell is positioned between crossed polarisers then, when in the off-state, polarised light produced by the first polariser travels through the homeotropi-cally aligned nematic medium unchanged, is absorbed by the analyser and the cell appears dark, see Figure 3.4. There is no effective birefringence (A efr = 0) since e see Chapter 2. 

The homogeneously aligned nematic liquid crystal mixture does not rotate the plane polarised light generated by the first polariser. Therefore, the light is absorbed by the analyser. Very efficient absorption of light takes place. 

Vertically Aligned Nematic capacitive threshold voltage (volts)... 

Cell Construction. The optical interference technique to be used here is a modification of that described by Gramsbergen and de Jeu. This method uses a thin paraUel-plate cell containing a planar aligned nematic liquid crystal sample. The sample cell consists of two parallel 15 X 50 mm microscope slides separated by a gap D in the range 50-100 /zm, which is determined by spacer shims. A description of its construction is given below. 

Schematic diagrams of die cell and apparatus for measuring die temperature dependence of die optical rotation of an aligned nematic liquid crystal cell. The birefringence cell is formed from microscope slides and die lines in the cell denote the rubbing direction for the glass plates and thus the direction for the nematic director n.

Xiang Y., Lin Z. Y., Li T., Shi J., Huang J. W. and Ji L. N., A homeotropical Alignment Nematic Liquid Crystal Doped with FeTPPCl will Dramatically Decrease the Critical Magnetic Field for the Bend Reorientation in a Static Magnetic Field. Mol. Cryst. Liq. Cryst. 339 (2000) pp. 139-143. 

X of a flow-aligning nematic with flow-alignment angle 9a in a steady shearing flow, with x the flow direction and... 

Figure 10.38 Temperature-shear-rate phase diagram for 8CB in the vicinity of the transition temperature Tan = 33.58°C from the nematic to the smectic A phase. At temperatures more than 5.5°C above Tan, 8CB is a flow-aligning nematic, and it orients close to the flow direction in orientation b. As the temperature is lowered, the nematic becomes a tumbler, preferring the a orientation, mixed with either b or c. The a orientation prevails even in the smectic-A state at temperatures within a degree of Tan, especially at high shear rates. At lower temperatures, the orientation is a mixture of a and c. The globes show the ranges of orientations obtained in each of the states ac, a, a i, and b inferred from x-ray scattering. (From Safinya et al. 1991, reprinted with permission from the American Physical Society.)...

Problem 10.7 Derive Eq. (10.4) for the alignment angle 6 of the director in shearing flow of a flow-aligning nematic. 

D. Baalss and S. Hess, Nonequilibrium Molecular Dynamics Studies on the Anisotropic Viscosity of Perfectly Aligned Nematic Liquid Crystals,... 

Zuniga, I. Leslie, F.M. Shear-flow instabilities in non-flow-aligning nematic liquid crystals. Liq. Cryst. 1989, 5, 725-734. 

Figure 100 The molecular organization within the aligned nematic phase of Kevlar.

AJV = Wj - JVjj is the net anisotropic anchoring constant. NPs tend to orient parallel to n for AW>0, and recent experiments suggest this condition. Furthermore, taking into account Eq. (16) the distribution probability function of the NPs within a homogeneously aligned nematic LC phase... 

Fig. 1. Schematic diagrams of the director field distortions black lines) around particles in an aligned nematic liquid crystal. For a normal anchoring of the liquid crystal molecules at the surface of the particles, there are two possible configurations, a Dipole configuration with a companion point defect (indicated by an arrow) located in the immediate vicinity of the particle, b Quadrupolar Saturn-ring configuration with a disclination ring surrounding the particle at the equator...

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