Nematic liquid crystals director alignment

An aligned monodomain of a nematic liquid crystal is characterized by a single director n. However, in imperfectly aligned or unaligned samples the director varies tlirough space. The appropriate tensor order parameter to describe the director field is then... 

Twisted nematic liquid crystal sandwiched between two glass plates, with the director aligned parallel to the plates, with one of the plates turned in its own plane about an axis normal to it. 

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. 

Finally, magnetic nanowires and other submicrometer-scale anisometric particles can also be manipulated and organized via controlled spatial variations in the alignment of nematic liquid crystals. Leheny and co-workers, for example, measured the elastic forces imposed on anisotropic Ni nanowires suspended in a nematic liquid crystal (here 5CB, Fig. 13a), and showed that by applying a magnetic field the nanowire reorients and distorts the director in the adjacent area [445, 446]. 

Liquid crystals may be classified as nematic, cholesteric, smectic, or columnar. Nematic liquid crystals are characterized only by orientational order. Molecules tend to be aligned with a director, as illustrated in Figure 16.1. 

A nematic liquid crystal of negative dielectric anisotropy is aligned with the director aligned orthogonal to the cell walls by means of a surfactant orientation layer, see Figure 3.4. One or two linear, elliptical or circular polarisers are... 

A standard TN-LCD consists of a nematic liquid crystal mixture of positive dielectric anisotropy contained in a cell with an alignment layer on both substrate surfaces, usually rubbed polyimide, crossed polarisers and a cell gap of 5- 0fim, see Figure 3.7. The nematic director is aligned parallel to the direction of rubbing in the azimuthal plane of the device. The alignment layer induces a small pretilt angle (6 1-3°) of the director in the zenithal plane. The... 

The nematic liquid crystal mixture containing the pleochroic dye is of positive dielectric anisotropy and is aligned parallel with the director parallel to the substrate surfaces. Therefore, plane polarised light is absorbed by the dye molecules in non-addressed areas of the display and they appear coloured. 

The use of birefringence to determine the behavior of 5( 7) is a natural choice since the principal characteristic of the nematic phase is optical birefringence i.e., the refractive index differs for light polarized parallel (/ n) or perpendicular (%) to the axis of molecular alignment. Eor a nematic liquid crystal, the director n specifies this optical z axis and / n = and = Dg are called the extraordinary and ordinary refractive indices, respectively. In general, rig > rig and the difference is the refractive index anisotropy (birefringence)... 

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.

FIGURE 7. Linear dichroic (LD) absorption spectra of phenol partially aligned in a uniaxiaUy oriented nematic liquid crystal . The curves indicate absorption measured with the electric vector of the linearly polarized radiation parallel (A) and perpendicular (B) to the director of the liquid crystalline sample. Reprinted with permission from Reference 198. Copyright (1998) American Chemical Society... 

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...

In this paper we have reviewed the structures appearing at onset of electro-convection in nematic liquid crystals. The influence of the relevant material parameters (ca and ao) and the role of the initial director alignment were explored. Our calculations using a linear stability analysis of the standard model of electroconvection (performed for zero frequency) revealed that four different scenarios characterized by different ranges of the wavenumber q can be identified (1) the Qf= 0 mode (a homogeneous deformation known as the Freedericksz transition) predicted and observed in cases C, D, E and H, which is... 

The nematic liquid crystal differs from a normal liquid in that it is composed of rod-like molecules with the long axes of neighbouring molecules aligned approximately parallel to one another. To allow for this anisotropic structure, we introduce a vector n to represent the direction of preferred orientation of the molecules in the neighbourhood of any point. This vector is called the director. Its orientation can change continuously and in a systematic manner from point to point in the medium (except at singularities). Thus external forces and Adds acting on the liquid crystal can result in a translational motion of the fluid as also in an orientational motion of the director. 

Fig. 6.5.2. Flow alignment of the director in nematic liquid crystals, (a) For rodshaped molecules the alignment angle 0 with respect to the flow direction lies between 0° and 45°, while (b) for disc-shaped molecules it lies between —90° and —45° (or equivalently between 90° and 135°).

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