Nematic liquid crystal director orientations

The nematic liquid crystal is orientationally soft, since restoring forces associated with deformation in the director field are very weak. This softness makes alignment of n in bulk samples occur even in very weak external magnetic or electric fields, F s H or E, or by interaction with boundary surfaces and flows in the liquid. This softness also allows for long wavelength thermal fluctuations in the director field. The Leslie viscosity parameters rather than the viscosity coefficients are the more natural quantities of interest for those methods that monitor the viscoelastic response of the nematic to director field modulations. Modulation of n in space and time manifests itself in variations of many bulk properties, e.g. the refractive index [27-37,41-44,48,51,94-106], electric susceptibility [38,39,107-110], or magnetic resonance spectra [40,45-47,111-113]. However, only a limited number of the viscosity parameters/coefficients can be precisely determined by these methods. 

Fig. 29. Schematic representation of a bend deformation (a) changes in the components of the director, n defining the orientation change (b) bend deformation of an oriented layer of a nematic liquid crystal.

Ratio of the shear stress, a, to the shear velocity gradient, y, for a nematic liquid crystal with a particular director orientation, denoted by /, under the action of an external field ... 

Note 1 The nematic liquid crystal must have a negative dielectric anisotropy (Af < 0), and a positive anisotropy (Aa > 0). The optical texture corresponding to the flow pattern consists of a set of regularly spaced, black and white stripes perpendicular to the initial direction of the director. These stripes are caused by the periodicity of the change in the refractive index for the extraordinary ray due to variations in the director orientation. 

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. 

MeOAn-ANI-3 NI in the nematic liquid crystal mixture E-7 (Merck) at two orientations of the liquid crystal director, L, taken 700 ns after a 420 nm laser pulse at 150 K. The narrow signal is an expansion of the radical pair signal, (b) Numerical differentiation of the B L L spectrum. 

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... 

The first nematic guest-host prototype nematic guest-host display device contained a nematic liquid crystal (4-butoxybenzoic acid) and a pleochroic dye (methyl red or indophenol blue) sandwiched between two (Nesa) electrodes dTn 12/im) rubbed uniaxially, but with no additional orientation layer, see Figure 3.14. One polariser was fixed to the front substrate surface with its direction of maximum absorption parallel to the rubbing direction and, therefore, the nematic director. 

The entropy production is consequently dependent on the orientation of the director. In a nematic liquid crystal consisting of prolate molecules A II >Aj l. The entropy production is consequently minimal in the perpendicular orientation. In a system consisting of oblate molecules the reverse is true, Ajj > A. Thus the entropy production is minimal in the parallel orientation. 

The nematics are similar to cholesteric liquid crystals in having just orientational long-range order, with the deviation that the director of the preferred orientation does not rotate (Fig. 2A). If, however, a chiral mesogen is dissolved in a nematic liquid crystal, the latter will transform into a cholesteric liquid crystal. 

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... 

The physics of shear waves in nematic liquid crystals is rather comph-cated. Because shear couples to reorientation, there are two separate modes— termed hydrodynamic and orientational —emanating from the oscillating crystal surface. The hydrodynamic mode mainly transports vorticity. This mode is known from simple hquids. The orientational modes mainly transport rotation of the director with regard to the background fluid. The penetration depth of the orientational mode is much smaller than the penetration depth of the hydrodynamic mode. While the amphtude of the orientational mode strongly depends on the strength of surface anchoring, the amplitude of the hydrodynamic mode does not [76]. 

Abstract A systematic overview of various electric-field induced pattern forming instabilities in nematic liquid crystals is given. Particular emphasis is laid on the characterization of the threshold voltage and the critical wavenumber of the resulting patterns. The standard hydrodynamic description of nematics predicts the occurrence of striped patterns (rolls) in five different wavenumber ranges, which depend on the anisotropies of the dielectric permittivity and of the electrical conductivity as well as on the initial director orientation (planar or homeotropic). Experiments have revealed two additional pattern types which are not captured by the standard model of electroconvection and which still need a theoretical explanation. 

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. 

Japan, Mar. 19-20, 2002, ed. M. Iwamoto, K. Kaneto and S. Mashiko, Elsevier Science B.V., Amsterdam, Netherlands, 2003 R 249 A. Sugimura and G.R. Luckhurst, Explanation of the Static and Dynamic Director Orientation in Thin Nematic Liquid Crystal Films Using Deuterium NMR Spectroscopy , p. 313... 

Fig. 5 Schematic depiction of the optical field propagating as an o-wave in a planar nematic liquid crystal cell, ba is the orientational fluctuation of the director axis. 

---