Director anisotropic material

In the nematic phase, the molecules are able to move in all directions. This means that there is no positional order of the centers of mass but, on the average, they keep their long axes locally parallel (the LC director tends to point in the same direction). Liquid crystal polymers are anisotropic materials so their physical characteristics are strongly dependent on the average alignment with the director (Acierno and Brostow 1996). 

EHrector field of an anisotropic material sandwiched between two parallel surfaces assuming uniform director field in the xy plane. The lilt angle 0is the lilt angle between n and the z axis. 9 and 0- are the surface tilt angles. 

Let us consider the possible deformations (splay, twist and bend) of anisotropic materials. As illustrated in Figure C.l, without loosing generality we choose the coordinate system so tirat the z-axis be parallel to the imdistorted director field (iiIIz ). 

Proton, deuteron and carbon spin relaxation measurements of liquid crystals have provided detailed information about the molecular motions of such anisotropic liquids (anisotropic rotation and translation diffusion of individual molecules), and about a peculiar feature of liquid crystalline phases, namely collective molecular reorientations or order fluctuations. Spin relaxation in liquid crystalline mesophases has challenged NMR groups since the early 1970s, shortly after the publication of theoretical predictions that order fluctuations of the director (OFD, OF), i.e. thermal excitations of the long-range orientational molecular alignment (director), may play an important unusual role in nuclear spin relaxation of ordered liquids. Unique to these materials, which are composed of rod-like or disc-like (i.e. strongly anisotropic molecules), it was predicted that such thermal fluctuations of the director should, at the frequencies of these fluctuation modes, produce rather peculiar Ti(p) dispersion profiles. For example in the case of uniaxial nematic... 

The electro-optic effects described above all refer to director reorientation induced through the dielectric properties (Af) of the polymer. If, however, a change in the optical properties is induced by a current flow resulting from the anisotropic electrical conductivity (A a) of the material, then the so-called electro-hydrodynamic effects are observed. ... 

Liquid crystals are organic compounds in a state of matter intermediate between that of an isotropic liquid and an anisotropic crystalline soUd. Most liquid crystalline molecules are markedly elongated and rod like. In nematic liquid crystalline materials, the directions of the long molecular axes (directors) are arranged parallel to each other. Adding a chiral molecule to a nematic phase causes a superstructure comparable to the steps of a spiral staircase. The structure can be understood as... 

In liquid erystals, the anisotropic nature of the materials means that there are at least two different components to the diffusion tensor. Measurements of unaligned liqujd crystal samples give a weighted average (D) of the two principle values [49] that describe diffusion parallel and perpendicular to the director... 

Nematic liquid crystals are 3D anisotropic fluids, and as such they have no translational order, i.e., they do not support extensional or shear strains. For this reason, the rheology of nematic liquid crystals is similar to conventional organic liquids with similar size of molecules. The main difference is due to the anisotropic nature of the materials the director distortion results in elastic responses, and the magnitude of the viscosity depend on the relative orientation of the director with respect to the velocity gradient. 

---