Surface orientational order, nematics

Barbero, G., Dozov, I., Palieme, J.F., Durand, G. Order electricity and surface orientation in nematic liquid crystals. Phys. Rev. Lett. 56, 2056-2059 (1986)... 

Recently, the surface tensor model has been used together with the dielectric continuum model to calculate the orientational order parameters of solutes in nematic solvents [8,9,27], Figure 2.32 shows the theoretical results for anthracene and anthraquinone in nematic solvents with different dielectric anisotropy. Considering only the surface tensor contribution, positive Szz and Sxx and negative are obtained, with Szz > Sxx > Syy. This corresponds to what could be expected on the basis of the molecular shape the long axis (z) is preferentially aligned with the director, and the normal to the... 

Relationship between elasticity and orientational order As remarked in chapter I, a uniformly oriented film of nematic liquid crystal may be prepared by prior treatment of the surfaces with which it is in contact. If the preferred orientation imposed by the surfaces is perturbed, let us say by a magnetic field, a curvature strain will be introduced in the medium. The theory of such a deformation will be discussed at length in 3.2 for the present it will suffice to state some of the important results. The free energy per unit volume of the deformed medium relative to the state of uniform orientation is... 

We now turn our attention very briefly to the nematic liquid crystal surface. A variety of experimental studies have established conclusively that orientationally ordered states, and in certain materials even density modulations, develop in the vicinity of the free surface. We describe below the salient features of these observations. 

This question can be answered by considering the experiments, where liquid crystals have been spread across solid crystal surfaces [15]. Macroscopic observations under a polarizing microscope reveal that crystal surfaces do orient nematic liquid crystals, which leads to the conjecture that there may be microscopic positional and orientational order at a crystal-liquid crystal interface. Indeed, this local interfacial order can be revealed in experiments with a Scanning Tunneling Microscope (STM), first perfoi med by Foster and Frommer [16]. An example of an STM image of a crystal-nematic liquid crystal interface is shown in Fig. 1.4 for the case of 8CB on graphite. One can... 

Another part of the NMR research of microconfined liquid crystals focused on the detection and measurements of the weak orientational order induced by solid surfaces above the transition temperature Tm where the bulk liquid crystal turns into the isotropic phase. The onset of orientational order on approaching Tni from above, surface wetting phenomena and the continuous or discontinuous nature of the transition in a cavity have been of particular interest. The contribution of NMR to the knowledge of these phenomena is the topic of the present article. As for the different names that have been used in the literatrrre to denote the high-temperature phase of microconfined liquid crystals, we will use the term isotropic as long as the phase transition into the nematic phase is discontinuous, but it is understood that truly isotropic is only the substance far away from the confining surfaces whereas a certain degree of orientational order persists next to the walls. 

There are basically two possible scenarios for the behavioxir of this prenematic mean field force when the separation between surfaces is reduced, and both depend on the degree of the surface-induced nematic order. For low surface-induced order, the magnitude of the attractive force just increases when approaching the isotropic-nematic phase transition from above. On the other hand, if a surface induces a high degree of LC orientation, the prenematic phase can spontaneously transform into the nematic phase, when the separation is decreased below a certain value. This is the nematic capillary condensation, that is discussed further on in this Chapter. 

The smectic layering transition [45] is a phenomenon, where smectic layers grow one by one, parallel to the surface, when the temperature approaches the isotropic-smectic phase transition from above. Ellipsometric observation of these steps is possible due to the coupling between the smectic order and the nematic (orientational) ordering. Consequently, the orientational order in... 

A qualitative picture, Fig. 10.4, shows the distance dependencies of the orientational order parameter for homeotropically aligned nematic liquid crystal at the solid substrate. The problem is to explain such dependencies [6]. The influence of the surface on the orientational order parameter may be discussed in terms of the modified Landau-de Gennes phase transition theory. Consider a semi-infinite nematic of area A being in contact with a substrate at z = 0 and uniform in the x and y directions. When writing the free energy density a surface term -W8(z)S must be added to the standard expansion of the bulk free energy density ... 

Fig. 10.4 Qualitative dependencies of the orientational order parameter of the nematic phase on the distance z from the sutface. The positive (a) and negative (b) surface potential IT has the form of the 5-function. The temperature is fixed...

---