Structure of Surface Layers

FIGURE 3.1. The interfacial layer of a nematic liquid crystal (z 0) in contact with a substrate (z 0). 

The most interesting case is a contact of a nematic liquid crystal with a solid substrate because in most devices a nematic is sandwiched between transparent 

1 Surface Induced Change in the Orientational Order Parameter 

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  

Here 8(z) is Dirac delta function, showing that the surface potential W = W (z = 0) is short-range, 9 is an angle between the longitudinal axis of a rod-like molecule and the director at the surface n. The surface potential W may be positive as in Fig. 10.4a or negative. Fig. 10.4b. In this consideration, a change of the 

Now we need two minimizations (i) over function S(z) with fixed So (z = 0) and (ii) over the boundary value Sq. In our case, the standard Euler equation 

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Phase transitions in overlayers or surfaces. The structure of surface layers may undergo a transition with temperature or coverage. Observation of changes in the diffraction pattern gives a qualitative analysis of a phase transition. Measurement of the intensity and the shape of the profile gives a quantitative analysis of phase boundaries and the influence of finite sizes on the transition. ... 

For the application of the explained surface pretreatment methods and their effects, the structure of surface layers typical for metal materials will be described below (Figure 7.7). 

Analysis of the Local Atomic Structure of Surface Layers from SEFS Experimental Data... 

Structure of Surface Layers The adsorption of fatty adds onto polar surfaces has been widely studied, and it usually results in a layer with the carboxyl group at the surface and the hydrocarbon chains oriented vertically to the surface. If the surface is microscopically smooth enough, and the chains sufficiently long, then the layer can be considered as semicrystalline. When packed in this way, the area occupied at the surface by one molecule of a saturated, linear, fatty add is about 0.21 nm. Fatty acids with branched chains, and those containing unsaturation, do not allow such close packing and hence occupy larger areas. Similar layers are believed to form on... 

The bulk properties are not influenced by the major modifications of the chemical structure of surface layers. 

The goodness of fit between the experimental points and the calculated curves from a model based on even a sophisticated advanced theory [52] does not prove the reality of the theoretical model, the assumed structure of surface layer, and the presumed interfacial reactions. Any model assumption has to be supported by independent experimental facts (e.g., surface spectroscopy, analysis of equilibrium bulk phase) to prove its reality. 

Surface energy was already shown to affect the chemical structure of surface layers. In theories of adhesion, the work of adhesion between two identical surfaces is given by this equation ... 

Morphological and chemical structure of surface layers depend on conditions of material formation. Two important principles are discussed here ... 

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