Optical texture dependence

In the following sections the electrochemical reactivity of single grains of polycrystalline Ti is explored by using the nl-droplet method. The results from electrochemical measurements and the optical laser techniques from the previous section are combined to yield a band structure model for anodically grown anodic oxide layers. Other applications of this method to study laser induced corrosion, texture dependent photocurrent and corrosion of anodic oxide films are described in Refs. [89,90 and 91]. 

Fissure propagation through the pore wall material of coke is also dependent on the size and orientation of anisotropic components present fissures propagate easily through large optical textures (>60 ym) while mosaic textures (1 to 10 ym) tend to act as fissure stops. Experimental verification of this aspect of fissure generation and propagation will be discussed below ... 

Azobenzenes have simple chemical structures and exhibit simple photoisomerization between the trans- and cis form. However, they can really provide various photofunctions by their combination with sophisticated systems such as Ch LCs. The Ch LCs have a good potential for display applications, because they show interesting optical properties depending on the helical structures and the textures. Many types of optical materials can be created by the use of the photoisomerization of chiral azobenzenes in the Ch LCs. New ideas, including design of molecules and systems, may allow us for future materials not only in the field of optics but also in other fields. 

Polymer liquid crystals generally form unusual textures, accompanied by complex and unstable superstructures. Such textures depend on a sample history, especially mechanical and thermal histories, and also on the surface nature of the vessels. We have described the structural characteristics of polymer liquid crystals under a shear force by means of rheo-optical methods. ... 

Texture transitions are particularly pronounced when an electric field is applied to materials having a large dielectric anisotropy. A planar texture undergoes transition to a quasihomeotropic optically transparent texture via intermediate structural defects [150,151]. The threshold voltage observed experimentally for a transition from a planar to a homeotropic texture depends on the layer thickness according to f/oc (for the Frederiks transition the threshold voltage is independent of thickness). A model that accounts for the experimental data (at least partly) has been developed by Parodi [152] who assumed the formation of transition layers between the surface and the bulk of a sample. A discrepancy between the calculated and observed periods of the texture instability may be due to a nonuniform... 

An example of an experiment involving continuous sample rotation with synchronized data acquisition is shown in Fig. 10 [123]. A thin liquid crystal cell filled with a side chain liquid crystal polymer was continuously rotated about an axis perpendicular to the magnetic field. The director behavior was followed by deuteron NMR as well as by polarizing microscopy. The optical texture and the orientation-dependent... 

For phase identification, therefore, it is essential to be familiar with the different birefringence textures each phase can exhibit. These textures depend on molecular alignment with respect to the optical axis and the polarizers. To illustrate this, we can consider the Schlieren texture of the nematic phase shown in Figure 2.22. 

Apart from high strength materials formed from nematic polymer fibres, most applications of nematic liquid crystals depend on their anisotropic optical properties. As a consequence the refractive indices of nematics are of prime importance in the development of materials for applications. The refractive indices are determined by the molecular polarisability coupled to the mientational order of the mesogens in the liquid crystal phase, so refractive indices can provide a direct probe of the order parameter. Furthermore the optical properties of liquid crystal films are frequently used to determine phase behaviour, identify phase types through characteristic optical textures or explore the properties of defects, and such experiments rely on the anisotropy in the refractive index of the material. The first tool of a liquid crystal scientist is the polarising microscope, which emphasises the importance of optical properties in general and refractive indices in particular to the stufy of liquid crystals. 

The appearance of focal conic structures depends on the direction of viewing. When the epitaxial interactions between the mesophase and the substrate surface, cause the axes of the focal conic units to lie vertical, the polygonal texture appears (Figs. 1.23 and 1.24). On the other hand, samples with focal conic units lying obliquely to the viewing direction, give the characteristic fan texture (Fig. 1.25). [A note of caution for decades, when faced with an undecipherable optical texture with a jumbled mass of domains, workers in the liquid crystal field have often taken the easy option and simply called it a focal conic fan texture to disguise the fact that they had no idea what it was. They have usually been correct, but not always, and mesophases of very different structures have been seriously misidentified on more than one occasion.]... 

A comparative study has been made by optical and electron microscopy of the anisotropic texture of several cokes from caking coals and pitches carbonized near their resolidification temperature. A simple technique made it possible to examine, by both methods, the same area of each sample and to identify the corresponding zones of the two very similar images. The anisotropy observed in polarized light appears in electron microscopy as differences in contrast resulting not from inequalities in electron absorption, but, as revealed by microdiffraction and dark Reid examinations, from diffraction phenomena depending on the general orientation of the carbon layers within each anisotropic area. 

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