Anisotropic modes

Although the ratio is not determined to a good accuracy when it reaches large values, interpretation of data shows that Tq is at least 150 or 200 times shorter than for each polymer under investigation. This implies that the anisotropic mode, which is observed in addition to the segmental reorientation in the NMR relaxation experiments, is indeed a very fast process. The high frequency of this motion is one of the main reasons for its assignment to a libration of the internuclear vector about its rest position. 

In this section we discuss electrohydrodynamic (EHD) instabilities, that is electric-field-induced phenomena that are caused by the flow of a liquid crystal (see also [8,219]. The reason for the flow is electrical conductivity, which has been disregarded in previous sections. The flow may arise either independently of the anisotropic properties of substance, as in isotropic liquids (isotropic modes of the electrohydrodynamic instability), or may be driven by the conductivity anisotropy, as in liquid crystals (anisotropic modes). The threshold for EHD instabilities depends on many parameters, such as the electrical and viscoelastic properties of substance, the temperature, and the applied field frequency. Due to flow distortion of the director alignment, the instability is usually accompanied by a characteristic optical... 

The other two instabilities shown in Fig. 28 may be observed only in liquid crystals (nematic, cholesteric, and smectic C). The first is the Carr-Helfrich instability, which is caused by a low-frequency electric field and occurs in the form of elongated vortices with their axis perpendicular to the original director alignment. The vortices cause a distortion of the director orientation, which is observed optically as a one-dimensional periodic pattern (Kapustin-Williams domains). The other anisotropic mode is observed only in highly conductive liquid crystals. For its interpretation the inertial term dvidt for the fluid velocity must be taken into account, which is why this mode may be called inertial mode. 

This anisotropic mode is observed at rather high frequencies (10 -10 Hz) in the case of... 

Carr-Helfrich one in nematics and may be studied quantitatively theoretically [280]. At high frequencies, an instability is observed with a characteristic frequency dependence of the threshold field Elayer thickness (fundamental domains). This has been regarded as an analog of the dielectric regime [282], but it can also be interpreted as the electrolytic mode [283] with some specific features. In some special cases a new domain mode is observed [284], which has been referred to the inertial (anisotropic) mode (discussed in Section 9.4.1.3 of this Chapter). 

For defect sizing by TOED, use of L waves involves a penalty in resolution of almost a factor of two at a given frequency because of difference in velocity as compared to shear waves and use of SV waves runs into difficulties because of the mode conversion problems. Further, problems due to couplant thickness variations, surface roughness affects, beam skewing and distortion problems in anisotropic welds can also be expected. On the contrary, SH waves are not affected... 

When we compare the VV with the VH configurations for the polarised light, the Raman intensity shows anisotropic behaviour. Most importantly, the A g mode at 165 cm is suppressed in the VH configuration, while the lower frequeney E g and E2g modes are not suppressed. This anisotropy is due to the... 

The thermal strain measurements described above have the common feature of anisotropic behaviour in a supposed isotropic state (cubic structure). These observations go well beyond the short-range, static strain fields associated with the lattice impurities responsible for Huang scattering. This then raises the question of the temperature at which the lattice symmetry changes and the implications of this for the central mode scattering. 

An indirect mode of anisotropic hyperfine interaction arises as a result of strong spin-orbit interaction (174)- Nuclear and electron spin magnetic moments are coupled to each other because both are coupled to the orbital magnetic moment. The Hamiltonian is... 

In the foregoing sections, the rate-enhancing effect of alkylammonium micelles has been extensively described. Similar effects can be expected for bilayer membranes of dialkylammonium salts. In addition, specific catalytic processes may be realized in this new system by taking advantage of the peculiar membrane structure. For example, catalyst molecules which are anisotropically bound to the membrane may act in very specific manners, and the liquid-crystalline nature of the bilayer membrane should provide unique microenvironments for catalysis. These are particularly interesting in relation to the mode of action of membrane-bound enzymes. 

Finally, lasers are examples of nonthermal and anisotropic radiation sources. The radiation field is concentrated in a few modes. Moreover, this radiation is emitted into a small solid angle. 

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