Anisotropic forces, effect

Noncentrosymmetric crystals may generate measurable anisotropic electrical effects in response to external forces snch as temperatnre changes... 

McKellar A R W and Welsh H L 1971 Anisotropic intermolecular force effects in spectra of H2- and D2-rare gas complexes J. Chem. Phys. 55 595-609... 

Orientation in crosslinked elastomers primarily reflects the configurational entropy and intramolecular conformational energy of the chains. However, as first shown by deuterium NMR experiments on silicone rubber (Deloche and Samulski, 1981 Sotta et al., 1987), unattached probe molecules and chains become oriented by virtue of their presence in a deformed network. This nematic coupling effect is brought about intermolecular interactions (excluded volume interactions and anisotropic forces) which can cause nematic coupling (Zemel and Roland, 1992a Tassin et al., 1990). The orientation is only locally effective, so it makes a negligible conttibution to the stress (Doi and Watanabe, 1991), and the chains retain their isotropic dimensions (Sotta et al., 1987). 

The third feature has to do with (the absence of) polarization effects in tw o-laser experiments. The hole-burning strength has been measured for different (parallel and mutually perpendicular) laser polarizations without any observable difference. Also, this third feature points to a strong coupling, this time between internal rotation of the constituents and the end-over-end rotation. In principle this observation is in agreement with recent discussions of anisotropic forces between SFg molecules. 

For a calculation of d. see R- H. Fowler. Statistical Thermodynamics. Second Edition, Cambridge University Press. 1956. p. 127. In Section 1.5a of Chapter 1 we defined the compressibility and cautioned that this compressibility can be applied rigorously only for gases, liquids, and isotropic solids. For anisotropic solids where the effect of pressure on the volume would not be the same in the three perpendicular directions, more sophisticated relationships are required. Poisson s ratio is the ratio of the strain of the transverse contraction to the strain of the parallel elongation when a rod is stretched by forces applied at the end of the rod in parallel with its axis. 

Wood is an anisotropic material that undergoes uneven dimensional changes and, under extreme variations of environmental conditions, becomes distorted and warped (see Chapter 10). Exposed to the atmosphere, wood is also susceptible to the mechanical forces of wind and rain, and the effects of solar radiation the latter, in particular, causes discoloration initially, and then photochemical degradation, which often results in the wood s total decomposition. Wood is also prone to consumption by bacteria, fungi, insects, and rodent animals (Unger et al. 2001). 

Calculations of forces may be improved in several ways. One is to pursue efforts towards the development of accurate classical, atomic-level force fields. A promising extension along these lines is to add nonadditive polarization effects to the usual pairwise additive description of interatomic interactions. This has been attempted in the past [35-39], but has not brought the expected and long-awaited improvements. This is not so much because polarization effects are not important, or pairwise additive models can account for them accurately in an average sense in all, even highly anisotropic environments. Instead, it seems more likely that the previously developed nonadditive potentials were not sufficiently accurate to offer an enhanced description of those systems in which induction phenomena play a crucial role. 

Fowkes, F.M. Surface effects of anisotropic London dispersion forces in n-alkanes, J. Phys. Chem., 84(5) 510-512, 1980. 

Physical properties of liquid crystals are generally anisotropic (see, for example, du Jeu, 1980). The anisotropic physical properties that are relevant to display devices are refractive index, dielectric permittivity and orientational elasticity (Raynes, 1983). A nematic LC has two principal refractive indices, Un and measured parallel and perpendicular to the nematic director respectively. The birefringence An = ny — rij is positive, typically around 0.25. The anisotropy in the dielectric permittivity which is given by As = II — Sj is the driving force for most electrooptic effects in LCs. The electric contribution to the free energy contains a term that depends on the angle between the director n and the electric field E and is given by... 

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