Liquid crystals additive effects

Polypeptides such as proteins are found to markedly enhance the HR effect [59]. The liquid crystal additive with a higher dielectric constant than that of the carrier fluid can also substantially increase the yield stress of Na-type /eolite/oil F.R fluid [60]. The yield stress of a 30 vol% zeolite NaY/silicone oil suspension with and without the liquid crystal additive, 4-heptyl-4 -cyance-biphenyl, is plotted as a function of the applied electric field in Figure 9. The addition of 1 vol% 4-heptyl-4 -cyance-biphenyl can increase the yield stress of zeolite NaY/siliconc oil suspension almost 4 times. A theory on the ER additive is proposed by taking into account the surface tension, dielectric and conduction effects on the ER fluid perfonnance [61]. Based on this theory, an additive should have a higher dielectric constant, lower conductivity, larger surface tension as opposed to that of the carrier fluid. 

In this section results will be given for the refractive indices of some specimra liquid crystals. First results for standard materials will be givoi to illustrate the temperature and wavelength dependence of the refractive indices of nematic liquid crystals. Additionally selected results are given for a variety of other mesogens, to illustrate the effect of molecular structure modifications on the refractive indices. Further results for materials of importance in display applications will be givoi in Chapter 11. 

The smectic layers may themselves compress or dilate in a full general theory of deformations in smectic liquid crystals. These effects have not been considered in the theory presented above because, in many instances, it may be assumed that layer compression or dilation may be neglected in basic planar layered geometries of SmC liquid crystals. Nevertheless, for the sake of completeness, we record for the interested reader some details from the literature. The additional term for smectic layer compression in SmA liquid crystals, which also serves as a first approximation to the compression term in SmC for the planar geometry pictured in Fig. 6.3, is of the form mentioned by de Gennes [106] and de Gennes and Frost [110, pp.345-346]... 

Peptides represent yet another family of intervening materials that provide a probe for conformational effects. Because peptides can aggregate as helices or as sheets, they produce a macromolecular order comparable to that of liquid crystals. Dipole effects in helical peptides are additive, so that a macroscopic dipole from the C- to the N-end of the peptide (or vice versa) can enhance or oppose the pulsed PET. 

Finally, there are groups of liquid crystals where, at the current time, force fields are not particularly useful. These include most metal-containing liquid crystals. Some attempts have been made to generalise traditional force fields to allow them to cover more of the periodic table [40, 43]. However, many of these attempts are simple extensions of the force fields used for simple organic systems, and do not attempt to take into account the additional strong polarisation effects that occur in many metal-containing liquid crystals, and which strongly influence both molecular structure and intermolecular interactions. 

In n-octane/aqueous systems at 27°C, TRS 10-80 has been shown to form a surfactant-rich third phase, or a thin film of liquid crystals (see Figure 1), with a sharp interfacial tension minimum of about 5x10 mN/m at 15 g/L NaCI concentration f131. Similarly, in this study the bitumen/aqueous tension behavior of TRS 10-80 and Sun Tech IV appeared not to be related to monolayer coverage at the interface (as in the case of Enordet C16 18) but rather was indicative of a surfactant-rich third phase between oil and water. The higher values for minimum interfacial tension observed for a heavy oil compared to a pure n-alkane were probably due to natural surfactants in the crude oil which somewhat hindered the formation of the surfactant-rich phase. This hypothesis needs to be tested, but the effect is not unlike that of the addition of SDS (which does not form liquid crystals) in partially solubilizing the third phase formed by TRS 10-80 or Aerosol OT at the alkane/brine interface Til.121. 

An important addition compared to previous models was the parameterization of the internucleosomal interaction potential in the form of an anisotropic attractive potential of the Lennard-Jones form, the so-called Gay-Berne potential [90]. Here, the depth and location of the potential minimum can be set independently for radial and axial interactions, effectively allowing the use of an ellipsoid as a good first-order approximation of the shape of the nucleosome. The potential had to be calibrated from independent experimental data, which exists, e.g., from the studies of mononucleosome liquid crystals by the Livolant group [44,46] (see above). The position of the potential minima in axial and radial direction were obtained from the periodicity of the liquid crystal in these directions, and the depth of the potential minimum was estimated from a simulation of liquid crystals using the same potential. 

Although, in theory, intrinsically thermochromic compounds should be the materials of choice for many apphcations, in practice, with the major exception of thermotropic liquid crystals (see Chapter 5, section 5.2.2), they often require quite high temperatures to effect the change, and this has limited their use. Additionally, there is also a problem with synthesising compounds to cover the desired colour gamut across the visible spectrum. Consequently, indirect systems, in which the chro-mophoric material reacts to changes in its environment brought about by heat, have... 

Micellar properties are affected by changes in the environment, eg, temperature, solvents, electrolytes, and solubilized components. These changes indude complicated phase changes, viscosity effects, gd formation, and liquefication of liquid crystals. Of the simpler changes, high concentrations of water-soluble alcohols in aqueous solution often dissolve micelles and in nonaqueous solvents addition of water frequendy causes a sharp increase in micellar size. 

Lopatina and Selinger recently presented a theory for the statistical mechanics of ferroelectric nanoparticles in liquid crystals, which explicitly shows that the presence of such nanoparticles not only increases the sensitivity to applied electric fields in the isotropic liquid phase (maybe also a possible explanation for lower values for in the nematic phase) but also 7 N/Iso [327]. Another computational study also supported many of the experimentally observed effects. Using molecular dynamics simulations, Pereira et al. concluded that interactions between permanent dipoles of the ferroelectric nanoparticles and liquid crystals are not sufficient to produce the experimentally found shift in 7 N/ so and that additional long-range interactions between field-induced dipoles of nematic liquid crystal molecules are required for such stabilization of the nematic phase [328]. 

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