Liquid crystal distortion

AH distortions of the nematic phase may be decomposed into three basic curvatures of the director, as depicted in Figure 6. Liquid crystals are unusual fluids in that such elastic curvatures may be sustained. Molecules of a tme Hquid would immediately reorient to flow out of an imposed mechanical shear. The force constants characterizing these distortions are very weak, making the material exceedingly sensitive and easy to perturb. 

In order to compensate for the distortions in the wavefront due to the atmosphere we must introduce a phase correction device into the optical beam. These phase correction devices operate by producing an optical path difference in the beam by varying either the refractive index of the phase corrector (refractive devices) or by introducing a variable geometrical path difference (reflective devices, i.e. deformable mirrors). Almost all AO systems use deformable mirrors, although there has been considerable research about liquid crystal devices in which the refractive index is electrically controlled. 

Currently, a wide variety of methods exists for calculating the molecular structure of large liquid crystal molecules which make use of pre-determined functional forms for the interactions in a molecule and semi-empirical information to parametrise the potentials. In general the interaction terms represent the energy cost of distorting bonds and bond angles from equilibrium. These can be expressed as... 

Coefficients K, Ki and in the expression for the distortion-Gibbs energy density, g, of a bulk nematic liquid crystal, with... 

According to one theoretical model, it can be assumed that the liquid crystal material is a continuum. It has been suggested that three types of distortions can take place in these structures ... 

Heat is the most common product of biological reaction. Heat measurement can avoid the color and turbidity interferences that are the concerns in photometry. Measurements by a calorimeter are cumbersome, but thermistors are simple to use. However, selectivity and drift need to be overcome in biosensor development. Changes in the density and surface properties of the molecules during biological reactions can be detected by the surface acoustic wave propagation or piezoelectric crystal distortion. Both techniques operate over a wide temperature range. Piezoelectric technique provides fast response and stable output. However, mass loading in liquid is a limitation of this method. 

Finally, magnetic nanowires and other submicrometer-scale anisometric particles can also be manipulated and organized via controlled spatial variations in the alignment of nematic liquid crystals. Leheny and co-workers, for example, measured the elastic forces imposed on anisotropic Ni nanowires suspended in a nematic liquid crystal (here 5CB, Fig. 13a), and showed that by applying a magnetic field the nanowire reorients and distorts the director in the adjacent area [445, 446]. 

The parameters of the JT distortions were calculated by the X -method for a series of crystals in good agreement with experimental melting temperatures [14]. The details of the theory and specific calculations seemingly require additional refinements, but the main idea of the JT origin of the liquid-crystal phase transition seems to be quite reasonable. This work thus makes an important next step toward a better understanding of the relation between the macroscopic property of SB and the microscopic electronic structure, the JT effect parameters. 

Similar to the situation for quadrupole-induced relaxation, the quadrupole splitting in liquid crystals is zero if the molecular symmetry is tetrahedral or higher. Electric field gradients are zero for such symmetries so there can be no quadrupolar interaction. However, one expects to see small splittings from tetrahedral or octahedral derivatives because of structural distortions. These predominately arise from specific interactions with extraneous materials such as lipophilic headgroups in surfactant systems, as seen, for instance, in both cationic and anionic octahedral cobalt(III) species [23], Much larger splittings will be expected from other structure... 

The formation of liquid crystals (LC) from suitably functionalized acetylenic SPMs has often been the motivation for their synthesis. A number of groups have sought to capitalize on the planar or nearly planar disk-like geometry of SPMs to promote the formation one-dimensional columnar stacks based on a combination of n-n stacking, van der Waals, dipole, and hydrophobic interactions. For example, Heiney, Moore, and coworkers have reported the high-resolution X-ray diffraction analysis (XRD) of a tubular, discotic LC based on SPM 133, which has been synthesized via a Sonogashira reaction [106]. These studies reveal an unanticipated distortion and... 

Various chlorine-substituted benzenes have been studied extensively by ED and MW and the most accurate results were obtained by joint analyses of ED intensities, rotational constants (from MW or high-resolution IR spectra) and dipolar coupling constants from liquid-crystal NMR spectra. Ab initio calculations have also been performed for some of these derivatives246 (Table 27). The primary interest in these studies is the degree of distortion of the benzene ring caused by chlorine substitution but in this context we will discuss only the variation of the C—Cl bond distances. In chlorobenzene this distance [173.90(23) pm] is very slightly longer than that in chloroethylene [173.0(4) pm], similar to the observation for C—F bonds. In the three disubstituted derivatives the C—Cl bond distances shorten only by a few tenths of a pm and are almost independent of the relative position of the two chlorines. These experimental results are reproduced correctly by the ab... 

Not only reaction rates, but also the positions of chemical equilibria can be influenced by liquid crystals as reaction media. A nice example is the ionization equilibrium of chloro-tris(4-methoxyphenyl)methane according to Ar3C-Cl Ar3C+ -I-C1 , which is more shifted in favour of the nearly planar triarylcarbenium ion in nematic liquid crystals as compared to in an isotropic reaction medium [868], Obviously, the discshaped carbenium ion fits better into the rod-Hke nematic phase than the tetrahedral covalent ionogen, which distorts the internal structure of the nematic liquid crystal. 

Yokoyama, H., and Van Sprang, H. A. A novel method for determining the anchoring energy function at a nematic liquid crystal-wall interface from director distortions at high fields./. Appl. Phys. 57, 4520 (1985). 

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