Nematic-like order

General Flow Properties for Nematic-like Ordered Fluids... 

Similar discrepancies have been frequently reported for colloidal spheres [131-135] and has led to the postulation of electrostatic attraction resulting in ordered domains of higher local concentration in coexistence with dilute non-ordered particles. For the present system, however, it would be very speculative to assign the slope of 0.44 observed for Cs+ counter-ions to a stronger attraction as compared to H+ which could possibly lead to a nematic-like order within the domains. Finally, another strange observation should be mentioned. The ratio qi /qj 3 of the first and second maximum for the highest concentrations for both, H+ and Cs+ counter-ions is equal to... 

Hibert and Solladie obtained 0.43% ee of ( + )-hexahelicene(21). More importantly, they found a measurable ee when the solvent pitch approaches infinity. These results indicate chat cholesteric order does influence the motions which lead from reactant to product (probably by affecting the equilibrium between the reactant conformations) but that oven nematic-like order, when combined with molecular asymmetry, can alter the balance between enantiomeric rate processes. Additionally, a part of the ee may arise from the partial circular polarization of the excitation light which results from selective reflection of one light component at the cholesteric surface. 

Fig. 1. Schematic representation of (a) nematic, (b) smectic and (c) cholesteric (or chiral nematic) liquid crystalline phases. In the nematic phase only orientational correlations are present with a mean alignment in the direction of the director n. In the smectic phase there are additional layer-like correlations between the molecules in planes perpendicular to the director. The planes, drawn as broken lines, are in reality due to density variations in the direction of the director. The interplane separation then corresponds to the period of these density waves. In the cholesteric phase the molecules lie in planes (defined by broken lines) twisted with respect to each other. Since the molecules in one plane exhibit nematic-like order with a mean alignment defined by the director n, the director traces out a right- or left-handed helix on translation through the cholesteric medium in a direction perpendicular to the planes. When the period of this helix is of the order of the wavelength of light, the cholesteric phase exhibits bright Bragg-like reflections. In these illustrations the space between the molecules (drawn as ellipsoids for simplicity) will be filled with the alkyl chains, etc., to give a fairly high packing...

FIGURE 4.6 Liquid crystal polymers can be side chain (top) or main chain (middle). In bulk, the liquid crystalline mesogens can align to form a birefrin-gent phase with nematic-like ordering (bottom). 

Cholesteric phase A liquid crystal phase characterized by nematic-like ordering with a helical twist. 

Chira.lNema.tlc, If the molecules of a Hquid crystal are opticaHy active (chiral), then the nematic phase is not formed. Instead of the director being locaHy constant as is the case for nematics, the director rotates in heHcal fashion throughout the sample. This chiral nematic phase is shown in Figure 7, where it can be seen that within any plane perpendicular to the heHcal axis the order is nematic-like. In other words, as in a nematic there is only orientational order in chiral nematic Hquid crystals, and no positional order. Keep in mind, however, that there are no planes of any sort in a chiral nematic Hquid crystal, since the director rotates continuously about the heHcal axis. The pitch of the helix formed by the director, ie, the distance it takes for the... 

Deloche, B., Samulski, E. T. Short-range nematic-like orientational order in strained elastomers a deuterium... 

The formalism sketched above has been used in the literature in more or less the same detail by many authors [87-92]. The predicted membrane structure that follows from this strategy has one essential problem the main gel-to-liquid phase transition known to occur in lipid membranes is not recovered. It is interesting to note that one of the first computer models of the bilayer membrane by Marcelja [93] did feature a first-order phase transition. This author included nematic-like interactions between the acyl tail, similar to that used in liquid crystals. This approach was abandoned for modelling membranes in later studies, because this transition was (unfortunately) lost when the molecules were described in more detail [87]. 

The directors (long molecular axes) of the constituent molecules in nematic phases are parallel to one another on average. This is the only order present in nematic liquid crystals, which are the most fluid type of liquid-crystalline phase. Molecules that form cholesteric phases must be optically active or contain an optically active dopant. As the phase name implies, the constituent molecules are frequently steroids and most commonly are cholesteric esters or halides. A conceptual model of the cholesteric phase includes layers of molecules in nematic-like positions, each layer being twisted slightly with respect to the ones above and below it. When the phase consists only of optically active molecules, the angle of twist between layers is typically less than one degree. Several subclasses of discotic phases exist. In all, the molecular planes of the constituent molecules are parallel. However, the discs can pack in nematic-like arrangements (ND) or in columns that are internally ordered (D ) or disordered (Dd) and may be stacked vertically,... 

In 2003, Sugimoto and Kanie published what appears to be the first claim on the formation of nematic-like one-dimensional ordering in liquid crystal-nanoparticle hybrid materials. Different shapes of Ti02 nanoparticles were hybridized with two structurally different rod-like liquid crystal amines (one based on a cyanobiphenyl and a second with a fluorinated cyclohexylbiphenyl core), but only the combination of spindle-like Ti02 nanorods (with an aspect ratio greater than 10) with the... 

Third, the fact that all ketones of general structure 8 undergo a continuous variation in their triplet state behavior (whether phase-dependent or not) at the bulk Sm-N solvent phase transition indicates that in spite of the dramatic change in bulk (thermodynamic) solvent order that occurs at the phase transition, the ketone s immediate solvation shell changes only slightly or not at all with respect to its restrictions on the conformational mobility of the solute. A nematic-like environment in the immediate vicinity of the solute in the smectic phase, with subtle differences in structure depending on the solute, could be suggested by this behavior (32). 

The constituent molecules of a cholesteric phase roust contain at least one center of chirality and frequently include a steroidal ring system. They are arranged with their long axes (directors) parallel, but without longitudinal order. A convenient description of a cholesteric arrangement (which is microscopically incorrect(3a)) places the molecules in nematic like "layers" which are very slightly twisted with respect to those above and below (Fig. 1). The angle and direction of... 

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