Nematic fluid

Fig. 12 Pn-4 (a, b) and Pn-8 (c, d) P-sheet variation measure by IR circles) and NMR triangles). 10 mg mL peptide solutions prepared in (a, c) D2O and (b, d) 130 mM NaCl in D2O. For Pii-4, I nematic gel, II flocculate, III nematic fluid, IV isotropic fluid. For Pn-8, I isotropic fluid, II biphasic solution. III nematic gel. Adapted from Carrick et al. [23]. Copyright 2007, with permission from Elsevier...

Referring to the plot of G vs. T for the isotropic and nematic fluids, a uni-molecular isomerization process would show the same characteristics if one of the isomers had a higher entropy of formation than the other. In the molecular case, however, the equilibrium constant at a given temperature would derive from the free energies on a per molecule basis, while for the phases this free energy is per collective volume of molecules. The similarities to a molecular isomerization, however, are more important than the differences for the purposes of this discussion. The transition from isotropic fluid to nematic LC can be considered a temperature-driven, or thermotropic, isomerization. 

Figure 6 Schematic diagram showing arrangement of fibrils in nematic fluids (a) and the fiber-like junctions in nematic gel states (b) (Aggeli et al., 2003a, b).

Polymers that exhibit liquid crystallinity, either in the melt or in their solutions, typically consist of comparatively rigid structures that confer high extension on the backbone of the macromolecule. This molecular feature is obviously conducive to the axial order that is the mark of a nematic fluid. ... 

S. Sarman and D. J. Evans, Statistical Mechanics of Viscous Flow in Nematic Fluids, J. Chem. Phys. 99 (1993) 9021. 

Consider now the steady laminar flow of a nematic fluid between two parallel plates. If the flow is along x and the velocity gradient along y the components of the velocity and the director are... 

A similar analysis may be applied to the partially ordered nematic fluids composed of molecules comprising the mesogenic unit and flexible chain segments. In the LC state, one must consider the orientation-dependent interactions in addition to those of the isotropic nature. As mentioned earlier, the volume dependence (1/V ) incorporated in the Maier-Saupe expression may be replaced by MV. In its modified form, Maier-Saupe potential can easily be accommodated by introducing an additional term in the conventional van der Waals expression ... 

The frequency dependences of the bend fiexoelectric coefficients were also measured for the same BC nematic fluid monomer, BC nematic swollen in a calamitic liquid crystal elastomer (BCN-LCE) and for the bent-core nematic elastomer (BCLCE) as shown in Fig. 3.14. One can see that for each material the fiexoelectric effect was found to be zero below 1 Hz, then the response increases abruptly up to 2 Hz and then decreases slightly. The apparent absence of the response below 1 Hz is probably due to screening by free ions. The slow decrease of the fiexoelectric coefficient at higher / is not yet clear. We assume, however, that it is not a measurement error, because 5CB showed a constant value in this frequency range. 

The reason for this is a flow of the nematic, which is lunched by the director rotation. The flow arises in the beginning of the director relaxation process when the elastic torque exerted on the director is very high near both interfaces due to a strong curvature of the director field. However, the curvature at the two interfaces has different sign, see Fig. 11.19a, where 89(z) = (ti/2) — 9. Therefore, the flow of nematic fluid coupled to the director rotation (backflow) at the two interfaces is... 

Brochard, F. (1977) Nematic Fluids Some Easy Demonstration Experiments, Collectif Orsay, College de France. Contemp. Phys. 18, 247... 

Defects can also be trapped by an object submerged within the liquid crystal. For example, Terentjev [29] gave a theoretical description of a -1/2 disclination line bound just outside the surface of a colloidal particle in a nematic fluid, as illustrated in Fig. 11. Here, also, one can imagine an applied field changing the equilibrium loop size. The response depends not only on viscosity but also on the elastic constants and the defect core energy. 

Nuclear magnetic resonance (NMR), in particular, deuterium NMR, has proven to be a valuable technique for determining the nature of molecular organization in liquid crystals. The utility of the NMR technique derives from the fact that the relevant NMR interactions are entirely intramolecular, i.e. the dominant interaction is that between the nuclear quadrupole moment of the deuteron and the local electric-field gradient (EFG) at the deuterium nucleus. The EFG tensor is a traceless, axially symmetric, second-rank tensor with its principal component along the C—D bond. In a nematic fluid rapid anisotropic reorientation incompletely averages the quadrupolar interaction tensor q, resulting in a nonzero projection similar to the result in Eq. (5.6) ... 

Fig. 5.14. A polymer dispersed liquid-crystal (PDLC) device consisting of a microdispersion of a low molecular weight nematic fluid (MLC) in a conventional transparent polymer host matrix sandwiched between thin coats of transparent, conducting tin oxide. On the left is shown the off state with a refractive-index mismatch between the dispersion and the host that scatters incident light. On the right is shown how an external electric held aligns the director of the nematic matching the refractive indices of the dispersion and the host, yielding an optically transparent medium.

---