Nematics dynamic effects, 161-7 effect

Thermotropic cholesterics have several practical applications, some of which are very widespread. Most of the liquid crystal displays produced use either the twisted nematic (see Figure 7.3) or the supertwisted nematic electrooptical effects.6 The liquid crystal materials used in these cells contain a chiral component (effectively a cholesteric phase) which determines the twisting direction. Cholesteric LCs can also be used for storage displays utilizing the dynamic scattering mode.7 Short-pitch cholesterics with temperature-dependent selective reflection in the visible region show different colors at different temperatures and are used for popular digital thermometers.8... 

Modified forms of these displays have been developed - e.g., the dye display which has a pleochroic dye dissolved in the nematic material and requires the use of just one polarizer - but we shall not be discussing them here. Analytical expressions have been derived which simplify the computational effort involved in optimizing the material and device parameters, but one has to rely on numerical modelling to give a complete description of the dynamical characteristics of these devices. Certain unusual dynamical effects observed in the TN device, e.g., the reverse-... 

The first example of a single Schiff base compound to exhibit nematic behavior at ambient temperatures was (8), prepared by Kelker and Scheurle, The electro-optic properties of the new compound, MBBA (p-methoxybenzylidene-p -butylaniline) were studied shortly thereafter and it was found that the material exhibited dynamic scattering. This compound has since become a model in a number of theoretical studies of the dynamic scattering effect,... 

T. Carlsson and F.M. Leslie, The development of theory for flow and dynamic effects for nematic liquid crystals, Liq. Cryst, 26, 1267-1280 (1999). 

We cannot call this wave a second sound keeping this term for the case of smectics, because the physical mechanism permitting the acoustic anisotropy is completely different in nematics and smectics. In nematics it comes from relaxation effects (probably relaxation of the nematic order parameter), i.e. it is a dynamical effect. 

The rapid rise in computer speed over recent years has led to atom-based simulations of liquid crystals becoming an important new area of research. Molecular mechanics and Monte Carlo studies of isolated liquid crystal molecules are now routine. However, care must be taken to model properly the influence of a nematic mean field if information about molecular structure in a mesophase is required. The current state-of-the-art consists of studies of (in the order of) 100 molecules in the bulk, in contact with a surface, or in a bilayer in contact with a solvent. Current simulation times can extend to around 10 ns and are sufficient to observe the growth of mesophases from an isotropic liquid. The results from a number of studies look very promising, and a wealth of structural and dynamic data now exists for bulk phases, monolayers and bilayers. Continued development of force fields for liquid crystals will be particularly important in the next few years, and particular emphasis must be placed on the development of all-atom force fields that are able to reproduce liquid phase densities for small molecules. Without these it will be difficult to obtain accurate phase transition temperatures. It will also be necessary to extend atomistic models to several thousand molecules to remove major system size effects which are present in all current work. This will be greatly facilitated by modern parallel simulation methods that allow molecular dynamics simulations to be carried out in parallel on multi-processor systems [115]. 

When r s, one has interconversion between operators Br and Bs, and Rrs is a cross-relaxation rate. Note that the cross-relaxation may or may not contain interference effects depending on the indices l and /, which keep track of interactions Cyj and C,. Cross-correlation rates and cross-relaxation rates have not been fully utilized in LC. However, there is a recent report41 on this subject using both the 13C chemical shielding anisotropy and C-H dipolar coupling relaxation mechanisms to study a nematic, and this may be a fruitful arena in gaining dynamic information for LC. We summarize below some well known (auto-)relaxation rates for various spin interactions commonly encountered in LC studies. 

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]. 

Generally large yield stress effects were dominant in the nematic melts, but they were strongly pre-history dependent. A three region flow curve for 15 mol % modified poly(pheny1-1,4-phenylene terephthalate) was probably due to a not completely molten system. Dynamic viscosity measurements showed strong pseudoplastic behaviour. Strain and time dependence phenomena were not observed. 

Figure 19.3. The time evolution of the morphology of an A3B7 surfactant melt, as predicted by dissipative particle dynamics [84], Mean field theory predicts the correct equilibrium hexagonal phase but does not provide any insights into how this equilibrium morphology is reached. The simulations showed that the dynamics of ordering is determined by the percolation of tubes, subsequently destabilized by a nematic or smectic phase transition and also that hydrodynamic effects are important in reaching equilibrium for this system. See the insert showing the colored figures for a better view.

Dynamic magnetic resonance techniques do not suffer from this deficiency. In particular, the use of CSL probes [8, 101, 102] offers the chance of determining the micro- and macroorder unequivocall This is demonstrated in Fig. 16. The spectra refer to the side chain polymers 2 (M = 14000) and two different temperatures (T = 382 K, left column T = 263 K, right column). Drastic spectral changes are observed when the sample is rotated (top row, q = 0 central row, q = 50°, bottom row, Q = 90°). Comparison of the angular variation in the nematic (left column spectra) and supercooled smectic phase (right column spectra) reveals the crucial effect of the microorder on the spectral feature. 

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