Reorientation dynamics nematic

The dynamics of optical reorientation in nematics has been studied much less extensively than the steady-state effects. The theoretical description of transient phenomena can be given in the framework of the non-equilibrium version of the continuum theory (Ericksen-Leslie hydrodynamic theory). 

Doane JW, Golemme A, West JL, Whitehead JB Jr, Wu BG (1988) Polymer dispersed liquid crystals for display application. Mol Cryst Liq Cryst 165 511-532 Drzaic PS (1988) Reorientation dynamics of polymer dispersed nematic liquid crystal films. Liq Cryst 3 1543-1559... 

Following the lines proposed above will give a prediction of the pattern formed above onset. For a transition from undulating lamellae to reorientated lamellae or to multilamellar vesicles, defects have to be created for topological reasons. Since the order parameter varies spatially in the vicinity of the defect core, a description of such a process must include the full (tensorial) nematic order parameter as macroscopic dynamic variables. 

It is observed that in the nematic phase of a liquid crystal, the solvation dynamics of coumarin 503 are biexponential [184a]. The slowest time constant decreases from 1670 ps at 311.5 K to 230 ps at 373 K. The solvation time is not affected by the nematic-isotropic phase transition. Thus, it appears that the local environment and not the long-range order controls the time-dependent Stokes shift. A theoretical model has been developed to explain the experimental findings. This model takes into account the reorientation of the probe as well as the fiuctuation of the local solvent polarization. Similar results are also obtained for rhodamine 700 in the isotropic phase of octylcyanobiphenyl [184b]. 

Using X-ray investigations and optical measurements the reorientation behavior of nematic monodomain samples has been studied in detail by Kundler and Finkelmann [12]. This reorientation behavior has been modeled using a bifurcation analysis of the macroscopic dynamic description by Wei-... 

The ellipsoidal Lennard-Jones potential has recently been applied in molecular dynamics studies of the angular velocity relaxation of hindered rotors, collective reorientation, and the stability of nematic-like orientational ordering. " The model was found to be a convenient and flexible representation of the interactions of polyatomic molecules. 

Fenchenko studied free induction decays and transverse relaxation in entangled polymer melts. He considered both the effects of the dipolar interactions between spins in different polymer chains and within an isolated segment along s single chain. Sebastiao and co-workers presented a unifying model for molecular dynamics and NMR relaxation for chiral and non-chiral nematic liquid crystals. The model included molecular rotations/ reorientations, translational self-diffusion as well as collective motions. For the chiral nematic phase, an additional relaxation mechanism was proposed, associated with rotations induced by translational diffusion along the helical axis. The model was applied to interpret experimental data, to which we return below. 

Liquid crystals are generally characterized by the strong correlation between molecules, which respond cooperatively to external perturbations. That strong molecular reorientation (or director reorientation) can be easily induced by a static electric or magnetic field is a well-known phenomenon. The same effect induced by optical fields was, however, only studied recently. " Unusually large nonlinear optical effects based on the optical-field-induced molecular reorientation have been observed in nematic liquid-crystal films under the illumination of one or more cw laser beams. In these cases, both the static and dynamical properties of this field-induced molecular motion are found to obey the Ericksen-Leslie continuum theory, which describe the collective molecular reorientation by the rotation of a director (average molecular orientation). 

Unlike the T-deformation, the director reorientation in S- and B-effects is always accompanied by the macroscopic flow of a nematic hquid crystal (backflow) with the velocity V = (V (z),0,0), where the z eods goes perpendicular to the substrates and SB-deformations take place in the x, 2(-plane. The velocity V includes only the rr-component, because the z-component is zero according to the continuity equation (div V = 0), and the y-component vanishes due to the symmetry of the problem (Fig. 4.6). The total system of dynamic equations for V z) and the director rotation angle 6 z) gives [37-39]... 

The results above show that the Frank moduli are determined mainly by the structure of mesogenic units which are similar for conventional nematics and thermotropic polymers (the situation changes considerably for the lyotropic solutions of long rod-like polymeric molecules, see the next section). On the other hand, the dynamics of reorientation are strongly influenced by the backbone. Field response and relaxation times depend dramatically on the molecular mass of a polymer though, in the first approximation, obey the same equations (4.30, 4.31). Figure 4.42 shows field-response times as a function of temperature for a comb-like acryl polymer H... 

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