Nematic Phase Reorientation Dynamics

For simplicity, we have again assumed that 0 is small and that we can use the one-elastic-constant approximation. Writing sin 0 0 and cos 0 = 1, Equation (8.55) becomes 

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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]. 

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. 

Dynamic four wave mixing experiments in the nematic phase were performed by Eichler and Macdonald [150] and Khoo et al. [151, 152] using picosecond lasers. They have observed that the short excitation pulse is followed by a delayed reorientation process, indicating a large inertial moment. The observed dynamics were explained by flow-alignment theory, taking into account translational motion of the molecules under the action of the optical field. Build-up and decay times of the diffraction grating were... 

Consider first an anisometric molecule with the longitudinal p, and transversal p, permanent dipole moments in an isotropic phase. There are two relaxation modes mode 1, rotations of p, around the long axis, and mode 2, reorientation of p,. Figure 10-1. The mode 1 has a smaller relaxation time, Tj < Tj, because of the smaller moments of inertia involved. When this isotropic fluid is cooled down into the NEC phase, the dynamics is affected by the appearance of the nematic potential associated with the orientational order along the director n. The mode 1 remains almost the same as in the isotropic phase, and contributes to both the parallel and perpendicular components of dielectric polarization (determined with respect to n). Mode 2 is associated with small changes of the angle between p, and n it contributes to the parallel component of dielectric polarization. Mode 3 is associated with conical rotations of p, around the director (as the axis of the cone) it is effective when the applied electric field is perpendicular to n and contributes... 

From Equation (8.61) we can see that 0(Xp) is appreciable only if is appreciable. In other words, if the laser pulse duration is short (e.g., nanosecond), it has to be very intense in order to induce an appreciable reorientation effect. In this respect and because the surface elastic torque is not involved, the dynamical response of a nematic liquid crystal is quite similar to its isotropic phase counterpart. However, the dependence on the geometric factor sin 2p is a reminder that the nematic phase is, nevertheless, an (ordered) aligned phase, and its overall response is dependent on the direction of incidence and the polarization of the laser. 

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