Forced Rayleigh scattering studies of tracer diffusion in a nematic liquid crystal: The relevance of complementary gratings

Abstract

We have employed forced Rayleigh scattering ͑FRS͒ to study the diffusion of an azo tracer molecule ͑methyl red͒ through a nematic liquid crystal ͑5CB͒. This system was first investigated in an important study by Hara et al. ͓Jpn. J. Appl. Phys. 23, 1420 ͑1984͔͒. Since that time, it has become clear that the presence of complementary ground-state and photoproduct FRS gratings can result in nonexponential profiles, and that complementary-grating effects are significant even when ''minor'' deviations from exponential decay are observed. We have investigated the methyl red/5CB system in order to evaluate the possible effects of complementary gratings. In the isotropic phase, we find that the presence of complementary gratings results in a nonmonotonic FRS signal, which significantly changes the values inferred for the isotropic diffusion coefficients. As a result, the previously reported discontinuity at the nematic/isotropic transition temperature (T NI ) is not present in the new data. On the other hand, in the nematic phase, the new experiments largely confirm the previous observations of single-exponential FRS decay and the non-Arrhenius temperature dependence of the nematic diffusion coefficients close to T NI . Finally, we have also observed that the decrease in the diffusion anisotropy with increasing temperature can be correlated with the 5CB nematic order parameter S(T) over the full nematic temperature range.

Hydrodynamic interactions in rod suspensions with orientational ordering

“…This has been observed before, 15 and is also observed for diffusion of spherical colloids in a nematic liquid crystal. 7,51 The larger value of the total, orientationally averaged diffusion coefficient D iso in Fig. 4c just inside the nematic phase as compared to the isotropic phase reflects the increase in free volume at the isotropic-to-nematic transition, in accordance with Onsager's theory.…”

Position-Dependent Three-Dimensional Diffusion in Nematic Liquid Crystal Monitored by Single-Particle Fluorescence Localization and Tracking

“…The average diffusion coefficients obtained in each section were plotted as a function of distance z 0 of the section from the cell wall in Figure e. Averaging further the diffusion coefficient along the direction y (along the LC director projection) over all sections and along x and z over all sections provides the diffusion anisotropy D ∥ / D ⊥ of 1.88, which is in good agreement with the NMR and FRS results for nematic LCs. ,,− …”

“…Forced Rayleigh scattering (FRS) is based on the measurement of the decay of a transient grating due to the mass diffusion . Applications of FRS in nematic LCs revealed the diffusion anisotropy values on the order of 1.3 to 1.6. − An alternative optical method is fluorescence recovery after photobleaching (FRAP), which monitors diffusion of fluorescent dye molecules into a spot previously darkened by a strong laser …”

Slow Single-Molecule Diffusion in Liquid Crystals

“…Other experimental techniques applied to nematics include single-molecule fluorescence correlation measurements (FCS) and the fluorescence recovery after photobleaching (FRAP) on thin free-standing films of a liquid crystal . Results on the time-dependent concentration spreading of dye-doped particles in 5CB and on forced Rayleigh scattering (FRS) − have been reported as well. Most of these methods, however, detect averaged properties of the liquid crystal from large prealigned sample regions, determined over long periods of time or at large particle ensembles.…”