Theory of light scattering by thin nematic liquid crystal films

Structural transitions in holographic polymer-dispersed liquid crystals

Dynamic light scattering was used to analyze the structural and dynamic properties of nematic director field within liquid crystal domains formed in holographic polymer-dispersed liquid crystal transmission gratings. Samples prepared from two different types of prepolymer mixture: one curable with visible (VIS) and another curable with UV light were investigated. In both formulations a critical slowing down of thermal director fluctuations, signifying the second-order structural transition of the nematic director field was observed in the vicinity of some critical external electric field as well as close to some critical temperature. For VIS samples also the size and the shape of phase separated droplets and viscoelastic and surface anchoring parameters of the liquid crystalline (LC) material forming the droplets were deduced. The viscoelastic constants were found to significantly deviate from the viscoelastic parameters of the pure LC mixture.

Saddle-splay-term-induced orientational instability in nematic-liquid-crystal cells and director fluctuations at substrates

We analyze stability of the planar orientational structure in a nematic-liquid-crystal cell with planar anchoring conditions at both substrates. Specifically, we study the instabilities of the ground state caused by surface elasticity with the saddle-splay elastic constant K24 violating the Ericksen inequalities. We express the surface part of static correlation functions as a functional integral over the fluctuation field induced by director fluctuations at confining walls and derive the stability conditions for the planar structure with respect to the fluctuation modes characterized by the in-plane wave numbers and by the parity. These conditions are analyzed in the cell thickness-fluctuation wavelength plane through the parametrization for the boundary curve of the instability region. For relatively small K24, the fluctuation mode of the critical wavelength is found to render the structure unstable when the thickness of the cell is below its critical value. The parity of the critical mode changes as the twist-splay ratio K(2)/K(1) is passing through unity. Further increase of K24 beyond the second threshold value, 4K(1)K(2)/(K1+K2), leads to the instability with respect to short wavelength fluctuations regardless of the cell thickness. We compute the critical thickness and the critical wavelength as a function of K24, the twist-splay ratio, and the azimuthal anchoring strength.

Azimuthal and zenithal anchoring of nematic liquid crystals

Temperature dependence of azimuthal and zenithal anchoring energy coefficients of the nematic liquid crystal 4-n-pentyl-4(')-cyanobiphenyl on rubbed nylon is measured using dynamic light scattering. The method is based on observations of director fluctuations in a planarly aligned wedge cell, where the anchoring energy coefficients can be obtained without any external torques acting on the liquid crystal during the measurement. We found that both anchoring coefficients decrease steadily on approaching the nematic-isotropic transition. Moreover, in the whole temperature range of the nematic phase, the ratio between the zenithal and the azimuthal anchoring coefficients is almost equal to the ratio between the splay and the twist Frank elastic constants. The same result is obtained also for the nematic phase of 4-n-octyl-4(')-cyanobiphenyl. This indicates that the aligning nylon layer directly affects only the monomolecular layer at the surface whereas the observed anchoring is governed by the elastic properties of the alkyl-cyanobiphenyl.

Dynamic light scattering measurements of azimuthal and zenithal anchoring of nematic liquid crystals

Dynamic light scattering is used to obtain the anchoring energy coefficients of nematic liquid crystal (4-n-pentyl-4(')-cyanobiphenyl) on rubbed nylon and photoactive poly-(vinyl-cinnamate). The anchoring coefficients are determined by measuring the relaxation time of the fundamental director fluctuation mode in a homogeneously aligned wedge cell as a function of cell thickness. The method is nonperturbative as no external torque is applied to the liquid crystal during the measurement. We show that by using two different scattering geometries, both azimuthal and zenithal anchoring energy coefficients can be measured on the same sample. The obtained zenithal anchoring coefficient is found to be, in contrast to previously reported results, approximately only two times larger than the azimuthal one. The effect of higher order fluctuation modes on the detected autocorrelation function is in good agreement with numerical calculations.

Aging of surface anchoring and surface viscosity of a nematic liquid crystal on photoaligning poly-(vinyl-cinnamate)

Dynamic light scattering was used to measure the azimuthal anchoring energy coefficient W(straight phi) of nematic liquid crystal (5CB) on photoaligning poly-(vinyl-cinnamate) layer. Measurements were repeated several times within two months. The results show that W(straight phi) increases in the first few days after filling the cell with liquid crystal. Then it remains approximately constant at W(straight phi)=5 x 10(-6) J/m(2) for at least two months. Also, presence of very large effective surface viscosity is observed. This phenomenon is of transient nature and attributed to swelling and dissolving of photosensitive polymer into the liquid crystal, which gives rise to an inhomogeneity of viscoelastic properties. Numerical modeling of the fluctuation spectrum shows that an inhomogeneous surface layer can account for the observed effective surface viscosity.

Dynamic light scattering as a probe of orientational dynamics in confined liquid crystals

The eigenmodes of director orientational fluctuations in nematic liquid crystals in confined geometries were studied both theoretically and experimentally by dynamic light-scattering tehnique. The fundamental mode of the orientational fluctuations shows a crossover from bulk behavior, dominated by bulk elastic constant K, to surface dominated one, in which the relaxation rate is determined by the ratio of surface anchoring strength W and viscosity eta. The contribution of surface viscosity zeta is also significant when its characteristic length zeta/eta becomes comparable to the size of the confined system. It was measured in nematic liquid crystal in cylindrical pores of polycarbonate (Nuclepore) membranes to be of the order of 10 nm.

Equilibrium structures and pretransitional fluctuations in a very thin hybrid nematic film

The stability of different structures of a nematic liquid crystal in a planar hybrid film is examined within the framework of a Gaussian description of order fluctuations. In a very thin film the director field is not bent smoothly but exhibits a steplike change if the anchorings at the confining substrates are strong (G approximately > or =10(-3) J/m(2)) and comparable in magnitude. A (dis)continuous structural transition to the bent-director state which occurs with increasing film thickness or decreasing temperature is governed by the lowest bending director fluctuation mode. Its relaxation rate exhibits a critical slowdown when the film thickness approaches the "supercooling" limit or transition point, respectively. The (dis)continuity of the structural transition depends on the temperature and film thickness. The upper limit for the corresponding tricritical point is determined. The lowest order parameter mode, which corresponds to fluctuations of the position of the central exchange region, is characterized by a nearly critical behavior of the relaxation rate. The spectra of the two noncritical biaxial fluctuation modes are degenerate, whereas the fluctuation profiles are just mirror images with respect to the middle plane of the film.

Interference patterns of scattering light induced by orientational fluctuations in an electric-field-biased nematic liquid-crystal film

A new light-scattering phenomenon from a planar aligned nematic liquid-crystal film is observed and studied. This new phenomenon exhibits ring patterns in the orthogonal polarization. A simple model based on optical interference has been developed, and its predictions agree well with experimental observation.