S.V. Pasechnik

Slow relaxation processes in nematic liquid crystals at weak surface anchoring

We present new results of experimental investigations of azimuthal director reorientation dynamics for a nematic liquid crystal on solid substrates. Two types of substrate with weak anchoring were studied: glass/polystyrene and glass/UV‐activated dye. Slow and fast relaxation processes were observed in both cases under the action of a strong ‘in‐plane’ electric field. The slow surface reorientation and memory effects were controlled by two parameters: the electric voltage and the excitation time. It was established that the increase of the excitation time results in a slowing of the relaxation of the system to the initial state after turning off the electric field. A phenomenological model of a gliding of easy axes is proposed to explain the slow relaxation process.

Anisotropic shear viscosity in nematic liquid crystals: new optical measurement method

We propose a new optical method and the experimental set-up for measuring the anisotropic shear viscosities of nematic liquid crystals (LCs). LC shear viscosities can be optimized to improve liquid crystal display (LCD) response times, e.g. in vertical aligned nematic (VAN) or bistable nematic displays (BND). In this case a strong back-flow effect essentially determines the LCD dynamic characteristics. A number of shear viscosity coefficients defines the LCD response time. The proposed method is based on the special type of a shear flow, namely, the decay flow, in the LC cell with suitably treated substrates instead of magnetic or electric field application. A linear regime of a quasi-stationary director motion induced by a pressure difference and a proper configuration of a LC cell produces decay flow conditions in the LC cell. We determine three principal shear viscosity coefficients by measuring relative time variations of the intensity of the light passed through LC cells. The shear viscosity coefficient measurements provide a new opportunity for the development of new LC mixtures with fast response times in VAN, BND and other important LCD types.

Effect of combined action of electric field and light on gliding of the easy axis in nematic liquid crystals

A new effect is described of the combined action of an electric field and light on the slow surface dynamics in a layer of nematic liquid crystal (NLC) contacted with a layer of dye pre‐treated by UV irradiation. Simultaneous application of a relatively weak “in‐plane” electric field and polarised light resulted in slow variation of the boundary orientation of the sample transmitted to the bulk of the layer. At the same time, the separate action of the two factors mentioned above did not produce any visible changes in the LC layer at the same intensity and time of application. After turning off both the electric field and light, extremely slow relaxation of the system to the initial state was observed. This effect depends on a number of control parameters (applied voltage, intensity of light, time of application, dose of a preliminary UV irradiation). The critical slowing down of this process (up to some weeks) via a proper choice of control parameters was established. The physical processes responsible for the combined effect were considered and applied to modify a previously proposed phenomenological model for the electrically induced slow azimuthal rotation of the easy axis of an NLC. The modified model was found to be in a qualitative agreement with the main experimental results.

Electrically assisted light-induced azimuthal gliding of the nematic liquid-crystal easy axis on photoaligned substrates

We study azimuthal gliding of the easy axis that occurs in nematic liquid crystals brought in contact with the photoaligned substrate (initially irradiated azo-dye film) under the action of reorienting UV light combined with in-plane electric field. For irradiation with the linearly polarized light, dynamics of easy axis reorientation is found to be faster as compared to the case of nonpolarized light. Another effect is that it slows down with the initial irradiation dose used to prepare the azo-dye film. This effect is interpreted by using the previously suggested phenomenological model. We present the theoretical results computed by solving the torque balance equations of the model that agree very well with the experimental data.

Photoinduced reordering in thin azo-dye films and light-induced reorientation dynamics of the nematic liquid-crystal easy axis.

We theoretically study the kinetics of photoinduced reordering triggered by linearly polarized (LP) reorienting light in thin azo-dye films that were initially illuminated with LP ultraviolet pumping beam. The process of reordering is treated as a rotational diffusion of molecules in the light intensity-dependent mean-field potential. The two-dimensional diffusion model which is based on the free energy rotational Fokker-Planck equation and describes the regime of in-plane reorientation is generalized to analyze the dynamics of the azo-dye order parameter tensor at varying polarization azimuth of the reorienting light. It is found that, in the photosteady state, the intensity of LP reorienting light determines the scalar order parameter (the largest eigenvalue of the order parameter tensor), whereas the steady state orientation of the corresponding eigenvector (the in-plane principal axis) depends solely on the polarization azimuth. We show that, under certain conditions, reorientation takes place only if the reorienting light intensity exceeds its critical value. Such threshold behavior is predicted to occur in the bistability region provided that the initial principal axis lies in the polarization plane of reorienting light. The model is used to interpret the experimental data on the light-induced azimuthal gliding of the liquid-crystal easy axis on photoaligned azo-dye substrates.

Optically controlled transmission of porous polyethylene terephthalate films filled with nematic liquid crystal.

We investigated the influence of blue light (lambda=450 nm) on the optical properties of porous polyethylene terephthalate (PET) films filled with a nematic liquid crystal (E7). In experiments, films of 12 microm thickness with randomly distributed open-end pores of well-defined diameters (170, 400, 850 nm) were studied. It was found that blue-light illumination of the porous PET films preliminarily treated by an azo dye solution resulted in strong intensity changes when the He-Ne laser beam passed through the sample. The observed effect can be attributed to the heating caused by the absorption of blue light by azo dye molecules.

Modeling Reorientation Dynamics of Electrically Assisted Light-Induced Gliding of Nematic Liquid-Crystal Easy Axis

The phenomenological torque balance model previously introduced to describe the electrically assisted light-induced gliding is generalized to study the reorientation dynamics of the nematic liquid crystal easy axis at photoaligned azo-dye films under the combined action of in-plane electric field and reorienting UV light linearly polarized at varying polarization azimuth, . We systematically examine the general properties of the torque balance model by performing analysising the bifurcations of equilibria at different values of the polarization azimuth and apply for the model to interpret the experimental results. These involve observation of the pronounced purely photoinduced reorientation at , as opposed to the case where the light polarization vector is parallel to the initial easy axis (), and the reorientation is almost entirely suppressed. In the regions between electrodes with nonzero electric field, the effects described by the model are that (a) the dynamics of reorientation slows down with and (b) the sense of easy axis rotation is independent of the sign of .

Effects of polarization azimuth in dynamics of electrically assisted light-induced gliding of nematic liquid-crystal easy axis

We experimentally study the reorientation dynamics of the nematic liquid crystal easy axis at photoaligned azo-dye films under the combined action of in-plane electric field and linearly polarized reorienting UV light at varying polarization azimuth, ϕp. At non-zero values of the azimuth, ϕp≠0, as opposed to the case where the polarization vector of the light is parallel to the initial easy axis (ϕp=0), the easy axis reorientation was observed to be most pronounced outside the interelectrode gaps. In the regions between electrodes with non-vanishing electric field, it is found that the dynamics of reorientation slows down with ϕp and the sense of easy axis rotation is independent of the sign of ϕp. A generalized version of the phenomenological model that was previously developed to describe the electrically assisted light-induced gliding is applied to interpret the experimental data.

Electro-kinetic phenomena in porous PET films filled with liquid crystals

Electro-kinetic phenomena (EKP) arising under dc electric field in nanoporous polyethylene terephthalate (PET) films, filled with a nematic liquid crystal (NLC) 4-n-pentyl-4ʹ-cyanobiphenyl (5CB), were investigated for the first time. The volumetric flow rate was found to be directly proportional to the applied voltage U in the range of for both isotropic and nematic phases. Such type of dependences correspond to quasi-Newtonian behaviour of a liquid crystal (LC) with orientation strongly stabilised by surfaces. The value of zeta potential (about –12 mV) was estimated from the data, obtained in isotropic phase in the approximation of a ‘plug’ velocity profile. The possible role of electrically induced flows in previously reported electrooptical response of PET films filled with LC is discussed. The obtained results can be used for elaboration of optofluidic devices of a new type.

Methods of controlling the optical properties of porous films filled with nematic liquid crystal

This paper discusses the optical properties of porous polyethylene terephthalate films filled with nematic liquid crystal. Various methods are investigated for controlling the transmittance of the films. The switching times of a liquid-crystal cell are measured under the action of an ac electric field and when the film is heated either by an electric current or by short-wavelength radiation in the optical region. It is experimentally determined that the cell’s response rate is significantly greater under the action of an electric field than under other external actions. In this case, the switching ON times are about 100 µs and are substantially independent of the pore diameter, while the switching OFF times vary from 200 to 700 µs as the pore diameter increases from 200 to 850 nm.