Valentin A. Tsvetkov

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.

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.

Oscillating Poiseuille flow in photo‐aligned liquid crystal cells

We present an experimental study of thin liquid crystal (LC) layers under the action of a harmonically varied pressure gradient. Optical measurements were performed to register the linear oscillations of a nematic director related to homeotropic and homeoplanar (hybrid) initial states. In the latter case one of the inner surfaces of the rectangular channels was treated by ultraviolet light to provide a relatively weak planar anchoring. The optical response of hybrid and homeotropic LC cells under an oscillating pressure gradient was investigated in relation to on the amplitude and frequency of the pressure gradient. A hydrodynamic model is developed taking into account the LC polar anchoring strength and the surface viscosity responsible for a fast LC surface dynamics. Our estimates show that the thickness of the boundary layer corresponding to the surface viscosity does not exceed 10−6 m, and further experiments are needed with thinner LC cells and higher frequency oscillations to achieve a more precise value. An oscillating Poiseuille flow in the hybrid cell was found to be useful for characterizing elastic and viscous properties of a weakly anchoring LC surface layer in a fast surface dynamic process.

Special optical geometry for measuring twist elastic module K22 and rotational viscosity γ1 of nematic liquid crystals

A special nontraditional optical geometry with a pure twist deformation induced by a homogeneous “in-plane” electric field in the layer of nematic liquid crystal (LC) is presented. A quantitative agreement of the theoretical and experimental results of the measured LC birefringence is obtained. A method for measuring the twist elastic module K22 and the rotational viscosity coefficient γ1 of nematic LC is proposed.

NEMATIC LIQUID CRYSTALS UNDER DECAY POUSEUILLE FLOW: NEW POSSIBILITIES FOR MEASUREMENT OF SHEAR VISCOSITY

Abstract The results of the optical investigation of a decay shear flow of a nematic liquid crystal are presented. The flow was produced by the varying hydrostatic pressure difference applied to the rectangular capillary. In the described experiment a quasi-stationary director motion took place. It allowed to control a pressure difference due to a measurement of an intensity of light, passing through the cell. The proposed procedure can be used to determine the shear viscosity of nematic liquid crystals.

Liquid Crystal in Rectangular Channels: New Possibilities for Three Dimensional Studies

The new construction of LC cell useful for a study both simple twist deformation and 3D structures in liquid crystals induced by boundaries and electric fields is proposed. One of the main advantages of the cell is a very high sensitivity of optical response to the small variations of the twist angle. It is possible to create homogeneous “in plane” electric field under special choice of dimensions of a rectangular channel filled with a liquid crystal. The new LC cell provides good perspectives for measurements of practically important viscous and elastic properties of liquid crystals including a rotational viscosity coefficient. The cell can be also used for a study of static and dynamic properties of liquid crystals at weak anchoring.

Usage of liquid crystals in optical sensors of mechanical forces and motion

Physical backgrounds for highly sensitive optical sensors of mechanical perturbations based on flow phenomena in liquid crystals are presented. It is shown that linear declinations of the optical axis of a nematic liquid crystal induced by a pressure gradient from the initial homeotropic orientation which are registered via polarized light can be considered as the basic mechanooptical effect for sensor applications. The ways of optimization of technical characteristics of liquid crystal sensors including usage of electric fields are discussed. The examples of sensors of acceleration, vibration and inclination based on the same principals are considered. It is shown that usage of liquid crystals provides an extremely high threshold sensitivity and electric control of the main technical parameters of optical sensors.

Anisotropic Spreading of Liquid Crystals and Isotropic Fluids on Anisometric Surface of DVD Discs

The first experimental data on spreading of liquid crystal droplets over the surface with a regular submicron relief are presented. It is established that such relief produces a motion of a precursor film in the direction of grooves. The dynamics of motion of a precursor film and a contact line is investigated via polarizing microscopy. The possible influence of anisotropic near-surface viscosity on spreading of liquid crystal droplets is discussed.