Sergey V. Pasechnik

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.

Ultrasonic studies of structural transformations and phase transitions in liquid crystal emulsions

The propagation velocity and absorption coefficient of ultrasound in the range of 0.6–150 MHz in a series of liquid crystal emulsions prepared on the basis of thermotropic liquid crystals and isotropic liquids were studied. The dispersity parameters of emulsions were controlled by polarization microscopy and dynamic light scattering. Features of the temperature dependences of the acoustic parameters of emulsions due to the structural transformations and nematic-isotropic liquid transition in disperse and dispersion phases of liquid crystal emulsions were established.

Light and phospholipid driven structural transitions in nematic microdroplets

We studied the UV-irradiation and phospholipid driven bipolar-radial structural transitions within azoxybenzene nematic liquid crystal (LC) droplets dispersed in water. It was found that the UV-irradiation induced trans-cis isomerisation of LC molecules could enable structural transitions into radial-type configurations at a critical UV-irradiation time tc. In particular, we show that under appropriate conditions, a value of tc could sensitively fingerprint the concentration of phospholipid molecules present in LC-water dispersions. This demonstrated proof-of-principle mechanism could be exploited for development of sensitive detectors for specific nanoparticles (NPs), where value of tc reveals concentration of NPs.

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.

Controlled nanoparticle targeting and nanoparticle-driven nematic structural transition

We study experimentally and theoretically controlled targeting of specific nanoparticles (NPs) to different regions within nematic liquid crystal. Using a simple mesoscopic Landau-de Gennes-type model in terms of a tensor nematic order parameter, we demonstrate a general mechanism which could be exploited for controlled targeting of NPs within a spatially nonhomogeneous nematic texture. Furthermore, we experimentally demonstrate using polarising microscopy that even a relatively low concentration of localised appropriate NPs could trigger a nematic structural transition. A simple estimate is derived to account for the observed transition.

Photoalignment in Rheology of Liquid Crystals

The application of photoalignment technique for rheological study of liquid crystals is considered. Such technique can be used for the initial preparation and rotation of mono domain samples of liquid crystals. It provides new possibilities in measurements of anisotropic viscosities of liquid crystals. Three shear and the rotational viscosities of nematic liquid crystals can be determined at additional usage of the electric field.

An ultrasonic investigation of the critical behaviour of the elastic moduli near the smectic C-smectic A phase transition

Abstract Results for the frequency-temperature dependence of the elastic moduli and the longitudinal sound velocity near the SC–SA transition are reported. The measurements were made at frequencies in the range 0.15–27 MHz. The interpretation of the results obtained was made using the theory of critical dynamics taking into consideration the bare anisotropy.

Influence of polar dopant on internal configuration of azoxybenzene nematic-in-water droplets

ABSTRACT We report the study of optical textures and director configurations within nematic-in-water microdroplets of the liquid-crystalline mixtures based on azoxybenzene and cyanobiphenyl (as a polar dopant). Both pure azoxybenzene and polar dopant materials exhibited bipolar configuration within liquid-crystalline droplets, whereas their mixtures at appropriate concentrations spontaneously formed radial droplets. It was found that increasing of the dopant’s concentration resulted in the forward tangential-homeotropic and reentrant homeotropic-tangential anchoring transitions. We also triggered bipolar-to-radial structural transition by UV irradiation providing trans-cis isomerisation of nematogens. Critical irradiation time needed for radial configuration formation was found to decrease with concentration of polar dopant. Mesoscoping modelling is proposed to explain main experimental results. The presented data are discussed for chemical and biological sensing applications. Graphical Abstract

Nonlinear Optical Response of Nematic Liquid Crystal on Varying Pressure Difference in the Presence of Electric Field

Abstract We are presenting the new results of an optical investigation of an initially homeotropic liquid crystal layer under action of sinusoidal pressure difference. The experiments were carried out by using the striped liquid crystal cell and the nematic mixture with a positive dielectric permittivity anisotropy. The non-linear optical response in the presence of stabilizing electric field (U) was studied and analysed.