Reorientation Of Director Field Research Articles

Photorefractive Effect in NLC Cells Caused by Anomalous Electrical Properties of ITO Electrodes

In a pure nematic liquid crystal (NLC) cell, optically induced charge carriers followed by transports in double border interfaces of orientant/LC and indium-tin-oxide (ITO)/orientant (or LC) can cause removal of screening of the static electric field inside the LC film. This is called surface photorefractive effect (SPR), which induces director field reorientation at a low DC electric field beyond the threshold at a reduced Fréedericksz transition and, as a result, a modulation of the LC effective refractive index. The studies conducted on the photoinduced opto-electrical responses in pure nematic LC cells biased with uniform static DC electric fields support the SPR effect (attributed to the photoelectric activation of the double interfaces). The SPR effect was further studied in LC cells with photoresponsive substrates, which act as a source of a bell-shaped electric field distribution in the LC film if no ITO electrode was employed. In an equipped cell with ITO, the photovoltaic electric field induces charge carrier redistribution in the ITO film, hence the SPR effect. This paper is aimed at highlighting all the evidences supporting ITO film as one of the fundamental sources of the SPR effect in pure NLC cells under the condition of applying low optical power and low DC voltage. An optically induced fringe electric field stemming from inhomogeneous photo-charge profiles near the electrode surfaces is expected in the LC film due to the semiconducting behavior of the ITO layer.

Features of Relaxation of a Stress Tensor in the Microscopic Volume of Nematic Phase under the Action of a Strong Electric Field

A numerical study of new regimes of reorientation of director field n, velocity v, and components of stress tensor σ ij (ij = x, y, z) of nematic liquid crystal (LC) encapsulated in a rectangular channel under the action of a strong electric field E directed at angle $$\alpha \left( {\sim\frac{\pi } {2}} \right)$$ to the horizontal surfaces bounding the LC channel is proposed. The numerical calculations performed in the framework of nonlinear generalization of the classical Eriksen-Leslie theory have shown that at certain relations between the torques and momenta affecting the unit LC volume and E ≫ Eth, transition periodic structures can emerge during reorientation of n, if the corresponding distortion mode has the fastest response, and, thus, suppress all other modes. Rotating domains originating within this process decrease the energy dissipation rate and create more favorable regimes of the director field reorientation, as compared with the uniform rotational displacement.

Особенности релаксации тензора напряжения в микроскопическом объеме нематической фазы под действием сильного электрического поля

AbstractA numerical study of new regimes of reorientation of director field n̂ , velocity v , and components of stress tensor σ_ ij ( ij = x , y , z ) of nematic liquid crystal (LC) encapsulated in a rectangular channel under the action of a strong electric field E directed at angle $$\alpha \left( {\sim\frac{\pi } {2}} \right)$$ α ( ∼ π 2 ) to the horizontal surfaces bounding the LC channel is proposed. The numerical calculations performed in the framework of nonlinear generalization of the classical Eriksen-Leslie theory have shown that at certain relations between the torques and momenta affecting the unit LC volume and E ≫ E _th, transition periodic structures can emerge during reorientation of n̂ , if the corresponding distortion mode has the fastest response, and, thus, suppress all other modes. Rotating domains originating within this process decrease the energy dissipation rate and create more favorable regimes of the director field reorientation, as compared with the uniform rotational displacement.

Formation of vortex flows in thin nematic cells

The director field reorientation and the formation of steady vortex flows in doubly and obliquely oriented thin liquid-crystal cells under the temperature gradient have been theoretically described within the nonlinear generalization of the classical Ericksen-Leslie theory for describing liquid crystal hydrodynamics. This generalization allowing the consideration of thermomechanical contributions to the shear stress and to the entropy balance equation has made it possible to describe the formation of a steady two-vortex flow in doubly oriented liquid-crystal cells.

TRANSIENT PATTERN DYNAMICS IN THE MAGNETIC NON-FRÉEDERICKSZ TWIST GEOMETRY

We present an analysis of the formation and evolution of the transient periodic pattern in the nematic director field reorientation in the magnetic non-Fréedericksz twist geometry and compare with experiments. The stability analysis of the magnetically induced director reorientation in a planar oriented nematic slab with rigid anchoring and with the viscoelastic parameters of 5CB shows that, when the magnetic field is not normal to the undisturbed director n 0 , the transient hydrodynamic instability sets in for a periodic pattern that is oblique with respect to n 0 , in agreement with our experimental results for a nematic slab of 5CB. The analysis of the formation of the stripes show the equivalent of a tricritical point separating supercritical from subcritical bifurcations when the critical wavevector at the transition is plotted as a function of the reduced field. The analysis of the evolution of the stripes show that the amplitude of the periodic perturbations only grows significantly near the Fréedericksz geometry. This is also in agreement with our experimental results, where the stripes are only observable in the vicinity of this geometry. Our results show that there is a favourable range of the magnetic field intensity for the instability growth which corresponds to a minimum of the obliqueness of the stripes. We also show that the time for reaching the maximum distortion amplitude decreases with increasing magnetic field while for given field it increases when the Freedericksz limiting case is approached.

Linear analysis of transient pattern evolution in the non-Freedericksz twist geometry

We present a study of the evolution of the transient periodic pattern in the nematic director field reorientation in the magnetic non-Fréedericksz twist geometry. The stability of the uniform director field reorientation with respect to periodic perturbations is studied as a function of the magnetic field H, the angle α between H and the initial homogeneous nematic director n0 (H not normal to n0) and the nematic viscoelastic parameters. The results predict that for α < π/2, the amplitude of the periodic modes becomes damped after a critical time and eventually fade away and consequently does not give way to periodic inversion walls as in the Freedericksz geometry (α = π/2). Also for α < π/2, it is predicted that the selected periodic modes have progressively smaller wave vectors as the director reorients back to equilibrium. The amplitude becomes damped earlier and the wave vector of the periodic pattern decreases faster with time when the magnetic field acts away from the normal to the initial director.

Linear analysis of pattern formation in nematics in oblique magnetic fields

The dynamics of nematic director field reorientation in non-Freedericksz geometries, after a magnetic field H is applied at an oblique angle relatively to the initial homogeneous director n 0 (H not normal to n 0), is studied considering a magnetic reorientation driven by hydrodynamic instabilities (with backflow). This study is carried out for bounded samples between two parallel plates with planar boundary conditions and with rigid anchoring. Linear stability and wave vector selection analysis predict that, when the angle of the magnetic to the initial director field is increased, for a given magnetic field intensity, two transitions from a homogeneous to a transient distorted director field reorientation can occur: a transition at a first critical angle to an aperiodic distorted director field and a transition at a second critical angle to a periodic distorted director field. It is shown that the periodic mode is cut off at a higher reduced field when the magnetic field acts away from the normal direction.