Orientational relaxation in free-standing smectic C film driven by rotating circular frame.

The pecularities in the cˆ-director reorientation in free-standing smectic C film without of defects and stretched between two circular frames, the rest outer and rotating inner, have been investigated theoretically based on the hydrodynamic theory including the cˆ-director motion and with accounting for backflow. Since the orientation of the cˆ-director is fixed at the rims of the smectic film, the shear flow induced by rotating frame winds up of the cˆ-director field. It is found that the higher shearing flow produces the greater twisting rotation of the cˆ-director around the normal to the smectic film directed in the opposite sense with respect to the direction of the angular velocity. Calculations also show that the relaxation dynamics of the cˆ-director field depends crucially on the curvature of the inner rotating frame.

Field-induced director dynamics in confined nematic liquid crystals imposed by a strong orthogonal electric field.

The field-induced director dynamics for a low molar mass nematic liquid crystal (LC) has been investigated theoretically based on the hydrodynamic theory including the director motion with appropriate boundary and initial conditions. Analysis of the numerical results for the turn-on process provides evidence for the appearance of the spatially periodic patterns in 4-n-pentyl-4'-cyanobiphenyl LC film, only in response to the suddenly applied strong electric field orthogonal to the magnetic field. It has been shown that at the values of the voltage of 200 V across the 194.7 µm LC film and the magnetic field of 7.05 T directed at the angle α=1.57(~89.99°) between two fields, there is a threshold value of the amplitude of the thermal fluctuations of the director over the LC sample which provides the nonuniform rotation mode rather than the uniform one, whereas the lower values both of the amplitude and the angle α [<1.565(~88.81°)] dominate the uniform mode. During the turn-off process, the reorientation of the director to its equilibrium orientation is characterized by the complex destruction of the initially periodic structure to a monodomain state.

Molecular structure and elastic properties of thermotropic liquid crystals: integrated molecular dynamics--statistical mechanical theory vs molecular field approach.

The connection between the molecular structure of liquid crystals and their elastic properties, which control the director deformations relevant for electro-optic applications, remains a challenging objective for theories and computations. Here, we compare two methods that have been proposed to this purpose, both characterized by a detailed molecular level description. One is an integrated molecular dynamics-statistical mechanical approach, where the bulk elastic constants of nematics are calculated from the direct correlation function (DCFs) and the single molecule orientational distribution function [D. A. McQuarrie, Statistical Mechanics (Harper & Row, New York, 1973)]. The latter is obtained from atomistic molecular dynamics trajectories, together with the radial distribution function, from which the DCF is then determined by solving the Ornstein-Zernike equation. The other approach is based on a molecular field theory, where the potential of mean torque experienced by a mesogen in the liquid crystal phase is parameterized according to its molecular surface. In this case, the calculation of elastic constants is combined with the Monte Carlo sampling of single molecule conformations. Using these different approaches, but the same description, at the level of molecular geometry and torsional potentials, we have investigated the elastic properties of the nematic phase of two typical mesogens, 4'-n-pentyloxy-4-cyanobiphenyl and 4'-n-heptyloxy-4-cyanobiphenyl. Both methods yield K3(bend) >K1 (splay) >K2 (twist), although there are some discrepancies in the average elastic constants and in their anisotropy. These are interpreted in terms of the different approximations and the different ways of accounting for the structural properties of molecules in the two approaches. In general, the results point to the role of the molecular shape, which is modulated by the conformational freedom and cannot be fully accounted for by a single descriptor such as the aspect ratio.

Dynamics of the modulated distortions in confined nematic liquid crystals.

The peculiarities in the dynamics of the director reorientation in confined nematic liquid crystals (LCs) under the influence of a strong electric field E have been investigated theoretically based on the hydrodynamic theory including the director motion with appropriate boundary and initial conditions. Analysis of the numerical results for the turn-on process provides an evidence for the appearance of the spatially periodic patterns in confined LC film, only in response to the suddenly applied strong E. It has been shown that there is a threshold value of the amplitude of the thermal fluctuations of the director over the LC sample which provides the nonuniform rotation mode rather than the uniform one, whereas the lower values of the amplitude dominate the uniform mode. During the turn-off process, the reorientation of the director to the direction preferred by the surfaces is characterized by the complex destruction of the initially periodic structure to a monodomain state.

Nonmechanical pumping principle in submicrosized devices.

Dynamic field pumping principle has been developed utilizing the interactions of both the director and velocity fields and temperature-redistribution across a two-dimensional (2D) homogeneously aligned liquid crystal (HALC) film under the influence both of a heat flow directed normal to the upper bounding surface, whereas on the lower bounding surface, the temperature is kept constant, and the normally directed electric field, due to electric double layers, i.e., a shielding layers that is naturally created within the liquid crystal (LC) near a charged surfaces. Calculations, based on the nonlinear extension of the classical Ericksen-Leslie theory, shows that the HALC material under the influence of the heat flow start moving in the horizontal direction. After turning off the heat flow, the HALC drop settles down to the rest, and the temperature field across the LC film is finally downfall to the value of temperature on the lower bounding surface. The role of hydrodynamic flow in the relaxation processes of the temperature field to its equilibrium distribution across the 2D HALC film, containing 4-n-pentyl-4(')-cyanobiphenyl, has been investigated for a number of dynamic regimes.

Thermal and flexoelectric effects on nematodynamics in a microvolume cylindrical cavity.

We have considered a homogeneously aligned liquid crystal (HALC) microvolume, confined between two infinitely long horizontal coaxial cylinders subjected to both a temperature gradient nabla T and radially applied electric field E. We have investigated dynamic field pumping, i.e., studied the interaction between director, velocity, electric fields, as well as, a radially applied temperature gradient, where the inner cylinder is kept at a lower temperature than the outer one. Flexoelectric polarization P has been taken into account as well, and modeled via the classical Meyer treatment. In order to elucidate the role of nabla T, E, and P in producing hydrodynamic flow, we have carried out a numerical study of a system of hydrodynamic equations including director reorientation, fluid flow, and temperature redistribution across the HALC cavity. Calculations show that there exists a range of parameter values (voltage and curvature of the inner cylinder) producing a kinklike orientation process in the system, as well as a nonstandard pumping regime with maximum flow near the hot cylinder.

Liquid-crystal pumping in a cylindrical capillary with radial temperature gradient.

Dynamic field pumping principle has been developed utilizing the interactions of both the director and velocity fields and a temperature gradient inverted deltaT. The orientational dynamics in the hybrid-oriented liquid-crystal (HOLC) microvolume confined between two infinitely long coaxial cylinders under the influence of the radially directed inverted deltaT has been investigated. We have carried out a numerical study of a system of hydrodynamic equations including director reorientation, fluid flow, and temperature redistribution across the HOLC cavity between two cylinders under the influence of inverted deltaT, when the liquid-crystal cavity is heated both from outer (inner) to inner (outer) bounding cylinders. Calculations show that under the influence of inverted deltaT the initially quiescent HOLC drop settles down to a stationary flow regime, with the horizontal u(st)(r) component of velocity. The effects of inverted deltaT , of the character of the preferred anchoring of the average molecular direction to the restricted cylinders, and of the size of the HOLC cavity on magnitude and direction of hydrodynamic flow--for a number of hydrodynamic regimes--has been investigated.

Director reorientation in a hybrid-oriented liquid-crystal film induced by thermomechanical effect.

We have carried out a numerical study of a system of hydrodynamic equations including director reorientation, fluid flow, and temperature redistribution across a two-dimensional (2D) hybrid-oriented liquid-crystal (HOLC) cell under the influence of a heat flow directed normal to the upper bounding surface, whereas on the rest boundaries the temperature is kept constant. Calculations based upon the nonlinear extension of the classical Ericksen-Leslie theory shows that the HOLC material under the influence of the heat flow, after some time, more than the time of relaxation, for instance, of the director field in the HOLC cell, settles down to the rest state regime, where the horizontal and vertical components of the velocity vector are equal to zero, and the temperature field across the LC cell finally reaches the value of temperature on the lower and two lateral bounding surfaces. The role of hydrodynamic flow in the relaxation processes of the temperature field to its equilibrium distribution across the 2D HOLC cell, containing 4-n-pentyl-4'-cyanobiphenyl, has been investigated, for a number of dynamic regimes.

Effect of electric field and temperature gradient on the orientational dynamics of liquid crystals in a microvolume cylindrical cavity.

We have considered a homogeneously aligned liquid crystal (HALC) microvolume confined between two infinitely long horizontal coaxial cylinders and investigated dynamic field pumping, i.e., studied the interactions between director, velocity, and electric E fields as well as a radially applied temperature gradient inverted Delta T, where the inner cylinder is kept at a lower temperature than the outer one. In order to elucidate the role of inverted Delta T in producing hydrodynamic flow u, we have carried out a numerical study of a system of hydrodynamic equations including director reorientation, fluid flow, and temperature redistribution across the HALC cavity. Calculations show that only under the influence of inverted Delta T does the initially quiescent HALC sample settle down to a stationary flow regime with horizontal component of velocity u(eq)(r). The effects of inverted Delta T and of the size of the HALC cavity on magnitude and direction of u(eq)(r) have been investigated for a number of hydrodynamic regimes. Calculations also showed that E influences only the director redistribution across the HALC but not the magnitude of the velocity u(eq)(r).

Orientational dynamics of the compressible nematic liquid crystals induced by a temperature gradient.

We have carried out a numerical study of a system of hydrodynamic equations including director reorientation, fluid flow, temperature, and density redistribution across a compressible hybrid-oriented liquid crystal (HOLC) cell under the influence of a temperature gradient nablaT directed normal to the restricting surfaces, when the sample is heated both from below and above. Calculations show that under the influence of nablaT the compressible HOLC sample settles down to a stationary flow regime, both with the horizontal u and vertical w components of velocity v, and u is directed in the opposite direction, approximately one order of magnitude less, than the one in the case of an incompressible HOLC cell. The role of hydrodynamic flow in the relaxation processes of the stress tensor components, for a number of dynamic regimes in the compressible HOLC cell containing 4-n-pentyl-4'-cyanobiphenyl, has been investigated.

Pretransitional anomalies in the orientational dynamics induced by temperature gradient in nematic hybrid-oriented cells.

We have carried out a numerical study of the system of hydrodynamic equations including director reorientation, fluid flow, and temperature redistribution across a hybrid-oriented liquid crystal cell (HOLCC), where the nematic sample is confined by two parallel horizontal surfaces, a temperature gradient is applied and directed normal to the boundaries, and the temperature on the lower cooler boundary is reduced towards T(NA). Calculations show that the temperature gradient causes the investigated system to settle down to a stationary flow regime in the horizontal direction. Presmectic anomalies in the vicinity of the lower boundary have been investigated as well, taking into account the fluctuations of the local smectic order parameter above T(NA). It has been shown that these fluctuations influence both magnitude and direction of the hydrodynamic flow, produced by the temperature gradient. We have also investigated the role of hydrodynamic flow in the resulting relaxation processes, for a number of dynamic regimes in a HOLCC containing 4-cyano-4'-octylbiphenyl.

Influence of the flow on the orientational dynamics induced by temperature gradient in nematic hybrid-oriented cells.

The numerical study of the system of hydrodynamic equations that include the director motion, fluid flow, and the temperature field redistribution across the hybrid-oriented liquid crystal (LC) cell under influence of the temperature gradient has been investigated. Calculations show that the horizontal LC layer being initially in the rest, if heated both from below or above, due to the temperature gradient, starts moving in the horizontal direction. It has been shown that the magnitude and direction of the hydrodynamic flow, excited by the temperature gradient, influence both the direction of the heat flow and the character of the preferred anchoring of the average molecular direction to the restricted surfaces. The role of the hydrodynamic flow in the relaxation processes, for a number of dynamic regime in 4-n-pentyl-4(')-cyanobiphenyl hybrid-oriented LC cell, also has been investigated.

Effect of backflow on the orientational and dissipation processes in Langmuir films.

The numerical study of the system of hydrodynamic equations that include both director motion and fluid flow, for a number of dynamic regimes in the 4-n -pentyl- 4' -cyanobiphenyl multilayer film on the water surface has been carried out. Calculations show that the relaxation time over which the torques exerted per unit of liquid crystals volume puts the director n[over] to be normal to the air-water interface, is one order of magnitude less in the case of accounting for the backflow effect than without accounting for that effect. The role of the charged water surface potential on the orientational relaxation process in the 5CB multilayer film on the water surface also has been investigated.

Intrinsic torsional reorientations in a twisted nematic liquid crystal cell.

The nature of the orientational relaxation process of the director n to its equilibrium orientation neq, in the twisted nematic cell, under the influence of an external electric field, is investigated. The influence of the electric, elastic, and viscous torques on the dynamics of the director is reflected in the relaxation of the director n to neq, with different relaxation times. It is shown that the relaxation time, both for the cases of a strong and weak anchoring, exhibits an anomalous increase with decreasing of an external electric field, whereas the influence of the azimuthal anchoring energy, in the case of the twisted nematic cell is characterized by a weak effect. It is also shown that these torques exerted on the director may excite the traveling wave spreading from one edge of the cell to their second edge. Calculations of the relaxation processes in the vicinity of a nematic-smectic- A (NA) phase transition temperature T(NA) , e.g., at a few tens of mK from T(NA) in the nematic phase, shows that the director distortion in the gap between two plates is maintained to be constant across the sample both in the case of a strong and weak anchoring.

Orientational relaxation phenomena in Langmuir-Blodgett films at the air-water interface.

The nature of the orientational relaxation process of the director n to its equilibrium orientation n(eq), in the mono-(multi)layer(s) Langmuir-Blodgett film, during the lateral compression in absence of flow, is investigated. The relaxation time, during compression of 4-n-pentyl-4(')-cyanobiphenyl mono-(multi)layer(s) film on the water surface, using the Ericksen-Leslie theory, has been calculated for the number of dynamic regimes.