Toshikuni Miyazaki

Surface-Stabilized Smectic A Phase in the Gay-Berne Model.

Constant temperature molecular dynamics simulations of the system of molecules with the Gay-Berne potential interacting with walls are carried out to study the effects of the walls on the positional and orientational smectic orders. The system is composed of molecules put into a space sandwiched by parallel walls with periodic boundary condition in the lateral directions, in which various types of conditions of the walls are tested. The system is shown to exhibit several phases, that is, isotropic, nematic, smectic and crystalline phases as the temperature decreases, in which two types of smectic phases, the surface-stabilized smectic A phase and the usual one observed at the bulk system, are found and also the phase transition between these is shown to occur. The degrees of the positional and orientational orders of the former are higher than those of the latter, which indicates that the walls work not only as the field of the directional anchoring but also as a symmetry breaking field inducing the smect...

Surface-Induced Spatial Ordering in Nematic and Smectic Phases of Gay-Berne Model

Abstract By constant temperature molecular dynamics simulation of Gay-Berne model with walls, the formation of liquid crystalline orders is studied, where typical three types of walls are tested. The wall is shown to work as a symmetry breaking field both for smectic order and nematic short range order of position. In addition to the surface-stabilised smectic A phase, smectic C phases stabilised topologically by the periodic boundary condition are obtained. In case these phases coexist it takes quite long time to achieve a uniform state of thermal equilibrium.

Layer Ordering and Phase Transition of the Surface Stabilized Smectic Phase at Thin System

The smectic layer ordering at quite thin system sandwiched by parallel walls is studied by generalizing the McMillan theory on bulk smectics. The walls work as an external field on the molecules adjacent to the walls, and every order of smectic layers is preserved finite even at high temperature because the effect of walls penetrates the whole system. The order shows crossover from a long range order to a surface sustained one near a bulk transition temperature, which is analyzed as the crossover phenomenon at the critical point of the bulk system. The present theory is applied to the phase transition between two types of smectic A phases observed at the simulated system of Gay-Berne model.

On the Surface Stabilized Smectic Phase at Very Thin System

Abstract The effect of the wall to the smectic layer ordering is studies at thin liquid crystalline system by generalizing the McMillan theory on smectics. Every order of smectic layers is proved not to vanish, and a crossover from a long range order to a surface sustained one is shown to occur near a bulk transition temperature. The dependence of transition temperature between smectic phase and nematic one at the parallel aligned case is obtained, where the transition temperature is determined from the global behaviour of the order averaged over the whole system.

The order of the system composed of polar Gay-Berne molecules with bend

To study the relation of the ordering to molecular shape, molecular dynamics simulation is carried out at the system of coupled Gay-Berne molecules which are dimers of two types of Gay-Berne particles connected by harmonic spring at each end. A strong dependence of the clearing temperature on the bend angle of molecule is derived. Because of the effect of flexible core and also of the mismatching of neighbouring molecules with bend, it is quite hard to obtain monodomain of smectic order.

Formation of Surface Stabilised Smectic Phases Driven by Electric Field under Homogeneous Anchoring

Abstract By constant temperature molecular dynamics simulation of the Gay-Berne system sandwiched by homogeneous anchoring walls, formation of surface stabilised order with homeotropically anchored layers (SmAII) driven by the electric field is studied. The phase SmAII is shown to be stabilised even in the anisotropic homogeneous anchoring condition where SmAII is never observed without the field, while in case dipole-dipole is not neglected the layer of SmAII is destabilised and eventually a sort of smectic C phase appears.

Phase Transition between Two Types of Smectic A Phases Driven by an Electric Field in Quite Thin System

The surface stabilized smectic A phase of homeotropic alignment (SmAII) appears even under the homogeneous anchoring due to an effect of walls as the symmetry breaking field to the smectic layer. As an analogue of the Freedericksz transition at nematic phase, a response of the transverse electric field at SmAII is studied by the constant temperature molecular dynamics simulation of the Gay-Berne system sandwiched by the walls, where each molecule has an electric dipole along the molecular long axis. At a threshold of the field strength, the system changes to the smectic A phase with layer normal being parallel to the electric field (SmAI) via an intermediate phase like a smectic C phase. By reducing the field, again SmAII appears directly, showing a hysteresis characteristic to the discontinuous transition. The dipole-dipole interaction is proved to stabilize the layer structure of smectics, while SmAI changes to smectic C phase in the electric field.

THE ORDER AND POLARISATION OF A SYSTEM COMPOSED OF BENT POLAR GAY-BERNE MOLECULES

NVT molecular dynamics simulation are carried out to study the relation between the molecular shape and dipole moment to the liquid crystalline ordering for a system of coupled Gay-Berne molecules. These are dimers of two types of Gay-Berne particles coupled by a harmonic spring. The appearance of the polarisation and biaxiality together with the location of isotropic-nematic clearing temperature are reported for various combinations of position and direction of the dipole attached to the molecule.

Structure, Stability and Response to the Electric Field of Antiferroelectric Smectic Materials

Abstract The structure and switching of five antiferroelectric mesogens are studied by the constant temperature-pressure molecular dynamics simulation. The conformations of the end chains in the smectic phase are compared with those at an isolated single molecule estimated by the method of molecular orbitals, where the bending angle of the chiral chain as well as the angle of the achiral one is shown to be enhanced. The tilt angles obtained here are in well agreement with the experimental ones. The response of the anticlinic alignment to the transverse electric field is also tested.

Molecular dynamics study on structure and stability of antiferroelectric smectic materials

Abstract Constant temperature-pressure molecular dynamics simulation is carried out to study the conformation and the stability of ten species of antiferroelectric smectic materials. The tilt angles agree fairly with those obtained experimentally. The conformations of the side chains in the smectic phase are compared with those at the single molecular state estimated by the method of molecular orbitais, where the bending angle of the chiral chain is suppressed, while the angle of the achiral chain is enhanced.