Mechanical Properties of Complex Structure Formed From Electro-Convection State of Smectic Liquid Crystal

In the present study, electro-rheological characteristics of a liquid crystal (8CB) in smectic-A phase were investigated utilizing a parallel-plate type rheometer under a stress control mode. Solid like behaviors of the liquid crystal under DC electric fields were mainly examined. Bingham-like properties were observed and yield stresses measured were affected with the electric field conditions. When the electric field strength was low, the yield stress was almost the same as that obtained under no electric field. Above a threshold of DC electric field strength, the yield stress increased. It was clarified that the increase in the yield stress was caused with the complex structure formed in cooling process from an electro-convection state in nematic phase. Mechanical property changes after deformation of the structure were also examined as changes in dynamic viscoelasticities under condition of very small strain amplitude and the yield stress. The properties were measured before and after the deformation and compared. Moreover, the deformed structure of the liquid crystal was visualized with a polarizing microscope. Since the initial structures formed after the cooling have unevenness, the strength of the structure varied widely. When the small deformation is applied, peculiar changes in the strength were observed, i.e. the G’ measured was increased or decreased after the deformation. Moreover, the values measured after the deformation had reproducibility despite of the scattered initial data. We observed growth of typical optical patterns in the visualization of the structure and it is considered that defects like focal conic domains were generated and developed. After large deformation, the strength of the structure decreased and the deformed structure had almost no elastic properties. The structures were changed to irregular flow structures.