Light Scattering from an Anisotropic Droplet and It''s Application in Polymer-Dispersed Liquid Crystal Transmission Study

• Main Authors: Wang-Yang Li, 李汪洋
• Other Authors: Shu-Hsia Chen
• Format: Others
• Language: en_US
• Published: 1998
• Online Access: http://ndltd.ncl.edu.tw/handle/22467491324798506933

博士 === 國立交通大學 === 光電工程所 === 87 === The subject of this thesis is to investigate the electrically controllable light scattering properties of nematic droplets in a polymer-dispersed liquid crystal (PDLC). The problem of light scattering from a liquid crystal droplet is quite complex because of the irregular liquid crystal director configurations, the nonuniform optical axis distributions inside the droplet make the droplet an optically inhomogeneous anisotropic scatter. In this research, we first used the finite difference and the relaxation methods to simulate the molecular director configurations in a spherical nematic droplet with the presence of an applied electric field. The simulation was performed in three dimensions using the equal elastic constant approximation and hard anchoring condition. We examined nematic droplets with normal and tangential anchoring boundary conditions. The deformation free energy of the nematic droplet was described in terms of tensor representation. The axial and bipolar structured nematic droplets are derived as the stable configuration for normal and tangential boundary conditions, respectively. When an external field is applied on the cell, the field is set back to zero, the initial director configuration can be retrieved as have been observed in many experimental works. The hysteresis phenomenon of the change of director configurations is observed under an increasing-then-decreasing external field sequence as recently reported in experimental observation. In addition, we found that the hysteresis can be reduced by lowering the values of dielectric anisotropy to elastic constant ratio of liquid crystal. The total scattering cross section was calculated to investigate the electrically controlled scattering properties of nematic droplets. The scattering properties of the nematic droplets are calculated using the anomalous-diffraction light scattering theory. The electrooptical properties of the nematic droplet are examined with respect to the size of nematic droplet, the dielectric anisotropy and the elastic constant of the nematic liquid crystals. Our numerical results showed that there exists an optimum size of a nematic droplet for maximum scattering. However, the maximum scattering does not always occur in the absence of external field because oscillation is found in the voltage-dependent scattering properties of large nematic droplets. Some large nematic droplets could have maximum scattering effects at nonzero bias field and the bias fields are found to be a decrease function of droplet size. Furthermore, we used a simple model for collecting the light scattering effects of individual droplets to estimate the transmission of a PDLC film. We showed that the hysteresis of voltage-dependent transmission of a PDLC film is caused by the bistability of director orientations. Besides, the hysteresis of voltage-dependent transmission was found to be reduced by using the liquid crystal with low values of dielectric anisotropy to elastic constant ratio. The dependences of threshold and saturation voltage of a PDLC film on the dielectric anisotropy and elasticity of liquid crystals were also examined. A simple relation between these parameters is obtained. Besides, the PDLC films with axial nematic droplets could have smaller threshold voltage, saturation voltage and transmission hysteresis compared with the PDLC films with bipolar nematic droplets, all other things being equal. Moreover, our experimental data testified our simulation prediction that the PDLC film with suitable size of nematic droplets could has low operation voltage and sufficient scattering power.