Research On The Influence Of Flexoelectric Effect On The Dynamic Response In Liquid Crystal Display And The Measurement Method To Flexoelectric Coefficient

The flexoelectric effect is the influence on the liquid crystal molecules deformation induced by the interaction between the flexoelectric polarization and electric field, especially the splay or the bend deformation of pear-shaped or banana-shaped liquid crystal molecules with permanent dipole moment. This effect can affect the dynamic response of liquid crystal display(LCD), which is decided by the flexoelectric coefficient of liquid crystal. It results in the variation of response time with flexoelectric coefficient. So, the measurement to the coefficient is very important. Based on this, the influence of flexoelectric effect on dynamic response of LCD and the exploration of measurement method to flexoelectric coefficient were studied in this dissertation, and three methods to determine the flexoelectric coefficient were put forward.Based on the elastic theory of liquid crystal and the variation principle, the relationship between the shift voltage in weak anchoring hybrid aligned nematic cell and flexoelectric coefficient was studied. By computer simulation, the dependence of the shift voltage on the sum of the flexoelectric coefficients and the anchoring energy strength was obtained. As a result, the flexoelectric coefficient can be determined by measuring the shift voltage.Liquid crystal cell itself may be seen as a capacitor. The effective dielectric constant of liquid crystal filled in this cell will change under an external applied voltage, which can result in the variation of the capacitance in liquid crystal cell. Based on liquid crystal elasticity theory and variation principle, the capacitance characteristics of parallel aligned nematic(PAN) and HAN cell were studied, where the analytical expressions of the capacitance for two kinds of cells were also deduced. The capacitance-voltage curves of this cell were simulated numerically.For PAN cell, when the absolute value of the sum of flexoelectric coefficients is the same and not different symbol, the c-U curve is consistent, and the influence of flexoelectric effect on the capacitance of liquid crystal cell is evident with the increasing of the absolute value of flexoelectric coefficient. For HAN cell, with the increase of the applied voltage, the capacitance increases linearly for e11+e3330; however, for e11+e33<0, the capacitance begins to decreases and then increases with increasing the applied voltage. Consequently, the flexoelectric coefficient may be determined by measuring the capacitance, and the detailed experimental design was proposed.Based on the multi-layer optical theory and the elastic theory of liquid crystal, the variation curves between the reflectivity and transmittance of the full-leaky liquid crystal waveguide due to the strong anchoring hybrid-aligned nematic liquid crystal and the internal angle(the angle of incidence for which light is incident on the liquid crystal layer) were calculated theoretically. For different sign of the sum of flexoelectric coefficients such as – or +, the variation curves of reflectivity or transmittance of the full-leaky liquid crystal waveguide will move a distance to the left or the right relative to the case of ignoring the flexoelectric effect, and the moved distance will vary with the different flexoelectric coefficients. The flexoelectric coefficients can be known from the moved distance.Based on the dynamic theory of liquid crystal and the finite-difference iterative method, the dynamic response process of strong anchoring hybrid aligned nematic(HAN) liquid crystal cell was studied with removing the applied voltage, and the time-dependent curves of transmittance through the liquid crystal cell were obtained by numerical simulation. The results indicate that the response time will be reduced with the increase of the absolute value of the sum of flexoelectric coefficients |e11+e33| and the response time for |e11+e33|=50p C/m is shortened half time relative to the case with flexoelectric effect ignored. Based on the self-assembly experimental installation, the dynamic response of strong anchoring HAN liquid crystal was measured. Through comparing the experimental curves of the dynamic response of liquid crystal for different positive pulse 3V, 4V, 5V, 6V with the corresponding theoretical curves with the flexoelectric effect considered, the sum of flexoelectric coefficient of liquid crystal material E7 is e11+e33=40p C/m, which is in agreement with the experimental measurement value reported in the literature.The fundamental cause of accelerating the dynamic response speed of liquid crystal induced by flexoelectric effect is that the deformation of liquid crystal director reduces obviously relative to ignoring this effect under the influence of applied voltage. When the external voltage applied into the liquid crystal cell is removed, therefore, the dynamic response of liquid crystal will be speeded up by the action of elastic deformation, which provides the certain method to solve the problem of response time long of liquid crystal display.