Study On Determination Of Flexoelectric Coefficient Of Nematic Liquid Crystal By Capacitance Method

The flexoelectric effect of nematic liquid crystal is the influence on the liquid crystalmolecules’ deformation induced by the interaction between the flexoelectric polarization ofpear-shaped and banana-shaped liquid crystal molecules with permanent dipole moment for thesplay or the bend deformation and electric field This effect can affect the distribution of liquidcrystal director in liquid crystal cell, and must affect the display, which is decided by theflexoelectric coefficients of nematic liquid crystal. So, the measurement to the coefficients isconcerned by researcher, and different measurement methods have been put forward. But,different methods gave different results even for the same liquid crystal material. Based on this, anovel method to determine the flexoelectric coefficient-capacitance method was put forward inthis paper through theoretical analysis and numerical simulation of flexoelectric effect to liquidcrystal cell capacitance.Considering a nematic liquid crystal cell as the model, under the action of external appliedvoltage, the effective dielectric constant of liquid crystal will change to result in the change ofliquid crystal cell capacitance. Firstly, based on the elastic theory of liquid crystal and variationalprinciple, the analytical expressions of capacitance in strong anchoring parallel aligned nematic(PAN) and hybrid aligned nematic (HAN) liquid crystal cell were deduced theoretically.Secondly, the distribution of director, capacitance-voltage curve and capacitance-flexoelectriccoefficient curve corresponding to these two kinds of cells were simulated numerically by usingthe Matlab software. In the end, the influence of flexoelectric effect on the capacitance of thesetwo cells was analyzed.For PAN cell, the capacitance increases with decreasesing of the sum (from negative topositive) of splay and bend flexoelectric coefficient corresponding to different voltage, elastic constants and dielectric constant. However, the capacitance of liquid crystal cell will increasewith increasing of the applied voltage and dielectric anisotropy Δε/and decreasesing of theelastic constants k33/k11. The capacitance-the sum of splay and bend flexoelectric coefficientcurves of the HAN cell is similar to the PNA cell. The difference is the change speed, and a lowpoint appears when the sum of flexoelectric coefficient is nearly zero.