Research On The Flexoelectric Effect And Flexoelectric Coefficient Of The Nematic Liquid Crystals

The flexoelectric effect is the polarization of liquid crystal and it is caused by the curvature deformations of the splay and bend types for liquid crystal molecules.Conversely,the interaction of this kind of polarization and electric field affects deformation of liquid crystal molecules.Flexoelectricity has attracted much research interest since it was discovered by R.B.Meyer in 1969.These studies were focused on exploring flexoelectric-optical characteristics of liquid crystal materials,the influence of flexoelectric effect on the liquid crystal display,the measurement of flexoelectric coefficient of liquid crystal materials using various experimental techniques and new application fields of the flexoelectric effect of liquid crystal materials.In 2006,P.Kuma et al.studied flexoelectric response at ±1 defect sites in namatic inversion walls,in a planarly aligned nematic liquid crystal cell where anchoring is weak under a direct-current field.The experimental phenomena indicated its flexoelectric effect.The flexoelectric effect can affect many of the characteristics of liquid crystal,such as director distribution,threshold voltage,electro-optical characteristics,viewable angle and so an.And the extent of the influence is decided by the flexoelectric coefficient of liquid crystal materials.So it is necessary to measurement the flexoelectric coefficient of liquid crystal materials.Based on the above two aspects,we research on the flexoelectric effect and flexoelectric coefficient of the nematic liquid crystals in this paper,mainly divided into the following three parts.The first part,we gave the theoretical analysis for the experimental phenomena of flexoelectric response at defect sites in nematic inversion walls conducted by P.Kumar et al.When a direct-current electric field normal to the liquid crystal substrate is applied to the parallel aligned liquid crystal cell,the rotation of ±1 defects in nematic inversion walls can be exhibited.After considering flexoelectric effect,the total free energy and equilibrium equations of the polar and deviation of azimuthal angle,used to describe the distribution of director,for the liquid crystal molecules around the +1 and-1 defects in the nematic inversion walls under the electric field were drived.And then polar and deviation of azimuthal angles were simulated.The angles of deviation of azimuthal are consistent with the experiment phenomena at the ±1 defects conducted by P.Kumar et al.The second part,the influence of the flexoelectric effect on the distribution of director for hybrid-aligned nematic(HAN)and hybrid-aligned nematic in-plane switching(HAN-IPS)liquid crystal cell under applied voltages were obtained by deriving the equilibrium equations of the tilt angle ? and azimuthal angle ?,used to describe the distribution of director,based on the elastic continuum theory with flexoelectric effect.The curves of reflectivity signals Rpp,Rss,Rsp and Rps versus internal angle(angle of incident light to the liquid crystal),namely liquid crystal optical waveguide modes,for HAN and HAN-IPS cell were then simulated using the distributions of director and multi-layer optical theory.Conclusions are as follows,there are clear and regular movements of the reflectivity signal Rpp for HAN cell with different flexoelectric coefficients e1+e3,and there are clear and regular movements of the reflectivity signals Rpp and Rss for HAN-IPS cell with determined flexoelectric coefficients e1+e3 and different flexoelectric coefficients e1-e3.The third part,the flexoelectric coefficients of liquid crystal materials MS-N01300-000 were measured using the fully leaky optical-guided technology.The experimental curves of reflectivity signals Rpp(refer to HAN cell)and Rss(refer to HAN-IPS cell)versus internal angle were obtained when a laser beam passed through the liquid crystal cell,the cell was embedded in pyramid-coupled waveguide with different alternating current with frequency 1kHz and direct current voltages in same value.Experimental results were then compared with theoretical results derived from elastic continuum theory and multi-layer optical theory of liquid crystal.Form the distance and direction of the shifts,the approximate values of the flexoelectric coefficients e1-e3 and e1-e3 of MS-N01300-000 are 2.5×10-11C/m and 5.5×10-11C/m,respectively.The two values were discussed.The fourth part,the approximate range of flexoelectric coefficients |e1-e3| for BCN liquid crystal material 7P-CF2O-ODBP is 4.8×10-11C/m~8.0×10-11C/m determined experimentally using critical voltage of flexodomain.The value was discussed.