Electric Field Effect Of Nematic Spherical Droplets Withconcentric Structure

Based on the experimental stimulation of J.H.Jiang,A.Fernández-Nieves,et al.,by using Landau-de Gennes theory,this paper simulated the polarized light display of ideal concentric structure,bipolar structure,radial structure and escape concentric structure in spherical liquid crystal droplet.The fine structure,size effect of spherical droplets of nematic escape concentric structure and the ideal concentric structure are studied,and their structural transformation and dynamic process under the action of electric field are given.The polarized light micrographs of the spherical liquid crystal droplets of the concentric structure,the bipolar structure,and the radial structure obtained from the simulation are consistent with the experimentally observed polarized light micrographs observed experimentally,thereby verifying the correctness of the simulation results,and then we simulated one for the escape concentric structure which provides a contrast material for the experimental observation.For the escaped concentric structure spherical droplet in nematic phase,results show that this structure is stable in the range of radius r≥6.5μm;when 5.2μm≤r<6.5μm,it is degenerate state between the bipolar structure and the escaped concentric structure;when r≤5.1μm,there is only bipolar structure.The escaped concentric structure satisfies C_∞symmetry but does not satisfy D_∞symmetry.When an electric field is present,in the range of 0≤E≤0.08V/μm,the liquid crystal molecules are gradually arranged in the direction of the electric field,based upon the escaped concentric structure;when0.09V/μm≤E≤0.1V/μm,the elastic energy,surface energy and electric field energy compete with each other to induce the first structural transition,which causes a similar bipolar structure in the droplet;when 0.11V/μm≤E<0.451V/μm,the directors are arranged in the direction perpendicular to the electric field near the equatorial plane,the upper and lower parts of the sphere are symmetrical and the directors are along the direction of the electric field;when the electric field strength reaches the threshold E_c1=0.451V/μm,the second structural transition is found for the first time,which occurs for the first time in the concentric structure.Most of the liquid crystal molecules in the droplets realize the rotation from the central axis to the boundary point by point in the direction of the electric field through the dynamic process.Inspired by J.Yan,and A.D.Rey,et.al.,the k₂₂（twisted elastic constant in Frank’s theory）was appropriately increased,and the concentric structure was obtained through simulation,which is similar to the structure described in J.H.Jiang’s experiments.It is worth noting that,near the+1 line defect,the system realized the structural transformation of the negative order parameter uniaxial state to the positive order parameter uniaxial state through a thin ordered reconstruction layer.The research found that:when r≥6.5μm,the concentric structure is stable;when 3.2μm<r<6.5μm,the escaped concentric structure is stable;when 2.4μm<r≤3.2μm,both the bipolar structure and the concentric structure exist and are bistable states;when r≤2.4μm,the bipolar structure is stable.Under the action of an external electric field,it was found that the configuration of the concentric structure did not change when the electric field strength was E<0.06V/μm,and when the electric field strength reached the threshold electric field E_c2=0.06V/μm,the structure of the liquid crystal system changed,and a field-escaped concentric structure was formed.During the formation process,it was found that the central core changed from a uniaxial state with a negative order parameter to a uniaxial state with a positive order parameter and the director became along the direction of the electric field.It is similar to the dynamic process of structural transformation under the action of the electric field for the escaped concentric structure.