Formation of a defect lattice in electroconvection of nematics

The formation of a defect lattice--i.e., a periodic orientational structure of numerous defect pairs--is experimentally investigated in the electroconvection of nematics. Specific twist structures of the director as a background field play an important role in the formation of a defect lattice. The observed formation sequence is as follows. With increasing applied voltage, normal rolls change into abnormal rolls due to the twisting deformation of the director. This process leads to belt-shaped domains along the abnormal rolls, in which the twist angle of the director alternates between positive and negative angles. The period of the defect lattice perpendicular to the rolls is equal to that of the domain structure in the abnormal rolls. Further increasing the applied voltage induces a secondary short-wavelength mode by the skewed varicose instability, which in turn induces defects. The periodicity of the defect lattice parallel to the rolls is due to the beating mode of the normal rolls and the secondary mode.