Formation and annihilation of electrically driven defects in nematic liquid crystals with negative dielectric anisotropy

Abstract

Orientationally ordered liquid crystals (LCs) exhibit remarkable physical anisotropy and responsiveness to external fields, which give rise to distinguished physical effects and have led to the emergence of a new generation of electric-optical applications. LCs are also renowned for their abundance of phases and topological defects, which are of significance in the researches of both fundamental science and practical technology. One simple approach to generate umbilic defects involves applying an electric field to a homeotropically aligned nematic LC with negative dielectric anisotropy △<i>ε</i>. However, the influence of material properties and external conditions on the dynamic process of nematic LC defects remains unclear. Here, we select seven kinds of nematic LCs with negative dielectrically anisotropy, ranging from -1.1 to -11.5, to explore the dynamics of electric-field-induced umbilics. By applying linearly increasing electric fields parallel to the molecular orientation of LCs, we systematically investigate the effects of material property (dielectric anisotropy) and external conditions (temperature and electric field parameters) on the formation and annihilation of umbilic defects. The experimental results show that the dynamic process of the umbilic defects formation in nematic LCs is independent of dielectric anisotropy, temperature, and electric field frequency, but follows the Kibble-Zurek mechanism, in which the density of generated umbilic defects exhibits a power-law scaling with the change of the electric ramp rate, with a scaling exponent of approximately 1/2. Interestingly, a stronger dielectric anisotropy leads to a higher density of umbilic defects. Additionally, a change in temperature has a significant impact on the density of umbilic defects as well, which higher temperature leads to greater defect density under the same external electric conditions. Furthermore, the annihilation rate of umbilic defects is closely related to the material properties and the ramp of the applied electric field. Specifically, the annihilation rate of umbilic defects becomes faster when dielectric anisotropy is stronger or the electric field ramp is larger. This study provides valuable insights into the relationship between the formation and annihilation of defects and material properties and external conditions in nematic LCs with dielectrically negative anisotropy. It contributes to advancing our comprehensive understanding of the dynamic process of topological defects in soft matter as well.