Topological defects in vertically aligned nematic liquid crystal cells with various pre-tilt angles

Po-Yao Chen^1*, Jieh-Wen Tsung¹

¹Department of eletrophysics, National Chiao-Tung University, Hsinchu, Taiwan

* Presenter:Po-Yao Chen, email:pychen1993.ep06g@nctu.edu.tw

In this study, we observe topological defects in nematic liquid crystal (NLC) operated by the vertical alignment mode. We find the pre-tilt angle of NLC on the substrate can alter the type of topological defects. The pre-tilt angle is regulated by the concentration of Hexadecyl trimethyl ammonium bromide (CTAB), which is the alignment layer coated on the substrate surface. When external electrical field is high enough, NLC reclines and gather at the center of the fish-bone electrode and generate the topological defects with topological charge (q) of +1 and -1. The total topological charge of them satisfy the conservation law ( ) where  is the Euler characteristic of a pixel in three dimension. We demonstrate if NLC in boundary is more parallel to substrate, the director field is messier in fish-bone electrode. The topological defects cannot be fixed at the center of a pixel. In contrast, NLC is perpendicular to substrate, it will be neatly arranged along the fine strips of the electrode. Defects stay stable at center of the electrode. The results show that the pre-tilt angle of NLC is crucial for defect formation.

Keywords: pre-tilt angle, vertical alignment mode, topological defect, nematic liquid crtstal