Résumé
Disclinations in nematic liquid crystals usually adopt a straight shape in order to minimize their elastic energy. Once created in the course of a nonequilibrium process such as a temperature quench from the isotropic to the nematic phase, the topologically stable disclinations of half-integer strength either annihilate each other in pairs of opposite strength or form topologically unstable disclinations of integer strength. In this article, we demonstrate that the annihilation process can be inhibited and the defects can be deformed by an applied electric field. We study the disclination lines in the deep uniaxial nematic phase, located at the boundary between two different types of walls, the so-called π wall (a planar soliton stabilized by the surface anchoring) and the Brochard-Léger (BL) wall stabilized by the applied electric field. By changing the electric voltage, one can control the energy of director deformations associated with the two walls and thus control the deformation and dynamics of the disclination line. At small voltages, the disclinations are straight lines connecting the opposite plates of the cell, located at the two ends of the π walls. The π walls tend to shrink. When the voltage increases above EF, the Fréedericksz threshold, the BL walls appear and connect pairs of disclinations along a path complementary to the π wall. At E>2EF, the BL walls store sufficient energy to prevent shrinking of the π walls. Reconstruction of the three-dimensional director configuration using a fluorescent confocal polarizing microscopy demonstrates that the disclinations are strongly bent in the region between the π and the BL walls. The distortions and the related dynamics are associated with the transformation of the BL wall into two surface disclination lines; we characterize it experimentally as a function of the applied electric field, the cell thickness, and the sample temperature. A simple model captures the essential details of the experimental data.