Formation of disclination lines near a free nematic interface

We have studied the nucleation and the physical properties of a -1/2 wedge disclination line near the free surface of a confined nematic liquid crysta l. The position of the disclination line has been related to the material p arameters (elastic constants, anchoring energy, and favored anchoring angle of the molecules at the free surface). The use of a planar model for the s tructure of the director field (whose predictions have been contrasted to t hose of a fully three-dimensional model) has allowed us to relate the exper imentally observed position of the disclination line to the relevant proper ties of the liquid crystals. In particular, we have been able to observe th e collapse of the disclination line due to a temperature-induced anchoring- angle transition, which has allowed us to rule out the presence of a real d isclination line near the nematic/isotropic front in directional growth exp eriments. [S1063-651X(99)03901-X].