Abstract
A topological study of the symmetry transformations on a surface inducing both tilted and homogeneous orientation in nematic liquid crystals is presented. It predicts the existence of two types of surface disclination lines, which have been observed experimentally. The evaluation of their strength (which does not satisfy the rule S = k × 1/2 where k is an integer) required a special definition of this quantity. Their relative positions and interactions are well interpreted in terms of grain boundaries on the surface considered as an ordered two-dimensional medium. The evaluation of the elastic energies of those lines can only be very approximate, due to our ignorance of the anchoring energies