Interaction of point defects in a nematic liquid

The structure of a topological dipole (two nematic point defects with oppos ite charges) is considered theoretically both in an infinite space and in a capillary. It is shown that the Frank elastic energy tends to concentrate within a spheroidal string if the defects are not too far from each other. The string thickness is stabilized by the 4th-order corrections to the Fran k energy and/or by the boundary conditions at the capillary surface. The de pendence of the interaction force on the distance between the defects is an alyzed. It is shown that both the force and the director structure are sens itive to the 4th-order gradient terms in the elastic energy.