Interaction of point defects in a nematic liquid

The structure of a topological dipole (two nematic point defects with opposite 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 Frank energy and/or by the boundary conditions at the capillary surface. The dependence of the interaction force on the distance between the defects is analyzed. It is shown that both the force and the director structure are sensitive to the 4th-order gradient terms in the elastic energy.