Topological dynamics of defects: boojums in nematic drops

The dynamics of creation, annihilation, and transformation of topological defects in a closed system with a vector-type order parameter is studied both theoretically and experimentally. The particular case of a drop of a nematic with varying boundary conditions is considered. In the theoretical part, the concept of a continuously defined topological description of surface defects (e.g., boojums) is introduced. It leads to conservation laws that control the dynamics of the restructuring of the director field in a nematic drop when the boundary conditions vary. It is shown experimentally that when the boundary conditions for a nematic drop are changed, the defects (boojums, hedgehogs, and disclinations) transform into each other and are created from or annihilated into the homogeneous state. All the processes can be described within the scope of the theoretical method developed in the paper.