  Stable array of point defects was generated in nematic liquid crystal. Point
defects with topological charge of +1 and -1 (hedgehogs) were generated in
vertically aligned liquid crystal cell. The hedgehogs were arranged in square
or hexagonal arrays, and the shape of defect, such as radial or circular
hedgehogs, were under the control of delicately patterned electrodes. Base on
the two-dimensional defect array discovered in experiments, three-dimensional
crystals of defects were postulated. The flux of the liquid crystal director
field was summarized for each unit cell. The analysis showed that the crystal
structure is periodic and stable if the total flux in a unit cell is zero. The
surface and bulk topological charges of each unit cells were analyzed. The
summation of topological charges of a unit cell obeys the topological rules of
a homeotropic droplet. The hidden conservation laws of flux and topological
charges in a soft crystal of defects were discovered, which can be implemented
in templates for self-assembled micro-structure and tissue design. Most
important of all, the conservation laws may answer why natural structures and
tissues grow in to a particular shape and form.
