Figure 2.

(a) A spherical colloidal particle with homeotropic surface anchoring forces the liquid crystal molecules to orient along its closed surface. The director field is represented by black lines. Far away from the particle, the nematic liquid crystal is uniform. However, because of the alignment of the liquid crystal along the closed surface, it is not possible to have a smooth director field everywhere. Instead, regions of disorder are created, where the molecules have no preferential direction to align with. These regions are topological defects. There are two topological defects (blue and red dots), separated by r_d. The first one is called a hyperbolic hedgehog (blue dot) and carries a topological charge $q=-1$. The second one is a virtual one and resides in the center of the particle. It is called a radial hedgehog and carries the opposite topological charge $q=+1$. These two topological defects form an elastic dipole denoted as ${\stackrel{\rightharpoonup }{p}}_{el}$. (b) Real image of a dipolar colloidal particle with a dark spot, which is a hyperbolic point defect. No polarizers were used when taking the photograph.