Topological defects in liquid crystals

With the long-range order present in liquid crystals comes the possibility of local breaking of this order, i.e. the introduction of a topological defect. In a defect point or line, directors with different orientation come together and at the defect the order thus breaks down. The situation is exactly analogous to the case of the north and south poles of the earth, where all the meridians come together into these two singular points on the earth at which you no longer can define what is north, south, east or west. In fact, the presence of these defects is inevitable when you have a tangential orientational field on a sphere, like the meridians on the earth, and thus exactly the same situation occurs with planar-aligned liquid crystals confined to a shell geometry. The generation of these topological defects, and the study of their types and arrangements as well as the possible uses of the defects, constitutes a major point in our research on liquid crystalline shells prepared by microfluidic techniques.

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Examples of positive-signed defects in the director field (lines in the left-side drawings) of half and unit size, together with polarizing microscopy texture examples of how they can look in a real sample.