Generation of umbilics by magnets and flows

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Abstract
Umbilics in a nematic layer can be seen as topological defects of a complex order parameter. Being analogous to vortex lines in superfluids or superconductors, they are much easier to handle. We describe classroom experiments on controlled generation of umbilics in a nematic layer with homeotropic anchoring conditions submitted to an electric field. For this purpose we submit nematic samples to magnetic fields created by small NdFeB magnets. Umbilics induced by applied fields are unveiled by observation between crossed polarisers in monochromatic or white light. We report also on the winding action of rotating localised magnetic fields and on the winding reversal induced by Poiseuille flows.

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