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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.
Acknowledgements
The corresponding author (PP) warmly thanks professors N. Clark and D. Walba for the invitation to attend the meeting ‘Frontiers of Soft Matter 2012’. The work on umbilics presented there as a talk and in the present paper was triggered by professors G. Salvetti and M.-O. Hebert who sent their students for a practice at the Laboratoire de Physique des Solides in Orsay. All authors thank them for this initiative. All authors are indebted to V. Klein, S. Saranga, J.L. Signoret and A. Lecchi for help in the realisation of the experimental setup and preparation of samples. We also thank H. Kitzerow and P. Keller who offered us nematic materials EN18 and MBBA used in this study. Perfection of the homeotropic anchoring was the sine qua non condition for successful experiments with umbilics: this problem was resolved thanks to D. Constantin and to P. Oswald who shared with us his expertise in this topic and provided us with an adequate polyimide solution.