Generation of umbilics by Poiseuille flows.

The Fredericks transition in homeotropic nematic layers submitted to an electric field can be described in terms of a 2D vectorial order parameter [Formula: see text]. We have shown previously that umbilics which are point defects of the field [Formula: see text] can be generated in a controlled manner by magnetic fields. Here we report on a serendipitous discovery of generation of umbilics during cleaning of a sample and on a series of experiments which were performed with the aim to unveil conditions necessary for a controlled reproduction of this phenomenon. From the analysis of all circumstances in which umbilics were generated we conclude that umbilics are generated by Poiseuille flows and appear in π-walls in points where these walls are orthogonal to the flow direction. We discuss theoretical aspects of the action of Poiseuille flows on the field [Formula: see text] and unveil the relationship [Formula: see text]→ system π-walls. Finally we explain why and where umbilics are generated in π-walls.