Eur. Phys. J. E (2014) 37: 24
https://doi.org/10.1140/epje/i2014-14024-3

Regular Article

Generation of umbilics by Poiseuille flows

Laboratoire de Physique des Solides, Université Paris-Sud, Bât. 510, 91405, Orsay Cedex, France

^* e-mail: pieranski@lps.u-psud.fr

Received: 15 July 2013
Revised: 10 September 2013
Accepted: 15 October 2013
Published online: 21 April 2014

Abstract

The Fredericks transition in homeotropic nematic layers submitted to an electric field can be described in terms of a 2D vectorial order parameter . We have shown previously that umbilics which are point defects of the field 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 and unveil the relationship → system π-walls. Finally we explain why and where umbilics are generated in π-walls.