Hedgehogs in the dowser state.

Abstract

We show how to easily generate point defects called hedgehogs, in the so-called quasi-planar texture --the dowser state-- of a nematic layer confined between surfaces with homeotropic anchoring conditions. We point out that the dowser texture can be preserved infinitely in spite of its higher energy with respect to the homogeneous homeotropic texture. For topological reasons the dowser state in a squeezed droplet must contain at least one hedgehog. We submitted this hedgehog to a rotating magnetic field and controlled the continuous evolution, transitioning continuously between radial, hyperbolic and circular hedgehogs, which, just as in previous experiments by Lavrentovich et al., are topologically equivalent states. The dynamics of this transformation is shown to be directly sensitive to energy costs of different geometric configurations of the hedgehog defect and therefore can be used as a rough probe for elastic constants; knowing the principal elastic constants K1,2,3, one can retrieve information about the K24 constant. We propose also a method of generation of hedgehog pairs by application of a Poiseuille flow to a dowser state wound by a rotating magnetic field.