Dynamics of magnetically induced defect walls in Nematic Lenses

We studied nematic liquid crystal (NLC) droplets with lens-shaped free surface placed on solid substrates under magnetic field applied parallel to the solid substrate. Both the solid substrate and the air pose homeotropic anchoring of the NLC director. At low magnetic-fields, the director field with a small radial in-plane component becomes distorted toward the field. At moderate fields it transforms into two regions with opposite tilt direction, separated by a wall. Polarizing optical microscopy (POM) with monochromatic light illumination reveals two sources of magnetic field driven fringe generation. At high fields, a Helfrich-type inversion defect wall arises normal to the magnetic field and moves outward. In addition to the experimental POM results, we present a simple theory and computer simulations that explain the magnetic field dependence of the director field and the time dependence of the outward motion of the inversion wall at constant magnetic fields.