Lens-shaped nematic liquid crystal droplets with negative dielectric anisotropy in electric and magnetic fields*

Recently plano-convex spherical lens-shaped liquid crystal sessile droplets with positive dielectric anisotropy were studied in magnetic and/or AC electric fields. Here we present experimental observations in magnetic, AC electric, combined fields and theoretical considerations of plano-convex lens-shaped nematic liquid crystal droplets with negative dielectric anisotropy. In purely magnetic field our results are the same as for positive materials, i.e., an inversion wall forms normal to the field that moves toward the periphery. In contrast to previous observations on nematic liquid crystals with positive dielectric anisotropy, in electric fields applied normal to the base plane, we do not find any wall formation, but only a radial director structure with a central defect line along the field. Above a threshold electric field, the radial symmetry becomes twisted due to elastic constant anisotropy. Applying a magnetic field perpendicular to the vertical electric field, a twisted inversion wall forms together with a vertical central defect line. When the electric field is applied parallel to the base plane of the droplet, a homeotropic central region forms along the electric field. When this field is applied together with a magnetic field applied in the same direction, the homeotropic central region becomes perpendicular to the applied field.