Electroconvection with and without the Carr-Helfrich effect in a series of nematics
Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, H-1525 Budapest, P.O.B.49, Hungary
Institut fur Physikalische Chemie, Martin Luther Universitat, D-06099 Halle/Saale, Germany
Experimental electroconvection investigations on four homologs with negative dielectric anisotropy and conductivity anisotropies of various signs are reported. The homolog with positive conductivity anisotropy follows the predictions of the standard theoretical model, while the homolog with negative conductivity anisotropy shows a new phenomenon i.e. non-standard electroconvection, not described by the Carl-Helfrich mechanism but manifesting in forms of stripes mainly parallel to the initial director thus having morphological similarity to the rolls of the standard EC. The remaining two homologs change the sign of their conductivity anisotropy with temperature that leads to a twofold character by crossing over from standard to non-standard electroconvection. Both the rolls of the standard electroconvection and the stripes of the non-standard case have been characterized and compared.