Electroconvection with and without the Carr-Helfrich effect in a series of nematics

Experimental electroconvection investigations on four homologs with negative dielectric anisotropy εa and conductivity anisotropies σa of various signs (the relative σ̂a σa σ ranges from -0.6 to +0.3) are reported. The homolog with positive conductivity anisotropy follows the predictions of the standard theoretical model, while the homolog with negative σa 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.