Electroconvection with and without the Carr-Helfrich effect in a series of nematic liquid crystals

Experimental electroconvection (EC) investigations on four homologs with negative dielectric anisotropy ϵ_a and conductivity anisotropies σ_a of various signs (the relative σ̂_a=σ_a∕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 σ_a shows a phenomenon, i.e., nonstandard electroconvection, not described by the Carr-Helfrich mechanism but manifesting in forms of stripes mainly parallel to the initial director, thus having a 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 nonstandard electroconvection. The patterns of both the standard and nonstandard EC have been characterized and compared.