Oblique Roll Instability in an Electroconvective Anisotropic Fluid

We have experimentally discovered that in a nematic liquid crystal subjected to an ac electric field, the first convective structure at low frequencies is in fact a set of oblique rolls, contrary to the accepted picture. We show that it is a new structure with a helical flow motion, and thus lower in symmetry than the usual normal rolls recovered at high frequencies. Besides indicating the limitations of the available theoretical models, these results clearly show that the highest-symmetry flow structure corresponds to the normal rolls.