Secondary instabilities in electroconvection in nematic liquid crystals

We report experimental results on the stability diagram of the normal-roll pattern in electroconvection in nematic liquid crystals. The stable wave-number bands of the normal rolls are determined for various frequencies of the applied voltage. We find that the mode destabilizing the normal rolls is of the zigzag type at low frequencies of the applied voltage and at higher frequencies mainly of the skewed varicose type. The change of the destablizing mode of the normal rolls is closely related to the change of the transition scenarios to weak turbulence. With increasing amplitude of the external voltage, at low frequencies the normal rolls are followed by a zigzaglike pattern and at higher frequencies they are followed by a breathing pattern consisting of a two-dimensional lattice of defects. Both secondary patterns are therefore on a different route to weak turbulence to which they finally bifurcate at higher voltages.