Instabilities and Transition to Defect Turbulence in Electrohydrodynamic Convection of Nematics

We report the stability diagram experimentally obtained for periodic roll structures in electrohydrodynamic convection of nematics. The stability diagram is composed of three types of the long-wavelength instabilities, the Eckhaus, the zig-zag and the skewed varicose instabilities. After characteristic transients these instabilities lead to stable normal-roll states or to time-dependent modulated structures. We find that among these instabilities the most important one is the zig-zag one in relation to the route to the defect turbulence.