Abstract
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.

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