We report experimental results on electrohydrodynamic convection of nematic liquid crystals having a free surface at the nematic-isotropic interface. The applied electric field is orthogonal to the free surface. In this configuration, the thickness of the nematic layer is linearly proportional to the reduced temperature. We observe that the threshold voltage for convective rolls varies significantly as the thickness is decreased, while the dimensionless wave vector remains constant. When convection occurs, the nematic-isotropic interface undergoes a periodic distortion, and we argue that this distortion is largely responsible for the relatively large variation of threshold voltage with layer thickness.
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