Electroconvection in freely suspended smectic-C and smectic-C* films

Electrohydrodynamic convection in freely suspended smectic-C (S-C) and smectic-C*(S-C*) films is investigated experimentally by means of reflection polarizing microscopy. A two-dimensional vortex Bow is observed when a de field Is applied in the plane of the smectic layers while the electrodes are in contact with the film. Measurements of the threshold voltage, the vortex velocity, and the vortex pattern wavelength are presented. In S-C films the convection is driven by the interaction of the applied electric field with space charges at the free surfaces of the films. The threshold voltage depends on the conductivity and is linearly related to the film thickness. Convective flow can be visualized directly from the optical textures. in S-C* films the threshold voltage is close to zero and the flow velocity is rather independent of film thickness, suggesting that bulk charges drive the convection.