Suppression of electroconvection by Couette shear in freely suspended smectic C* films

We present the first experiments on electrohydrodynamic convection with a s uperposed Couette shear in freely suspended annular smectic C* films. Simil ar experiments on smectic A films have been carried out previously. In thos e experiments, surface-charge driven electroconvection in combination with shear leads to a pattern of travelling convective vortices. Travelling vort ex patterns are also observed in smectic C* films. In contrast to smectic A films, however, the flow field in C* films is coupled to c-director orient ation effects that allow a direct visualization of the periodic flow patter n. The critical voltage for the onset of convection was determined by elect ric current measurements and compared with corresponding results for smecti c A films. Measurements at various shear rates show that the onset is suppr essed by the shear.