Velocity and current measurements in electroconvecting smectic films

We have studied electrically driven convective flow in thin, freely suspended films of smectic-A liquid crystal. Here we report measurements of the two-dimensional velocity field of this flow as the driving voltage is varied. Our results are well described by the first few terms of an appropriate mode expansion over a range of the experimental control parameter, from the onset of convection to the appearance of unsteady flow substantially above onset. The behavior of the mode amplitudes near onset indicates that the bifurcation to convection is forward. We also present measurements of the current through the film, which show an enhanced charge transport due to convection, and measurements of the pattern wavelength at and above onset. We argue that a reduction in the onset wavelength observed above a cutoff frequency is related to the relaxation time of the charge distribution in the film.