Acoustic streaming in 2D freely suspended smectic liquid crystal film

We study a horizontal streaming excited by means of a low frequency and intensity acoustic wave in 2D freely suspended film of thermotropic smectic liquid crystal. Acoustic pressure induces fast periodic transverse oscillations of the film, which produce in-plane stationary couples of vortices slowly rotating in opposite directions owing to hydrodynamic nonlinearity. The parameters of vortices are measured using a new method, based on tracking disk-shaped island defects. The horizontal motion occurs only when the amplitude of acoustic pressure exceeds threshold value, which can be explained by Bingham-like behavior of the smectic film. The measurements above threshold are in a good agreement with existing theoretical predictions. We have demonstrated experimentally that in-plane flow is well controlled by changing the acoustic pressure, excitation frequency and geometry of the film. The observations open a way to use the phenomenon in various applications.