Ultrasonically-induced optical effect in a nematic liquid crystal

A series of lines parallel to the liquid-crystal boundary have been observed using crossed polarizers on a homeotropically aligned nematic liquid-crystal cell excited by ultrasonic bulk waves of about 1 MHz. We propose that the physical mechanism responsible for the rotation of the molecules of the liquid crystal from their perpendicular position is the acoustic streaming which occurs in any viscous liquid. Theoretical analysis of the model shows that the lines should be separated by one quarter of the acoustic wavelength and the intensity of the light reflected from the cell through crossed polarizers should be proportional to sin²(BV⁴). where V is the excitation voltage applied to the transducer and B is a constant. Both predictions are supported by our experimental results.