Acoustic Torque Acting upon Nematic Liquid Crystals

There is plenty of experimental evidence that the propagation of an ultrasonic wave in a nematic liquid crystal affects the director n, which represents the average molecular orientation, thus producing detectable optical effects. There have been several attempts to explain these observations on the basis of a coherent variational theory. We consider here a general theory for nematoacoustics that incorporates flow effects and that has been recently proposed in E.G. Virga (Phys. Rev. E 80:031705, 2009). An explicit application of the proposed theory to a simple computable case was given in G. De Matteis and E.G. Virga (Phys. Rev. E 83:011703, 2011) by linearizing the corresponding balance equations derived from the basic theory. This study was done in order to estimate phenomenological parameters involved in the theory and by using available experimental data. After reviewing the results previously obtained, as a further application of the governing equations, we consider here an approximate equation for the flowless nematodynamics by introducing a second-order acoustic torque acting upon the nematic and we solve the obtained equation in a standard geometry of confined liquid crystal.