Director precession and nonlinear waves in nematic liquid crystals under oscillatory compression
  T. Börzsönyi,  Á. Buka,  A. P. Krekhov, O. A. Scaldin and L. Kramer
[Phys.Rep. 337, 171 (2000)]  and  [SPIE, 3319, 129-132 (1998)]
In oscillatory compression (one of the plates is moving perpendicular to the plane of the sample with a typical frequency of f = 1-100 kHz) a slow precession of the director is generated if the director is tilted (homeotropic boundary conditions, externally induced tilt by an electric field for substances with negative dielectric anisotropy). By increasing the tilt anlge the precession reverses, which was reproducible in several samples using different materials (see Figure LHS). The small spatial variation of the precession frequency leads to the continous increase of the director gradient in the sample which is observed in the form of propagating (phase) waves as observed by crossed polarizers (inset of Figure LHS). The Freedericksz threshold and the critical voltage (corresponding to the reversal of the precession) depends on the amplitude of the applied compression (see figure RHS).