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).