Aims and techniques:
Experimental studies were performed on orientational phenomena
in nematic liquid crystals under oscillatory shear.
The dynamic behavior of a homeotropically oriented thin
nematic layer was detected by optical methods using a computer controlled
shear apparatus and digital image processing.
Since the relaxation of the director distortions in nematic
liquid crystals is much slower than the relaxation of velocity fluctuations,
orientational instabilities take place at much smaller shear
rate than the laminar-turbulent transition. We measured the threshold of
the "roll instability" as a function of the
frequency of the rectilinear oscillatory shear
in a homeotropically oriented nematic layer. The results are compared with
numerical linear stability analysis. [Phys.Rev.E.
58, 7419 (1998)]
In elliptic shear a director
precession is observed that is much slower than the frequency
of the excitation. It depends on the tilt of the director that is controlled
by an electric field across the sample. A nonmonotonous behavior is found
experimentally where the different regimes correspond to different types
of pattern formation [Phys.
Rev. Lett. 84, 1934 (2000)].
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 similar 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 various materials. [Phys.
Rep. 337, 171 (2000)].