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.

Latest results:

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)].