Nematic liquid crystals under oscillatory shear flow

This contribution deals with three types of orientational phenomena generat ed by oscillatory flow in nematic liquid crystals: (1) Uniform orientationa l instabilities in the geometry where the director is initially oriented wi thin the flow plane. The instability occurs under rectilinear Poiseuille fl ow and not under Couette flow. (2) The first instability under rectilinear Couette flow is a bifurcation to a stationary roll pattern with the roll ax is perpendicular to the flow direction. The threshold amplitude has been me asured for homeotropic alignment and calculated by linear stability analysi s. (3) An interesting slow director precession and nonlinear waves which ar ise in homeotropically aligned nematics exposed to an elliptical shear how in the bend Freedericksz distorted state. The experimental data are compare d with a theoretical analysis. The precession can also be generated by osci llatory compression. (C) 2000 Elsevier Science B.V. All rights reserved.