Nematic liquid crystals under oscillatory shear flow

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