Poiseuille flow in nematics : experimental study of the instabilities

Abstract

This work discusses experimentally the hydrodynamic instabilities of an oriented nematic film contained in a plane Poiseuille flow cell such that the director is initially perpendicular to the velocity and velocity gradient. Homogeneous instabilities having a uniform distortion in the plane of the boundary plates are obtained with or without distortion of the flow profile. The thresholds are analysed and compared with the theoretical analysis given in the following paper. The effect of alternating flow and of the application of external fields on the nature of the mode and on the threshold is analyzed with attention to the transition from the homogeneous to roll instabilities. No detailed theory exists for these problems and the analysis is carried with reference to the simple shear flow case although marked differences are observed