MOLECULAR-ORIENTATION AND INSTABILITY IN PLANE POISEUILLE FLOW OF A LIQUID-CRYSTALLINE POLYMER

A common theological hypothesis, that the stress in a fluid element is only a function of its own deformation history, is rendered questiona ble in liquid-crystalline polymers (LCPs) due to the presence of disto rtional elasticity, through which neighboring fluid elements may direc tly influence one another. However, the fine defect texture in LCPs ha s led to the suggestion that fluid properties may be averaged over a m esoscopic length scale, intermediate between the molecular and macrosc opic, so that averaged measures of fluid structure and stress at this scale depend only on their own deformation history [R. G. Larson and M . Doi, J. Rheol. 35, 539 (1991)]. We describe an experimental test of this hypothesis. If true, it should be possible to use theological and rheo-optical data obtained in simple shear flow to predict the veloci ty and molecular orientation fields in a nonhomogeneous shear flow. Qu antitative flow birefringence experiments are conducted on a liquid-cr ystalline solution of poly(benzyl glutamate), in plane Poiseuille flow . At low flow rates, the data support the local response hypothesis. A s flow rate is increased, however, a profound instability occurs that is unanticipated based on behavior reported in homogeneous simple shea r flow. The instability is characterized by large wavelength disturban ces in structure oriented perpendicular to the flow direction that are clearly visible to the naked eye. With increasing flow rate, these st ructures decrease in size and become increasingly chaotic. Despite the onset of the instability, time-averaged measurements of average orien tation may be qualitatively predicted based on simple shear flow data.