ON THE ORIGIN OF NEGATIVE NORMAL STRESSES IN SHEARED OR LYOTROPIC LIQUID-CRYSTALS

In steady shear flow, sustained negative primary normal-stress differences have been reported for several liquid crystalline systems, in most detail for m-cresol solutions of helicoidal polypeptides and for copolyesters. A theoretical model is proposed in which 'dumbbell' molecules are constrained from rotating freely in the shear flow by the intermolecular potential that produces liquid crystalline order. In steady state, the molecules are imperfectly aligned, corresponding to a shortening or buckling of molecular layers. The experimentally observed range for the polypeptide solutions is wider, probably because of polydispersity (imperfect monodispersity), and the theory underestimates the magnitude of observed negative normal stress. The simple model identifies dimensionless variables that will be important in any detailed microrheological theory based on calculating the distribution of molecular orientations.