Lyotropic Liquid Crystals Under Simple Couette and Oscillatory Shear

Abstract

Small angle neutron scattering (SANS) has been utilized to investigate the flow properties of the discotic micellar isotropic (I), nematic (N) and lamellar (L α) phases in aqueous solutions of cesium perfluoro-octanate. Under simple shear, no preferred orientation was observed in the I phase. The N phase aligned with the nematic director along the shear gradient velocity, providing the first evidence for a positive value of the viscosity parameter, α2. The L α phase oriented with lamellae parallel to the shear plane. A reorientation of the director near the N-L α transition was witnessed upon cooling from the N phase at a constant shear rate and is attributed, primarily to a change in the value of α2 as a result of increasing lamellar correlations in the nematic phase. The application of oscillatory shear induced significant shifts in the phase boundaries, including nematic-like order some 7 K into the quiescent I phase.