SHEAR ORIENTATION OF A MICELLAR HEXAGONAL LIQUID-CRYSTALLINE PHASE - A RHEO-SMALL ANGLE LIGHT-SCATTERING STUDY

The hexagonal phase, H-1, of aqueous mixtures of a nonionic surfactant was investigated by combined theological and small angle Light scatte ring experiments. The isotropic to hexagonal phase transition could be detected by changes in polarized and depolarized scattering as well a s by elastic properties of the lyotropic liquid crystalline phase. She ar orientation was observed under different theological conditions nam ely constant stress or constant rate. An orientation correlation perpe ndicular to the flow direction was observed at short creep times or lo w shear rates. After long creep times or at high shear rates, an orien tation parallel to the direction of flow was reached. After cessation of flow only little structure relaxation was found; i.e. both states o f orientation were very stable. The results are discussed in compariso n to the behavior of nematic polymer solutions. The observed orientati on perpendicular to the direction of flow seems to be a unique propert y of the hexagonal phase and might be caused by the high elasticity of this mesophase.