SOLID-LIKE DIRECTOR REORIENTATION IN SHEARED HEXAGONAL LYOTROPIC LIQUID-CRYSTALS AS STUDIED BY NUCLEAR-MAGNETIC-RESONANCE

The shear-induced reorientation of the hexagonal surfactant mesophase is investigated by deuterium nuclear magnetic resonance spectroscopy. The NMR spectra of the sheared samples, measured in situ with two diff erent shear geometries, either cone and plate or coaxial cylinders, pr ove that the hexagonal axis (the director) aligns parallel to the flow . Shear experiments starting from a monodomain sample, with the direct or initially aligned along the velocity gradient, exhibit an increasin g tilt of the director as the shear strain gamma grows. The tilt angle is given by arctan gamma. This makes evident that the hexagonal lyome sophase behaves like a solid. The analysis of nuclear magnetic resonan ce spectra and textures observed by polarizing microscopy shows an und ulation of the columns in both the quiescent and the sheared state. Th e amplitude of the undulations decreases with increasing shear strain.