Solid-Like Director Reorientation in Sheared Hexagonal Lyotropic Liquid Crystals as Studied by Nuclear Maginetic 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 different shear geometries, either cone and plate or coaxial cylinders, prove that the hexagonal axis (the director) aligns parallel to the flow. Shear experiments starting from a monodomain sample, with the director initially aligned along the velocity gradient, exhibit an increasing tilt of the director as the shear strain γ grows. The tilt angle is given by arctan γ. This makes evident that the hexagonal lyomesophase behaves like a solid. The analysis of nuclear magnetic resonance spectra and textures observed by polarizing microscopy shows an undulation of the columns in both the quiescent and the sheared state. The amplitude of the undulations decreases with increasing shear strain.