J. Phys. II France
Volume 7, Numéro 3, March 1997
Page(s) 421 - 432
DOI: 10.1051/jp2:1997135
J. Phys. II France 7 (1997) 421-432

Solid-Like Director Reorientation in Sheared Hexagonal Lyotropic Liquid Crystals as Studied by Nuclear Maginetic Resonance

Stefan Müller, Peter Fischer and Claudia Schmidt

Institut für Makromolekulare Chemie, Universität Freiburg, Sonnenstr. 5, 79104 Freiburg, Germany

(Received 4 October 1996, received in final form 9 December 1996, accepted 9 December 1996)

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 $\gamma$ grows. The tilt angle is given by arctan $\gamma$. 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.

© Les Editions de Physique 1997