Rheology and nuclear magnetic resonance measurements under shear of sodiumdodecyl sulfate/decanol/water nematics

We report on the rheology and deuteron nuclear magnetic resonance (NMR) mea surements performed on the nematic lyotropic system sodium dodecyl sulfate, decanol, and water. For total surfactant and cosurfactant concentrations o f about 30% by weight, we prepared nematic calamitic N-C and nematic discot ic N-D solutions. We have studied the flow behavior of the N-C and N-D phas es in terms of the transient shear stress using different rheological histo ries. In flow-reversal experiments, the transient response exhibits nonperi odic oscillations in the region of small deformations (gamma = 20), which i s followed by a long-lasting relaxation toward the stationary state. In the pseudo-Newtonian regime of the viscosity; we have found that the stress, w hen normalized to its long term value and plotted versus deformation, is sh ear-rate independent. H-2 NMR measurements at rest and under shear were per formed on nematic calamitic and discotic solutions prepared in D2O. At rest , the NMR spectra exhibit well-resolved doublet structures, which indicate an alignment of the director along the axis of the magnetic field. At shear rates large enough ((gamma) over dot > 0.3 s(-1)) alignment is achieved wi th respect to the flow velocity. One finds alignment angles of 12 degrees a nd 105 degrees for the directors of the calamitic and discotic phase, respe ctively. Combining the results obtained by rheology and by NMR, we provide strong evidence that the N-C and N-D nematics of the sodium dodecyl sulfate / decanol system are textured nematics of the flow-aligning type. (C) 2001 The Society of Rheology.