Using a dynamic light scattering technique, we measure the damping rate of thermal fluctuations of the nematic director for the so-called disklike nematic N(D) phase of both the ternary lyotropic K-laurate-1-decanol-H(2)O system and the quaternary one of similar composition except for the addition of salt (K(2)SO(4)). By varying the scattering angle in suitable geometries and polarizations, we are able to measure the orientational diffusivities associated with the pure deformations of splay and twist. A previous study made in the N(D) phase of the same ternary system yielded a large deviation between the splay and twist diffusivities. The effect was then interpreted in terms of the anisotropy between their associated viscosities due to induced flows, or backflow. In the present work we observe a strong increase of the backflow as an effect of the added salt. In addition, we make auxiliary measurements of shear viscosity and magnetic instabilities, which help to characterize the effect of the salt in the orientational diffusivities as they are mixed quantities involving elastic constants and viscosity coefficients.

We report rheological studies of micellar nematic phases of potassium laurate/ decanol/ water mixtures with and without addition of salt (K 2 SO 4 ) at a fixed temperature of 20 • C. Using a cone-plate rheometer, we verified that the ternary mixture (i.e., without salt) exhibiting the discotic (N D ) nematic phase showed only a slight pseudo-plastic (or shear-thinning) behavior. In contrast, a pronounced pseudoplastic effect was observed in the N D phase of the quaternary samples. On the other hand, all the N C samples (salted or unsalted) showed a quasi-Newtonian behavior. In addition, we have made viscosity measurements using a (variation of a) rotating Couette flow viscometer. Finally, we also performed density measurements, using a density cell attached to the viscometer. Our results showed a contrasting behavior between the two types of nematic phases in two aspects. Firstly, regarding the ternary samples only, the apparent viscosity η at moderate shear rates was lower in the N C than in the N D phase. The second aspect, regarding both ternary and quaternary samples, is that η in the N D phase dramatically increased as the salt concentration increased, while in the N C phase η was quite insensitive to the addition of salt. In order to analyse the results, we have used two different theoretical frameworks, namely the nematodynamics in the first aspect and the classical theory of electrolyte solutions in the second one.