Shear viscosity and rheology of ternary and quaternary lyotropic liquid crystals in discotic and calamitic nematic phases

Abstract

We report rheological studies of micellar nematic phases of potassium laurate/ decanol/ water mixtures with and without addition of salt (K2SO4) 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 (ND) nematic phase showed only a slight pseudo-plastic (or shear-thinning) behavior. In contrast, a pronounced pseudo-plastic effect was observed in the ND phase of the quaternary samples. On the other hand, all the NC 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 NC than in the ND phase. The second aspect, regarding both ternary and quaternary samples, is that η in the ND phase dramatically increased as the salt concentration increased, while in the NC 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.