Rheological properties of anisotropic charged colloidal suspensions

Abstract

In this paper, we have investigated the viscoelastic properties of aqueous clay suspensions. The complex modulus has been measured on a very extended scale ranging from 10⁻⁵ to 10² rad s⁻¹. For this purpose, we used two sets of experiments: a controlled shear rate and a controlled stress rheometer. The data show that the relaxation mechanisms are very slow, and the noticeable variation of the viscoelastic moduli is located in the very narrow range of our frequency scale. We have pointed out that the elastic modulus evolves monotically with the particle concentration and follows the power law: G=A(C-C ₀)^α. Our results show that only the threshold C ₀ varies with the ionic strength I. This power law holds in the concentration regime where the suspensions are either optically isotropic or birefringent viscoelastic solids. The observation of flow birefringence of isotropic gels leads to conclude that the Laponite suspensions are composed of micro-sized nematic clusters. This conclusion is in good agreement with the large relaxation times measured and with the hierarchical structure of these suspensions.