Microstructure and thermodynamic properties of aqueous alumina nanofluids

Abstract

In this work, an assessment of the shear-thinning behaviour of alumina colloidal suspensions based on thermodynamics parameters was carried out. For this purpose, two activation kinetic models, based on the dependence of viscosity with temperature, were analyzed. The first was a modified version of the Andrade-Eyring equation, while the second considered the stress contributing to the activation energy, resulting in a modified version of the Arrhenius relationship. In both models, the activation Gibbs energy change was determined, and a decrease of the activation Gibbs energy change in the Newtonian region compared to the shear-thinning zone was obtained. The stress assisted activation kinetic model indicated a very high increase of the activation volume in the shear-thinning flow compared to the Newtonian regime. This parameter consisted of two contributions: enthalpic and entropic, respectively, from which the enthalpy and entropy changes of distinct flow zones were estimated. The decrease in the entropy change (and entropic forces too) as a consequence of small changes in the suspension microstructure was in agreement with the results obtained by other authors. The determination of thermodynamic parameters from rheological measurements has demonstrated to be an effective method for the analysis and assessment of non-Newtonian behaviours.