An activation energy approach for viscous flow: A complementary tool for the study of microstructural evolutions in sheared suspensions

Abstract

The dynamic viscosity of low-molecular-weight polypropylene glycol-based SiO2 nanofluids was measured in the temperature range 10−50°C, with varying solid volume concentrations (0.07−0.10), and a shear rate of 0.1−300s−1. A weak shear-thinning region followed by temperature- and solid concentration-dependent shear-thickening behavior was observed. The onset of shear-thickening behavior moved to higher shear rate values when the temperature increased and the particle concentration decreased. The temperature dependence of the viscosity at given shear rate values was fitted by the Andrade-Eyring equation. An analogy to chemical reactions, considered to be processes that change from reactants to products, with sheared particle suspensions that change from initial to final microstructure state, is proposed. This activation energy approach for a viscous flow is used to elucidate the shear-thickening mechanism (order-disorder or hydroclusters) shown by hydrophobic fumed silica/low-molecular-weight polypropylene glycol suspensions. Activation enthalpy is interpreted as a measure of the potential energy barrier that has to be overcome when a suspension changes from the initial to the final microstructural state. Positive (negative) activation enthalpy indicates adsorption (liberation) of energy in chemical reactions, which is interpreted as a phase transition from a more to less (less to more) structured state in particle suspensions. Activation entropy is interpreted as a measure of order in the suspension microstructure. Positive (negative) activation entropy is an expression of a dissociative (associative) mechanism between reactant molecules, which can be interpreted as the microstructure breaking down (buildup) in particle suspensions.