Abstract
The viscous flow behavior of a 12.5 vol% hydrophobic fumed silica (Aerosil® R816) suspension in polypropylene glycol of low molecular weight (PPG400) was studied in a stress-controlled rheometer. The steady flow curve showed shear thickening between two shear thinning regions. Time-dependent viscosity response provoked by step changes in shear stresses corresponding to the shear-thickening region apparently agrees with thixotropic behavior just after the very initial stages are surpassed. Almost instantaneous jamming can justify misinterpretation of the results.
Highlights
When shear steps are applied on time-dependent viscous (TDV) fluids, steady states are reached with some delay
0.01 for 10 s at each shear stress value. As it can be seen, the suspension exhibits shear-thickening behavior between two shear-thinning regions. This complex rheological behavior has been observed in suspensions of both, hydrophilic and hydrophobic fumed silica particles, dispersed in a variety of polar organic liquids such as polypropylene glycol and glycerol [24,33,34]
The overall conclusion of these experiments is that shear-thickening suspensions can show apparent thixotropic behavior due to the fact that the formation of hydroclusters is virtually “instantaneous” when a sudden step-up shear stress is applied, and it is followed by a progressive erosion of the hydroclusters due to Brownian thermal diffusion until the steady hydroclusters size is achieved
Summary
When shear steps are applied on time-dependent viscous (TDV) fluids, steady states are reached with some delay. In accordance with the accepted definitions, the viscosity of thixotropic materials decreases with time when they are sheared at constant shear, followed by gradual recovery when shear is removed [1,2]. Anti-thixotropic materials must exhibit an increase of the viscosity with time under constant shear [3], followed by gradual decrease of the viscosity when shear is removed. The macroscopic viscous behavior of inelastic time-dependent fluids is associated with shear- and time-dependent changes in microstructures, such as was justified long ago by Ostwald when he studied the viscosity changes of colloidal dispersions [5]. In the case of TDV behaviors, shear work done on the material must lead to both breakdown and buildup of microstructure. A large number of products, such as foodstuff, personal care products, paints, fresh cement, adhesives, crude oils, natural and drilling muds, slurries, and various colloidal suspensions are some examples of fluids in which TDV behavior can be measured [8,9,10,11]
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