Effects of pH on shear thinning and thickening behaviors of fumed silica suspensions

The effects of pH on the initial shear thinning and subsequent thickening behaviors of fumed silica suspensions with increased shear rate were investigated. The critical shear stress observed at the onset of the shear thickening of suspensions can be tuned through the pH of added aqueous solutions. The increase in pH of the addition shifts the onset of shear thickening to higher shear stresses. This finding is consistent with the trend of prediction models that involve the hydrocluster mechanism for shear thickening. A high critical shear stress at a relatively high pH corresponds to a high value of low-shear-rate viscosity, coincident with a high friction coefficient between two silica surfaces that are lubricated with high pH solutions. The correlation between low-shear-rate viscosity and critical shear stress of suspensions depends on whether the added dilute solution is HCl or NaOH.