Shear-Induced Changes in Rheological Reponses and Neutron Scattering Properties of Hydrophobic Silica Suspensions at Low Silica Concentrations

Abstract

Rheological responses of hydrophobic fumed silica powders, whose surface silanol groups were modified by hexadecane, suspended in 1,4-dioxane at lower silica concentrations than 6.8 vol% have been investigated as a function of the silica concentration. Transient shear stress behavior before attaining the steady-state shear stress could be classified into three regimes as follows, irrespective of the silica concentration: at the lower shear rates than ca. 0.3 s⁻¹ a stress overshoot was observed, at the shear rate ranges from 0.3 to 30 s⁻¹ sustained oscillations in shear stresses were exhibited and these oscillations were first observed for the suspensions at the low particle concentrations, and beyond the shear rate of 30 s⁻¹ a sigmoid decrease of the shear stress with increasing time, that is, structural breakdown, was observed. At the steady state the silica suspensions showed shear thickening. Small angle neutron scattering (SANS) measurements of the silica suspension under shear flow provided that changes in the SANS intensities were well correlated with the shear thickening behavior. However, shear thinning behavior at higher shear rates did not cause any changes in the SANS intensities.

Keywords:

• Hydrophobic fumed silica
• shear thickening
• shear thinning
• small angle neutron scattering
• stress overshoot
• sustained oscillations
• transient shear stress

Acknowledgments

The authors thank Professor Yoshiaki Takahashi (Kyushu University) for performing the small angle neutron scattering measurements. This work is supported in part by the Grant-in-Aid for Scientific Research on Priority Area “Soft Matter Physics” (2006–2010) from the Ministry of Education, Culture, Sports, and Technology of Japan.