Effect of silica particle surface chemistry on the shear thickening behaviour of concentrated colloidal suspensions

We have found that heat treating silica particles can have a large effect on the shear thickening properties of suspensions comprised of 550 nm silica particles in ethylene glycol (EG). The shear thickening effect becomes stronger after heat treating the particles at the temperatures studied (120 and 220 degrees C) especially at the higher temperature, indicating that the interparticle interactions in these suspensions are changed by heat treatment. Having confirmed the existence of hydroxyl groups on the surface of the silica particles using differential scanning calorimetry, x-ray photoelectron spectroscopy showed that heat treatment increases the valence of the Si element on the surface, resulting in a higher density of hydroxyl groups. We believe that a denser solvation layer on the surface of heat-treated particles, formed by adsorbing EG molecules to the particle surface through hydrogen bonds between EG and hydroxyl groups, results in a higher short-range repulsive force, which is responsible for the enhanced shear thickening behaviour.