Temperature and pressure effects on stability and gelation properties of silica suspensions

The kinetics of gelation and flow properties of fumed silica suspensions were investigated in pure water and a water/glycerol mixture, as a function of temperature and pressure. In pure water, the gelation time was determined by following the evolution of the storage and loss moduli versus time at several frequencies and temperatures between 18 and 35 degrees C. Increasing the temperature increased the aggregation kinetics and reduced the gelation time, while the elastic character and relaxation exponent showed more liquid-like viscoelastic properties. The fractal dimensions of the flocs in the gel network decreased when the temperature increased, showing a looser and more open gel network at the higher temperature. In the 50/50 mixture of water/glycerol, the flow behaviour showed a transition from more viscoelastic solid to a shear induced gel and highly Newtonian behaviour when the temperature increased from 18 to 50 degrees C. The irreversible shear thickening behaviour at the intermediate temperature (25 degrees C) was explained by the domination of hydrodynamic forces, originating from thermal fluctuations of the medium, in balance with the colloidal interaction forces. When increasing the pressure from 1 to 150 bars at constant temperature, the suspensions showed highly shear thickening effects in the whole pressure range. The onset shear rate of the thickening was accelerated from ambient pressure up to 150 bars. Most anomalies of the temperature and pressure effects on the suspensions stability were attributed to the conformational changes of the water and glycerol molecules and the degree and range of the hydrogen bondings, resulting in a change in the extent of silica bridging during destabilisation of suspensions. (C) 2011 Elsevier B.V. All rights reserved.