Shear-induced restructuring of colloidal silica gels

Continuous oscillatory experiments were used to investigate the viscoelasti c properties and structural evolution of silica gels obtained from unsheare d and presheared sols, at pH 8. The processes controlling structural evolut ion before, at and after the gel point are discussed. Although the viscoela stic properties of the aged gels are comparable (implying similar structure s on length scales of several microns), their chemical properties and micro structures (<1 mum) are significantly different. This is directly related t o the different aggregate structures formed in the unsheared and presheared systems (50 and 700 nm, respectively) and the associated concentration of surface hydroxyls available for cross-linking and formation of stable silox ane bonds.