Shear-induced restructuring of concentrated colloidal silica gels

The viscometric properties and structure of concentrated sheared colloidal gels, formed at a pH = 8 from a commercial aqueous sol of 7 nm diameter silica particles, were investigated. As the system gels under a constant shear rate, the viscosity first increases, then peaks when the shear stress reaches a shear-rate-independent threshold value of about 350 Pa, and finally decreases asymptotically to approximately one tenth the peak value. This low-viscosity state can apparently be held indefinitely by maintaining the shear, but the system gels once the shear is removed. The structural origins of this viscometric behaviour were studied using in situ small-angle neutron scattering (SANS). An abrupt change in the character of the SANS intensity was observed at the time corresponding to the peak in the viscosity. Samples of the gelling system were extracted and diluted at various times after gel initiation. Effective hydrodynamic diameters of these extractants were estimated by dynamic light scattering, and their Guinier diameters were estimated by SANS when appropriate. We conclude that the drop in viscosity corresponds to a structural densification of growing clusters of the colloidal particles, and that this transformation occurs once the critical stress is reached.