The viscometric properties and structure of concentrated sheared colloidal
gels, formed at a pH = 8 from a commercial aqueous sol of 7 nm diameter sil
ica particles, were investigated. As the system gels under a constant shear
rate, the viscosity first increases, then peaks when the shear stress reac
hes a shear-rate-independent threshold value of about 350 Pa, and finally d
ecreases asymptotically to approximately one tenth the peak value. This low
-viscosity state can apparently be held indefinitely by maintaining the she
ar, but the system gels once the shear is removed. The structural origins o
f 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. Sampl
es 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 est
imated by SANS when appropriate. We conclude that the drop in viscosity cor
responds to a structural densification of growing clusters of the colloidal
particles, and that this transformation occurs once the critical stress is
reached.