Shear-induced microstructural evolution of a thermoreversible colloidal gel

We report a study of the effect of shear deformation on the static structure factor, S(q), of a thermoreversible gel of organophilic colloidal silica (a = 40 nm) in the solvent hexadecane. Small- and wide-angle light scattering measurements show that the quiescent structure of these gels is consistent with that of fractal clusters with dimension cl = 2.4(independent of volume fraction, phi) and finite radius (xi), which is a function of phi for the range 0.01 < phi < 0.1. Upon application of low shear rate deformation(gamma less than or equal to 30 s(-1)), we observe an increase in xi and d, relative to the quiescent conditions. For this to be the case, mass conservation requires that the number density of clusters be dramatically reduced upon shearing. The increase of d and xi and the concomitant decrease in the number density of clusters point to the profound effect of shear on the long-range structure of colloidal gels. At high shear rates (gamma > 30 s(-1)) we observe anisotropy of S(q) in the now-vorticity plane. The observed two-lobe butterfly patterns are oriented in the now direction for all cp studied. The anisotropy persists after cessation of shear, although some partial relaxation is observed at the highest shear rate studied (gamma = 120 s(-1)). Start-up of steady shear experiments performed for phi = 0.035 reveals a monotonic increase of S(qi at low q (aq = 0.032), which is consistent with an increase in both the fractal dimension, d, and cluster radius, xi. Comparison of the time evolution of S(aq=0.032) with transient theological measurements performed under the same conditions reveals that the monotonic increase in S(aq = 0.032) occurs on a time scale identical to that required for the stress response to attain steady state.