Shear-induced microstructural evolution of a thermoreversible colloidal gel

We report a study of the effect of shear deformation on the static structur e factor, S(q), of a thermoreversible gel of organophilic colloidal silica (a = 40 nm) in the solvent hexadecane. Small- and wide-angle light scatteri ng measurements show that the quiescent structure of these gels is consiste nt with that of fractal clusters with dimension cl = 2.4(independent of vol ume fraction, phi) and finite radius (xi), which is a function of phi for t he range 0.01 < phi < 0.1. Upon application of low shear rate deformation(g amma less than or equal to 30 s(-1)), we observe an increase in xi and d, r elative to the quiescent conditions. For this to be the case, mass conserva tion requires that the number density of clusters be dramatically reduced u pon 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 lon g-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 studi ed. 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 wit h an increase in both the fractal dimension, d, and cluster radius, xi. Com parison of the time evolution of S(aq=0.032) with transient theological mea surements performed under the same conditions reveals that the monotonic in crease in S(aq = 0.032) occurs on a time scale identical to that required f or the stress response to attain steady state.