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 d = 2.4 (independent of volume fraction, φ) and finite radius (ξ), which is a function of φ for the range 0.01 < φ < 0.1. Upon application of low shear rate deformation (γ̇ ≤ 30 s - 1 ), we observe an increase in ξ 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 ξ 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 (γ̇ > 30 s - 1 ) we observe anisotropy of S ( q ) in the flow−vorticity plane. The observed two-lobe butterfly patterns are oriented in the flow direction for all φ studied. The anisotropy persists after cessation of shear, although some partial relaxation is observed at the highest shear rate studied (γ̇ = 120 s - 1 ). Start-up of steady shear experiments performed for φ = 0.035 reveals a monotonic increase of S ( q ) at low q ( aq = 0.032), which is consistent with an increase in both the fractal dimension, d, and cluster radius, ξ. Comparison of the time evolution of S ( aq =0.032) with transient rheological 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.