Shear-Induced Microstructural Evolution of a Thermoreversible Colloidal Gel

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Abstract

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 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.

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