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Shear Induced Structural Changes in a Gel-Forming Suspension Studied by Light Scattering and Rheology

1996; American Chemical Society; Volume: 12; Issue: 12 Linguagem: Inglês

10.1021/la951504r

1520-5827

Henk Verduin, Bérenger Gans, Jan K. G. Dhont,

Material Dynamics and Properties

We investigated the shear induced microstructural changes around the gel point, using constant stress and constant shear rheology, small angle light scattering, and turbidity measurements. The colloidal system that is used is a dispersion of colloidal stearyl silica spheres, dispersed in benzene, which shows gelation upon cooling at a temperature of 18.2 °C and a volume fraction of 0.25. At the gel temperature the dispersion develops a yield stress, which increases with decreasing temperature. The high-shear and low-shear viscosity show a sharp discontinuity at the gel temperature. Constant shear measurements in the gel phase show shear thickening at very low shear rates and shear thinning at intermediate shear rates. Modeling the shear-thinning behavior according to a power law (η ∼ γ̇-b) results in values for the exponent varying from b = 0.43 at 0.4 °C below the gel temperature to 0.77 at 0.2 °C above the gel temperature. These values are in the same range as suggested by Wessel/Ball (Phys. Rev. A 1992, 46, R3008) and Doi/Chen (J. Chem. Phys. 1989, 91, 2656). Small angle light scattering reveals a high degree of anisotropy induced by the shear flow, both below and above the gel point. More or less regularly spaced large scale structures are induced by the shear flow, which are responsible for the observed shear-thickening behavior. With increasing shear rate, these large structures are destroyed, giving rise to shear thinning. As the shear flow is suddenly turned off, the anisotropic structures relax to equilibrium above the gel temperature but effectively persist below the gel temperature. We also report a decrease of the cloud point temperature with the shear rate according to ΔT = Aγ̇0.53, for γ̇ ≥ 0.3 s-1, which is quite similar to the case for critical fluids and polymer mixtures. However, contrary to the case for critical fluids and polymer mixtures, for very small shear rates (γ̇ < 0.3 s-1) the cloud point temperature is found to increase.