1. Introduction to colloid science and rheology 2. Hydrodynamic effects 3. Brownian hard spheres 4. Stable colloidal suspensions 5. Non-spherical particles 6. Weakly flocculated suspensions 7. Thixotropy 8. Shear thickening 9. Rheometry of suspensions 10. Suspensions in viscoelastic media 11. Advanced topics.

Colloidal Suspension Rheology

In contrast to recent publications suggesting that particle cluster formation alone can play an important role in the shear thickening flow behavior of concentrated colloidal suspensions, we believe that there is little if any substantive evidence to prove it. To support this view, we use data from various studies, including data from studies concluding that layered flow is not involved. One reason for the confusion seems to center around the inability of various light-scattering and neutron-scattering techniques to show particle layering before shear thickening when the layers are not well defined. In this regard, one should understand that layered flow can occur without rigorous ordering of particles within the layers, and as the flowing suspension approaches the point of instability, the hydrodynamic forces driving for the instability will jostle the particles within the layers sufficiently to make it even harder to see the layering and any ordering, if it exists, within the layers. Having these views,...

Explanations for the cause of shear thickening in concentrated colloidal suspensions

The particle size dependence of the reversible shear thickening transition in dense colloidal suspensions is explored. Five suspensions of monodisperse silica are synthesized via the Stober synthesis. The physicochemical properties of the dispersions are quantified using transmission electron microscopy, dynamic light scattering, small angle light scattering, electrophoresis, and viscometry. Rheology measurements indicate a critical stress marking the onset of reversible shear thickening that depends on the dispersion’s particle size, concentration, polydispersity, and interparticle interactions. A simplified two particle force balance between the interparticle repulsive forces and the hydrodynamic compressive forces is used to derive a scaling relationship between this critical shear stress and the suspension properties. The scaling is tested against the fully characterized silica dispersions, which span nearly a decade in particle size. Furthermore, bimodal mixtures of the dispersions are employed to ev...

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The effects of particle size on reversible shear thickening of concentrated colloidal dispersions

Yield stress measurements were performed on a TiO 2 pigment suspension with the vane in both a rate controlled and a stress controlled mode. In the rate controlled mode, a constant rotational speed is applied to the vane immersed in the suspension, and the resulting stress is measured as a function of time. In the stress controlled mode, a constant stress or a constant stress-rate is applied to the vane immersed in the suspension, and the resulting creep angle is measured as a function of time. In both modes the yield stress is determined as the minimum stress required for continuous rotation of the vane. A Haake Rheometer and a modified Weissenberg Rheogoniometer facilitated the controlled rate measurements. The significant difference between these instruments is that the Weissenberg torsion bar is 550 times stiffer than the Haake spring. To ensure a comparable time frame of measurement with the Haake, it was necessary to use lower rotational speeds in the operation of the Weissenberg. Although agreement in the measured yield stress within 5% was established between the Haake at a rotational speed of 0.021 rad s −1 and the Weissenberg at a rotational speed of 0.0063 rad s −1 , the shapes of the respective stress-time profiles were strikingly different. The Haake exhibited a largely elastic response, whereas the Weissenberg exhibited a largely viscoelastic response. The yield stress measured by both the Haake and the Weissenberg corresponds to the transition stress between viscoelastic and fully viscous flow. A Bohlin Rheometer facilitated the stress controlled measurements. The yield stress measured by the Bohlin was up to 13% lower than that measured by the Haake and the Weissenberg, and seemed to correspond more closely to the transition stress between fully elastic and viscoelastic flow.

Yield stress measurements with the vane

The shear induced microstructure for electrostatic and Brownian suspensions are compared using in situ small angle neutron scattering (SANS). The dispersions consist of 75 nm Stober silica coated with 3-(trimethoxysilyl) propyl methacrylate (TPM) and have a zeta potential of −42.6±4.7 mV. Neutralizing the surface charge with 0.066 M¯ nitric acid yields stable hard-sphere dispersions. SANS is conducted over a range of shear rates on the charge-stabilized and Brownian suspensions to test the order–disorder transition and hydrocluster mechanisms for shear thickening, and demonstrate the influence of stabilizing forces on the shear induced microstructure evolution. Through treatment of the colloidal micromechanics, shear induced changes in the microstructure are correlated to the hydrodynamic component of the shear stress and the thermodynamic component of the normal stress, i.e., the method of “Rheo-SANS” is developed. The results demonstrate that hydrocluster formation accompanies the shear thickening trans...

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Flow-small angle neutron scattering measurements of colloidal dispersion microstructure evolution through the shear thickening transition

The rheological and microstructural properties of dense suspensions of uniform, charge stabilized colloidal spheres with diameters greater than 200 nm are investigated at volume fractions just below the ordering transition up to 0.6. Shear stresses marking static and dynamic yielding, discontinuous shear thinning, and shear thickening are weakly dependent on volume fraction and particle size when scaled on the crystal’s elastic modulus G0. As shear stress is increased microstructures evolve through similar states independent of volume fraction. As rest, presheared suspensions exhibit long‐range orientational order. Above the dynamic yield stress, the suspensions deform with a polycrystalline microstructure which, at intermediate shear rates, evolves to hexagonally close‐packed planes lying parallel to the rheometer walls. At higher shear rates the suspensions melt. Thickening is only observed above a volume fraction of 0.4–0.5, depending on particle size and at shear rates above that where the hexagonal c...

M. K. Chow

Papers

Nonequilibrium behavior of dense suspensions of uniform particles: Volume fraction and size dependence of rheology and microstructure