Showing papers by "Richard Buscall published in 1987"

The consolidation of concentrated suspensions. Part 1.—The theory of sedimentation

Abstract: The concentration or consolidation of suspensions of fine particles under the influence of a gravitational field has been analysed. The rate and extent of consolidation depends upon a balance of three forces, the gravitational driving force, the viscous drag force associated with flow of liquid in the sediment and a particle or network stress developed as a result of direct particle–particle interactions. In the case of colloidally stable suspensions, this particle stress is the osmotic pressure of the particles; in the case of flocculated or coagulated suspensions, it is the elastic stress developed in the network of particles. A constitutive equation is suggested for irreversibly flocculated suspensions undergoing consolidation which embodies the concept of a concentration-dependent yield stress Py(ϕ). This is then used to analyse the sedimentation behaviour of flocculated sediments and to derive expressions for the initial sedimentation rate. The initial rate of change of sediment height with time in a uniform gravitational or centrifugal field is given approximately by: [graphic ommitted] where B=Δρgϕ0H0/Py(ϕ0), u0 is the sedimentation rate of an isolated particle, ϕ0 is the initial (uniform) volume fraction of solids, r(ϕ0) is a dimensionless hydrodynamic interaction parameter, Δρ is the difference in density between solid and liquid, g is the gravitational or centrifugal acceleration and H0 is the initial sediment height. The theory accounts correctly for the equilibrium consolidation behaviour of strongly flocculated suspensions, and preliminary experimental data suggest that it is not inconsistent with their dynamic behaviour. The estimation of the yield stress Py(ϕ) from a batch centrifuge experiment is also described.

The rheology of strongly-flocculated suspensions

Abstract: The rheology of strongly-flocculated dispersions of colloidal particles has been investigated at particle concentrations where a continuous network is formed rather than a collection of discrete flocs. Such networks are shown to possess a true yield stress in both shear and in uniaxial compression (as realised in a centrifuge). Properties measured as a function of particle concentration and particle size include the yield stresses in shear (σ y ) and compression ( P y ); the limiting and strain-dependent, instantaneous shear moduli G O and G (γ); the elastic recovery at finite strains, and the rate of centrifugally-driven compaction. The yield stresses and moduli appear to show a power-law dependence on particle concentration with G O and P y , having the same power-law index and σ y a somewhat lower one. The data are in part consistent with predictions based on the idea that the networks have a heterogeneous structure comprising a collection of interconnected fractal aggregates. The behaviour as a function of particle size and concentration is however not completely scaleable as might be expected on this basis. Thus, whereas the shear yield stress could be scaled to remove its dependence on particle radius a and volume fraction φ (over the measured range 0.25 μm ⩽ a ⩽ 3.4 μm; 0.05 ⩽ φ ⩽ 0.25) as could the strain dependent modulus (0.25 ⩽ a ⩽ 1.3 μm; 0.08 ⩽ 0.25), the particle-size and concentration dependence of P y and G O could only be scaled for particles with radii between 0.16 and 0.5 μm, smaller and larger particles having different and much higher power-law index in respect of their concentration dependencies. In the case of the smaller particles the failure of the scaling is thought to be due to an anomaly since these particles distort significantly under the influence of the strong van der Waals forces and this causes the aggregates to be more compact then they otherwise would be. The reasons for the failure at larger sizes is not clear.