Paul D. Mills

Bio: Paul D. Mills is an academic researcher from University of Swaziland. The author has contributed to research in topics: Colloidal silica & Particle. The author has an hindex of 5, co-authored 7 publications receiving 627 citations.

Scaling behaviour of the rheology of aggregate networks formed from colloidal particles

Abstract: The instantaneous shear modulus G and compactive strength Py of aggregate networks formed from silica particles with a mean diameter of 26 nm have been determined as a function of particle concentration. The data are compared with similar data obtained earlier for a range of polystyrene spheres with diameters between 60 and 960 nm and with compactive strength data obtained for polystyrene spheres at higher volume fractions by Sutherland. It is shown that clusters of submicron spheres formed by rapid aggregation become spacefilling and form a network at a critical volume fraction Φg of ca. 0.05. Above this concentration the data for Py and G suggest that aggregate networks show universal behaviour which is consistent with the scalings G∼ϕµ, dPy(ϕ)//d ln ϕ∼G(ϕ), with µ= 4 ± 0.5. This latter value for the exponent agrees well with that predicted by Ball and Brown by assuming the clusters comprising the network are fractal. For diffusion-limited cluster–cluster aggregation (DCA) they obtained a value of µ= 3.6. The data for Py imply a particle size dependence of the type Py∼am with m between –2 and –3, where a is the particle radius. More data are required to establish the precise dependence; the observed trend is, however, not inconsistent with what might be expected from a consideration of interparticle forces which implies a scaling of a–2.3. The scaling behaviour of the yield stress in shear flow and the dependence of the shear modulus on strain for non-negligible strains is also discussed.

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.

Viscoelastic and shear viscosity studies of colloidal silica particles dispersed in monoethylene glycol (MEG), diethylene glycol (DEG), and dodecane stabilized by dodecyl hexaethylene glycol monoether (C12E6).

Abstract: Silica dispersions stabilized by a nonionic surfactant, dodecyl hexaethylene glycol monoether (C12E6), were studied using rheological measurements. The viscosity−shear rate flow behavior of silica in monoethylene glycol (MEG) is shear thinning at low shear rates, leading to a Newtonian plateau at high shear rates for all dispersions studied. All rheological properties showed an increase above a critical surfactant concentration. The dispersions were stable at low levels of C12E6 concentrations because of electrostatic repulsions as deduced from the zeta potentials of silica that were on the order of about −30 to −65 mV in monoethylene glycol (MEG). Instability on further addition of C12E6 to the silica particles, a phenomenon normally obtained with high-molecular-weight polymers, was observed in MEG. Viscoelatic measurements of silica in monoethylene glycol at various surfactant concentrations showed a predominantly viscous response at low frequency and a predominantly elastic response at high frequencies...

Colloidal Processing of Ceramics

Abstract: Colloidal processing of ceramics is reviewed with an emphasis on interparticle forces, suspension rheology, consolidation techniques, and drying behavior. Particular attention is given to the scientific concepts that underpin the fabrication of particulate-derived ceramic components. The complex interplay between suspension stability and its structural evolution during colloidal processing is highlighted.

Universality in colloid aggregation

Abstract: THE aggregation of colloidal particles is of fundamental importance in colloid science and its applications. The recent application of scaling concepts1,2 has resulted in a much deeper understanding of the structure of colloidal aggregates and the kinetics of their formation. Two distinct, limiting regimes of irreversible colloid aggregation have been identified3. Diffusion-limited colloid aggregation occurs when there is negligible repulsive force between the colloidal particles, so that the aggregation rate is limited solely by the time taken for clusters to encounter each other by diffusion. Reaction-limited colloid aggregation occurs when there is still a substantial, but not insurmountable, repulsive force beween the particles, so that the aggregation rate is limited by the time taken for two clusters to overcome this repulsive barrier by thermal activation. These regimes correspond to the limiting cases of rapid and slow colloid aggregation that have long been recognized in colloid science4. An intriguing possibility suggested by recent work is that each of these limiting regimes of colloid aggregation is universal, independent of the chemical details of the particular colloid system. Here we investigate the aggregation of three chemically different colloidal systems under both diffusion-limited and reaction-limited aggregation conditions. A scaling analysis of light-scattering data is used to compare the behaviour and provides convincing experimental evidence that the two regimes of aggregation are indeed universal.

Colloidal Suspension Rheology

Abstract: 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.

Exploiting particle shape in solid stabilized emulsions

Abstract: Pickering–Ramsden emulsions and other forms of particle stabilized soft materials have received quite some attention recently because of the relative ease of formulation and the possibility to create novel materials. There is, however, a clear need for approaches that are versatile and efficient. In the present work the effect of aspect ratio of particles on the stability of both water-in-oil and oil-in-water emulsions is investigated experimentally. Two types of non-spherical particles are used. Hydrophobic prolate ellipsoids with aspect ratios ranging from 1 to 9 are obtained by stretching polystyrene latex particles. Hydrophilic spindle type hematite particles have been synthesized with aspect ratios ranging from 1 to 6. A strong dependence of emulsion stability on the aspect ratio of the particles is observed. Optical as well as cryogenic scanning electron microscopy are used to visualize the droplet morphology and particulate structure and reveal fairly densely packed monolayers of ellipsoids, consistent with the mechanism of limited coalescence. Yet stable emulsions are only obtained for particles with a sufficient aspect ratio. Surface rheology on planar monolayers demonstrates the pronounced effect of aspect ratio on the surface moduli. The magnitude of the interfacial viscoelastic properties is shown to strongly depend on the aspect ratio at a given surface coverage. This is most probably due to an increased effective coverage and the occurrence of strong attractive shape induced capillary interactions. The dependence of the surface rheological properties on the aspect ratio of the particles rationalizes the observed emulsion stability as the surface rheological properties play a role in the coalescence process. The results demonstrate that interfaces with controlled surface rheology, as obtained by using shape induced capillary forces and packing effects, can be used for the rational design of Pickering emulsions and other types of high interface materials.