MSACT Research and Consulting

Exeter, United Kingdom

MSACT Research and Consulting

Exeter, United Kingdom

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Stickland A.D.,University of Melbourne | Stickland A.D.,University of Leeds | Stickland A.D.,MSACT Research and Consulting | Kumar A.,University of Melbourne | And 17 more authors.
Rheologica Acta | Year: 2015

Measuring yielding in cohesive suspensions is often hampered by slip at measurement surfaces. This paper presents creep data for strongly flocculated suspensions obtained using vane-in-cup tools with differing cup-to-vane diameter ratios. The three suspensions were titania and alumina aggregated at their isoelectric points and polymer-flocculated alumina. The aim was to find the diameter ratio where slip or premature yielding at the cup wall had no effect on the transient behaviour. The large diameter ratio results showed readily understandable material behaviour comprising linear viscoelasticity at low stresses, strain-softening close to yielding, time-dependent yield across a range of stresses and then viscous flow. Tests in small ratio geometries however showed more complex responses. Effects attributed to the cup wall included delayed softening, slip, multiple yielding and stick–slip events, and unsteady flow. The conclusion was that cups have to be relatively large to eliminate wall artefacts. A diameter ratio of three was sufficient in practice, although the minimum ratio must be material dependent. © 2015, Springer-Verlag Berlin Heidelberg.


Lester D.R.,CSIRO | Lester D.R.,Institute of Chemical Technology | Buscall R.,MSACT Research and Consulting | Buscall R.,University of Melbourne
Journal of Non-Newtonian Fluid Mechanics | Year: 2015

The batch settling test is widely used to estimate the compressive rheology of strongly flocculated colloidal suspensions, in particular the compressive yield strength and hydraulic permeability. Recently it has been discovered (Lester et al., 2014) that wall adhesion effects in these tests may be significantly greater than previously appreciated, which can introduce unbounded errors in the estimation of these rheological functions. Whilst a methodology to solve the underlying static problem and correct for wall adhesion effects has been developed, this method is quite complex and unwieldy, involving solution of a 2D visco-elastic constitutive model for strong colloidal gels. In this paper we develop a highly simplified 1D visco-plastic approximation to the visco-elastic model which admits analytic expressions for the equilibrium solids concentration profile and bed height. These expressions facilitate robust estimation of the compressive yield and wall adhesion strength via nonlinear regression of experimental data in the presence of small measurement errors. © 2015.


Buscall R.,MSACT Research and Consulting | Buscall R.,University of Melbourne | Scales P.J.,University of Melbourne | Stickland A.D.,University of Melbourne | And 2 more authors.
Journal of Non-Newtonian Fluid Mechanics | Year: 2015

An experimental system has been found recently, a set of coagulated CaCO3 suspensions, which shows very variable yield behaviour depending upon how it is tested and, specifically, at what rate it is sheared. At Péclet numbers (Pe)>1 it behaves as a simple Herschel-Bulkley liquid, whereas at Pe<1 highly non-monotonic flow curves are seen. In controlled stress testing it shows hysteresis and shear banding and in the usual type of controlled stress scan routinely used to measure flow curves, it can show very erratic and irreproducible behaviour. All of these features appear to arise from a dependence of the solid phase, or yield stress, on the prevailing rate of shear at the yield point. Stress growth curves obtained from step strain-rate testing showed that rate-dependence was a consequence of Péclet number dependent strain softening. At very low Pe, yield was cooperative and the yield strain was order-one, whereas as Pe approached unity, the yield strain reduced to that needed to break inter-particle bonds, causing the yield stress to be greatly reduced.It is suspected that rate-dependent yield could well be the rule rather than the exception for cohesive suspensions more generally. If so, then the Herschel-Bulkley equation can usefully be generalized to read σ=σ0g(γ)+σiso+kγn (in simple shear). The proposition that rate-dependent yield could be general for cohesive suspensions is amenable to critical experimental testing by a range of means and along lines suggested. © 2015 Elsevier B.V.


Anderson R.,University of Melbourne | Buscall R.,MSACT Research and Consulting | Eldridge R.,University of Melbourne | Mulvaney P.,University of Melbourne | Scales P.J.,University of Melbourne
Colloids and Surfaces A: Physicochemical and Engineering Aspects | Year: 2014

Ag salt nanoparticles were manufactured at high concentration in water in the presence of a series of copolymers of poly(ethylene glycol) and maleic anhydride (PEG-MA). The Ostwald ripening behaviour was then examined using the PEG chain length of the copolymer as a variable. The ripening rate was observed to be very slow compared to previous literature studies of bare or surfactant stabilised particles in dilute solution. Despite the hindrance to ripening caused by the presence of the adsorbed copolymer, the predicted rate was shown to fit closely to Lifshitz-Slyozov-Wagner (LSW) theory. Particle ripening decreased with increasing PEG chain length. The data highlight the important role of stabiliser chain length for the stability and shelf-life of concentrated nanoparticle dispersions. © 2014 Elsevier B.V.


Anderson R.,University of Melbourne | Buscall R.,MSACT Research and Consulting | Eldridge R.,University of Melbourne | Mulvaney P.,University of Melbourne | Scales P.,University of Melbourne
RSC Advances | Year: 2014

Silver nanoparticles have been synthesised at high concentration (>1 mol L-1) in water using a series of comb-graft copolymers. The copolymers were synthesised and then derivatised from methoxypoly(ethylene glycol) acrylate and maleic anhydride. The degree of polymerisation (number of backbone units), the nature of the adsorption group and length (MW) of the steric component were all found to be critical to the stabilisation of the nanoparticles during synthesis. The results demonstrate that a minimum steric length is required to prevent particle aggregation during synthesis. For Ag particles less than 30 nm in diameter, this corresponded to a polyethylene glycol (PEG) chain with a molecular weight of 454 g mol-1. It was shown that stabiliser molecules with a low number of backbone units were readily displaced, whereas stabilisers that possess numerous adsorption groups bind irreversibly to the particle surface. Adsorption groups were modified to test both chemisorption and physisorption functionality. Chemisorbed species were shown to be more effective stabilisers whereby concentrated Ag nanoparticles were synthesised at a smaller particle size. This was attributed to the adsorption strength and greater diffusive capabilities of the molecule. © the Partner Organisations 2014.


Anderson R.,University of Melbourne | Buscall R.,MSACT Research and Consulting | Eldridge R.,University of Melbourne | Mulvaney P.,University of Melbourne | Scales P.,University of Melbourne
RSC Advances | Year: 2014

The synthesis of a range of metal nanoparticles (Ag, Au, Pd, Pt) in water through reduction from the acid soluble salt at abnormally high concentrations (>1 mol L-1) is demonstrated using a comb polymer of methoxypoly(ethylene glycol) acrylate and maleic anhydride (PEG-MA) as the particle stabiliser during particle formation. The results show that at high concentrations in water, the general growth mechanism in these systems is through aggregation of nuclei of an approximate diameter of 0.6 nm. Aggregation resulted in formation of single crystals up to a particle diameter of approximately 5 nm but thereafter, further aggregation resulted in polygonal twinned particles. Continued aggregation caused agglomerate particles to be formed at larger sizes (>30 nm). Stabiliser adsorption was found to be critical to size control whereby the aggregation process was interrupted, preventing further growth. A case of the synthesis of Ag nanoparticles with a mean size of 8 nm at concentrations of up to 2.5 mol L-1 is elaborated. © 2014 the Partner Organisations.

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