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Journal ArticleDOI

Effects of microstructure on the compressive yield stress

01 Jan 2000-Aiche Journal (John Wiley & Sons, Ltd)-Vol. 46, Iss: 1, pp 72-78

AbstractThe effects of microstructure on the compressie properties of aggregated alumina suspensions are determined by intentionally introducing heterogeneities into the suspen- sion. Suspensions are prepared at a higholume fraction and diluted with low shear hand mixing to a series of initial concentrations. As the initial concentration is in- creased, larger heterogeneities are introduced, and the suspension becomes more com- pressible relatie to the compressie yield stress of the uniform suspension. A simple model is proposed in which the heterogeneous suspensions compress by rearrangement ( of the dense aggregates until a critical concentration f , which coincides with the c ) ¤olume fraction prior to dilution is reached. Aboe f , the suspensions consolidate c ( identically to the uniform suspension. With a single fitting parameter the size of the ) heterogeneities , the model shows semiquantitatie agreement with the experimental data.

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Citations
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Journal ArticleDOI
Abstract: The ability to pattern materials in three dimensions is critical for several technological applications, including composites, microfluidics, photonics, and tissue engineering. Direct-write assembly allows one to design and rapidly fabricate materials in complex 3D shapes without the need for expensive tooling, dies, or lithographic masks. Here, recent advances in direct ink writing are reviewed with an emphasis on the push towards finer feature sizes. Opportunities and challenges associated with direct ink writing are also highlighted.

926 citations


Journal ArticleDOI
Abstract: The ability to pattern ceramic materials in three dimensions (3D) is critical for structural, functional, and biomedical applications. One facile approach is direct ink writing (DIW), in which 3D structures are built layer-by-layer through the deposition of colloidal- or polymer-based inks. This approach allows one to design and rapidly fabricate ceramic materials in complex 3D shapes without the need for expensive tooling, dies, or lithographic masks. In this feature article, we present both droplet- and filament-based DIW techniques. We focus on the various ink designs and their corresponding rheological behavior, ink deposition mechanics, potential shapes and the toolpaths required, and representative examples of 3D ceramic structures assembled by each technique. The opportunities and challenges associated with DIW are also highlighted.

502 citations


Journal ArticleDOI
30 Jan 2007-Langmuir
TL;DR: This study synthesized dispersants with a molecular architecture that enables better control over the particle adlayer thickness that facilitates the fabrication of a wide range of products and intermediates in materials technology, cosmetics, pharmacy, and in all areas where concentrated nanoparticle suspensions are required.
Abstract: The stabilization of nanoparticles in concentrated aqueous suspensions is required in many manufacturing technologies and industrial products. Nanoparticles are commonly stabilized through the adsorption of a dispersant layer around the particle surface. The formation of a dispersant layer (adlayer) of appropriate thickness is crucial for the stabilization of suspensions containing high nanoparticle concentrations. Thick adlayers result in an excessive excluded volume around the particles, whereas thin adlayers lead to particle agglomeration. Both effects reduce the maximum concentration of nanoparticles in the suspension. However, conventional dispersants do not allow for a systematic control of the adlayer thickness on the particle surface. In this study, we synthesized dispersants with a molecular architecture that enables better control over the particle adlayer thickness. By tailoring the chemistry and length of these novel dispersants, we were able to prepare fluid suspensions (viscosity < 1 Pa.s at 100 s-1) with more than 40 vol % of 65-nm alumina particles in water, as opposed to the 30 vol % achieved with a state-of-the-art dispersing agent. This remarkably high concentration facilitates the fabrication of a wide range of products and intermediates in materials technology, cosmetics, pharmacy, and in all other areas where concentrated nanoparticle suspensions are required. On the basis of the proposed molecular architecture, one can also envisage other similar molecules that could be successfully applied for the functionalization of surfaces for biosensing, chromatography, medical imaging, drug delivery, and aqueous lubrication, among others.

221 citations


Journal ArticleDOI
TL;DR: The best dewaterability is observed for activated sludge that contains strong, compact flocs without single cells and dissolved extracellular polymeric substances.
Abstract: Biological wastewater treatment removes organic materials, nitrogen, and phosphorus from wastewater using microbial biomass (activated sludge, biofilm, granules) which is separated from the liquid in a clarifier or by a membrane. Part of this biomass (excess sludge) is transported to digesters for bioenergy production and then dewatered, it is dewatered directly, often by using belt filters or decanter centrifuges before further handling, or it is dewatered by sludge mineralization beds. Sludge is generally difficult to dewater, but great variations in dewaterability are observed for sludges from different wastewater treatment plants as a consequence of differences in plant design and physical-chemical factors. This review gives an overview of key parameters affecting sludge dewatering, i.e. filtration and consolidation. The best dewaterability is observed for activated sludge that contains strong, compact flocs without single cells and dissolved extracellular polymeric substances. Polyvalent ions such as calcium ions improve floc strength and dewaterability, whereas sodium ions (e.g. from road salt, sea water intrusion, and industry) reduce dewaterability because flocs disintegrate at high conductivity. Dewaterability dramatically decreases at high pH due to floc disintegration. Storage under anaerobic conditions lowers dewaterability. High shear levels destroy the flocs and reduce dewaterability. Thus, pumping and mixing should be gentle and in pipes without sharp bends.

163 citations


Journal ArticleDOI
Abstract: To characterize and optimize the compressi®e dewatering of suspensions, rapidly determinable material parameters are required. A mathematical framework exists for char() acterization of dewatering using three parameters: the compressi®e yield stress P , the y () ( ) hindered settling function r , and a solids diffusi®ity D .An ew®ariation on traditional constant pressure, batch filtration tests is described along with a computer-controlled filtration apparatus that enables complete characterization of a suspension with respect to dewatering in a matter of hours. The testing methods are ®alidated experimentally on flocculated zirconia suspensions along with calculation of the material dewatering parameters. Application of the calculated parameters to modeling of a real filtration test shows excellent agreement. Practical implications of the modeling are also discussed.

116 citations


References
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Journal ArticleDOI
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.

383 citations


"Effects of microstructure on the co..." refers background in this paper

  • ...Buscall and White 1987 describe a constitutive model for the compressive behavior of aggregated suspensions....

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  • ...Miller et al., 1996 , or 2 measuring the equilibrium height at Ž .a series of speeds Buscall, 1982; Buscall and White, 1987 ....

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Journal ArticleDOI
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.

342 citations


"Effects of microstructure on the co..." refers background or methods in this paper

  • ...While the term gelation is more precisely used for systems with interparticle interactions on the order of a few kT, it is often applied more broadly to systems where there is a liquid to solid transition, even in the presence of much stronger attracŽ .tions Buscall et al., 1988 ....

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  • ...Buscall et al. 1988 Ž .and Meeten 1994 present data on the compressive behavior of polystyrene latex, silica, and two clays, bentonite and attaŽ .pulgite....

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Journal ArticleDOI
Abstract: The consolidation behavior of flocculated alumina suspensions has been analyzed as a function of the interparticle energy. Consolidation was performed by a centrifugal force field or by gravity, and both the time-dependent and equilibrium density profiles were measured by a gamma-ray absorption technique. The interparicle energy at contact was controlled by adsorbing fatty acids of varying molecular weight at the alumina/decalin interface. We found that strongly attractive interactions result in a particle network which resists consolidation and shows compressible behavior over a large stress range. The most weakly flocculated suspension showed an essentially incompressible, homogeneous density profile after consolidation at different centrifugal speeds. We also found a significant variation in the maximum volume fraction, φm, obtained, with φm∼ 0.54 for the most strongly flocculated suspension to φm∼ 0.63 for the most weakly flocculated suspension. The compresive yield stresses show a behavior which can be fitted to a modified power law. In this paper, we discuss possible correlations between the fitting parameters and physical properties of the flocculated suspensions.

193 citations


"Effects of microstructure on the co..." refers background or methods in this paper

  • ...In centrifugation, compressive yield stress data can be obtained Ž .by two methods: 1 measuring the volume fraction profile of Žthe centrifuge bed at a single speed Bergstrom et al., 1992;¨ ....

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  • ...Bergstrom et al. 1992 studied the compressive behavior¨ Ž .of alumina particles suspended in decalin....

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Journal ArticleDOI
Abstract: The shear and compressive properties of aggregated alumina particles are determined as functions of volume fraction and the strength of the interparticle attraction. Over a range of volume fractions, yield stresses, τy, elastic moduli, the strain delimiting the extent of the linear elastic response, and compressive yield stress, Py, are well described by power-law functions of volume fraction, while the role of interparticle attractions can be accounted for by expressing these mechanical properties as (ϕ/ϕg − 1)n, where ϕg captures the strength of particle attractions and n the microstructure. The links between compressive and shear properties are well described by linear elastic models where the Py and τy are a function of Poisson's ratio which, for the suspensions investigated, has a value near 0.49.

164 citations


"Effects of microstructure on the co..." refers background in this paper

  • ...…curve is characteristic of the Žcompressive behavior observed in other systems Bergstrom¨ January 2000 Vol. 46, No. 1 AIChE Journal74 The curves presented are for suspensions with homoge- Ž u.neous microstructures that is, P .y et al., 1992; Channell and Zukoski, 1997; Miller et al., 1996; ....

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  • ...Similar effects are also observed for a variety of systems as the strength of Žthe particle network is varied Channell and Zukoski, 1997; ....

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  • ...The ability of small shear stresses to disrupt heterogeneities is a result of the shear yield stress being much smaller than the Ž .compressive yield stress Channell and Zukoski, 1997 and the continuous nature of the shearing action....

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Journal ArticleDOI
Abstract: Solid-liquid separation operations leading to the concentration and isolation of fine particles dispersed in liquids are important in the chemical and mineral processing industries. In spite of this, the procedures available for the prediction of equipment performance remain crude. Almost all major mineral and chemical processing companies now have a clear priority in RD a major part of the problem faced by these industries relates to the effect of management of waste slurries. The processes that manage final waste slurries are often classical “end-of-pipe” solutions. One of the key aims of the present broad program is to understand how to manipulate the structure of slurries within the process so that finally it is possible to engineer clear liquor and simultaneously manageable or tractable waste solids. The best way to process such wastes relies on understanding how to control the compressibility and viscosity of these materials. A generalized approach to understanding and prediction of solid-liquid separation methods based on the measurement of fundamental material properties is reviewed here. This is of value in designing more efficient methods and ultimately to optimizing the performance of solid-liquid separation methods and the selection of flocculants for any given slurry. The model identifies two key parameters, the compressional yield stress Py(φ) and the hindered settling factor r(φ) and laboratory test procedures for the direct measurement of both have been developed. The application of this model to a variety of thickening and filtration processes is demonstrated and a direct relationship between the model parameters and the conventional cake resistance as utilised by current filtration engineers is provided.

162 citations


"Effects of microstructure on the co..." refers background in this paper

  • ...At this point, the applied load is the compressive yield stress at that volume fracŽ .tion Lange and Miller, 1987; Landman and White, 1994 ....

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