Effects of microstructure on the compressive yield stress
TL;DR: In this paper, the effects of microstructure on the compressie properties of aggregated alumina suspensions are determined by intentionally introducing heterogeneities into the suspen- sion.
Abstract: The 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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Dissertation•
01 Jan 2008
TL;DR: In this article, the influence of processing on the microstructure and the rheological properties of tomato paste suspensions was studied using light microscopy and particle size distribution analysis, and the way in which particles of varying size are packed in a specified volume at different concentrations was estimated in terms of the compressive volume fraction.
Abstract: Food processing comprises operations such as dilution (changing the concentration), homogenisation (changing the particle size), and subsequent pumping (shearing), among
others. It is thus of great interest to gain a better understanding of the mechanisms governing the creation and disruption of structures during these engineering operations, and the way in which they are related to the textural and rheological properties of the material.
The influence of processing on the microstructure and the rheological properties of tomato paste suspensions has been studied. The microstructure was characterised using light microscopy and particle size distribution analysis. The way in which particles of varying size are packed in a specified volume at different concentrations was estimated in terms of the compressive volume fraction. The rheological properties were studied using small-amplitude oscillatory tests, giving the elastic (G') and viscous (G'') moduli, as well as steady shear measurements, giving the viscosity (etha). In the latter case both a rotational
and a tube viscometer were used.
The results indicate that tomato suspensions consist of a collection of whole cells and cell wall material forming a network (G'>G''). During the process of homogenisation,
the particles are broken down, resulting in a smoother and more evenly distributed network of finer particles. The effectiveness of homogenisation in decreasing particle size
seemed to be governed by the inherent susceptibility of the particles to breakage (i.e. the type of paste), the viscosity of the suspending medium, and the concentration of particles. Higher viscosities and concentrations were found to prevent breakage to some extent.
The presence of larger amounts of fine particles in the homogenised suspensions had a considerable effect on the rheological properties. The yield stress was found to increase, and time-dependent effects became more apparent. At low deformations (gamma < 20), the system consisting of finer particles exhibited rheopectic behaviour (increasing viscosity with time), which was suggested to be caused partly by the rotation of the particles induced by the flow, and partly by the remaining elastic behaviour at stresses close to the yield stress. At larger deformations (gamma<1000), the non-homogenised system exhibited steady-state viscosity, while in the homogenised system it continued to decrease. The unstable behaviour observed in homogenised systems at large deformations gave an indication of particle rearrangement under flow conditions. Micrographs of homogenised suspensions subjected to shearing showed the formation of flocs consisting of densely packed particles that could easily orient in the shearing direction. At high concentrations, the changes in the microstructure caused by homogenisation and shearing were better reflected by the compressive volume fraction than by the elastic modulus.
Tube viscometer measurements showed the presence of wall slip in highly concentrated tomato suspensions, which tended to disappear at lower concentrations. The wall slip,
which could be as high as 70% of the flow rate, was estimated using both the classical Mooney approach and an inverse numerical method, and the performance of these two
methods was compared. The performance of the methods was complicated by the relatively poor reproducibility of the data. Steady shear rheological measurements obtained using a rotational rheometer with different geometries (concentric cylinders, vane, vane-vane) and tube viscometer measurements agreed when no slip was present, and the vane and vane-vane geometries were found to be free of wall slip effects.
Finally, the applicability of the Cox-Merz rule (superposition of oscillatory and steady shear data) seemed to be limited to systems that do not form a network (G' G''. However, the dynamic and steady shear data obtained for tomato suspensions coincided when using a shifting factor of about 0.1 on the frequency , which was fairly constant for a large range of tomato paste concentrations (from 100 to 30%, all with G'>G'').
24 citations
TL;DR: In this paper, the temporal evolution of specific resistance α (considered as the most representative fouling parameter) is investigated, aiming to elucidate differences and possible similarities in the different filtration modes.
23 citations
TL;DR: In this paper, the compressive yield stresses of sediment beds from flocs induced by 10% charged polymers were observed to increase with an increase in polymer dose, initial solids concentration, and background electrolyte concentration at all volume fraction.
17 citations
TL;DR: In this article, the microstructure details and the mechanical response under uniaxial compression of a strongly aggregating colloidal dispersion were investigated under the presence of sliding and rolling resistance (RR) and preparation conditions.
Abstract: We investigate the microstructure details and the mechanical response under uniaxial compression of a strongly aggregating colloidal dispersion The numerical simulations account for short-range interparticle attraction, normal and tangential deformation at particle contacts, sliding and rolling resistance (RR), and preparation conditions The compression rates are small so that hydrodynamic interactions are negligible In the absence of RR, the average coordination number varies only slightly with compaction while the variation is significant in its presence The particle contact distribution is isotropic throughout the consolidation process, irrespective of the magnitude of the parameters In this limit of strong aggregation, the elastic modulus is a weak function of the magnitude of attraction It is shown that the yield strain does not change significantly during the entire consolidation process, and the value in the presence of RR is marginally higher than in its absence However, the yield stress increases with volume fraction which is a direct consequence of the increased elastic modulus The yield stress, both in the presence and absence of RR, scales similarly with volume fraction The overall power law exponent of 57 of the yield stress in the presence of RR as a function of volume fraction is in good agreement with previous simulation results
17 citations
TL;DR: In this paper, the history of each mud sample is also an important factor influencing the rheological behavior, as the state of the clay fabric is dependent on the shear stresses experienced previously by the sample.
Abstract: Natural mud usually exhibits non-Newtonian rheological behaviors like viscoelasticity, thixotropy and yield stress. The history of each mud sample is also an important factor influencing the rheological behavior, as the state of the clay fabric – for a same composition – is dependent on the shear stresses experienced previously by the sample. Several rheological tests including stress ramp-up, oscillatory frequency sweep and structural recovery tests were performed, in order to analyse the rheological fingerprint of the mud samples collected from two different locations of the Port of Hamburg. The yield stress, storage moduli and structural recovery of mud from the same location was studied as a function of density for two series of samples. One series consisted of samples (“natural samples”) taken in-situ as a function of depth (with increasing density as a function of depth) whereas the other one (“diluted samples”) consisted of samples whereby the density was varied by adding in-situ water to the natural sample having the highest density. Significant differences in rheological characteristics were found between the natural and diluted mud samples, that were attributed to the state/composition of the mud's fabric in each situation but also to the structural rearrangements caused by the preparation of diluted samples.
17 citations
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TL;DR: In this paper, the concentration or consolidation of suspensions of fine particles under the influence of a gravitational field has been analyzed and a constitutive equation is suggested for irreversibly flocculated suspensions undergoing consolidation which embodies the concept of a concentration-dependent yield stress Py(ϕ).
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.
403 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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TL;DR: 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 as mentioned in this paper.
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.
357 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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TL;DR: In this paper, the consolidation behavior of flocculated alumina suspensions has been analyzed as a function of the interparticle energy, and it has been shown that strongly attractive interactions result in a particle network which resists consolidation and shows compressible behavior over a large stress range.
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.
194 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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TL;DR: In this paper, a generalized approach to understand and prediction of solid-liquid separation methods based on the measurement of fundamental material properties is reviewed and applied to a variety of thickening and filtration processes.
169 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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TL;DR: In this article, the shear and compressive properties of aggregated alumina particles are determined as functions of volume fraction and the strength of the interparticle attraction, and 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.
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.
167 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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