Fiber suspensions

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The role of attractive interactions on shear thinning in non-Brownian fiber suspensions

The role of attractive interactions on shear thinning in non-Brownian fiber suspensions

This work is focused on shear thinning behavior of suspensions of rigid non-Brownian fibers dispersed in a Newtonian liquid. The work consists in developing a new theoretical model and conducting accurate experimental measurements. The shear thinning is expected to be caused by adhesive interactions between fibers. Experiments on polyamide (PA) fibers (present work) and carbon nanotube (CNT) suspensions [Khalkhal et al., J. Rheol. 55, 153-175 (2011)] have revealed the following features: (a) the flow curves exhibit a pronounced pseudo-plastic behavior interpreted in terms of the progressive aggregate destruction at the increasing shear rate; (b) the enhancement of the shear thinning with an increasing particle volume fraction is observed and explained by an increase of the strength of effective interactions between particles, as their concentration increases; (c) a weak yield stress of the PA fiber suspensions is detected in a controlled-stress mode and explained by the liquid-solid transition as the concentration of aggregates (constituted by fibers) approaches the close packing limit; (d) the shear thinning is much stronger in CNT suspensions because the adhesive interactions play a more important role between nano-sized CNT particles than between micron-sized PA fibers. A theoretical model considering the coexistence of transient aggregates with free non- aggregated fibers has been developed. The model allows viscosity calculations in terms of the aggregation parameter – the ratio of adhesive to hydrodynamic forces. It captures qualitatively the above-mentioned shear thinning behaviors and fits reasonably well to the experimental data on both PA fiber and CNT suspensions.
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Normal stress differences in non-Brownian fiber suspensions

Normal stress differences in non-Brownian fiber suspensions

viscosity reported here is actually the apparent viscosity that is the ratio of the shear stress at the rim to the shear rate the rim and not the true viscosity that should have been obtained by deconvoluting the data with various shear stress and rates at the rim. Nevertheless, it clearly appears that fiber suspensions are shear-thinning when the fiber concentration is high enough. The higher the fiber concentration and aspect ratio, the more pronounced is the shear-thinning behaviour (see, for instance, [? ]). Such a shear thinning behaviour has often been reported [? ? ? ? ? ] whereas its origin has not received any clear explanation. Most models either theoretical [? ? ] or numerical [? ? ? ] do not report such a shear-thinning behaviour because the effect of short-range hydrodynamic forces and of direct mechanical contacts that are all supposed to be proportional to the shear rate, leading to a linear scaling of the shear stress with the shear rate. Shear thinning can only occur if a characteristic time different from 1/ ˙γ is involved in the dynamics of the suspension. In particular, this is the case if the contacts are adhesive or if the friction is not Coulombic (i.e. a non-linear frictional law). The ratio of the characteristic adhesive force F to the characteristic hydrodynamic force can be evaluated [? ]:
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Non-linear viscoelastic response of magnetic fiber suspensions in oscillatory shear

Non-linear viscoelastic response of magnetic fiber suspensions in oscillatory shear

aggregates. Optical microscopy observations [Fig. 1b] revealed a relatively large quantity of these structures in fiber suspensions, compared to suspensions of spherical particles. This could explain an enhanced viscous response of the fiber suspensions. Of course, the variety of the intricate structures observed in fiber suspensions could generate a large spectrum of relaxation times, not accounted in our theory. However, our single-relaxation time model is the first necessary step to the understanding of the nonlinear viscoelastic response of magnetic fiber suspensions. Note that at large oscillation amplitudes, the motion of pivoting and bridging aggregates can be restricted by the neighboring aggregates, so that they could progressively stick to each other and form thick clusters with a reduced mobility. This could cause irreversible changes of the suspension structure provided that the Brownian motion is absent. Nevertheless, our experiments with increasing and decreasing stress ramps did not reveal a significant hysteresis of the shear moduli. This indicates that the structure can be efficiently reformed by the shear flow, at least in the short time scale, or irreversible transformations might have occurred at longer times. Note finally that the chains of different length are expected to oscillate out-of-phase relative to each other. So, the hydrodynamic screening effects, not accounted in our theory, should be more significant than in the case of a steady shear flow, for which the chains are considered to be more or less parallel to each other [12]. A detailed investigation into these points will be conducted in future.
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Steady shear flow of magnetic fiber suspensions: theory and comparison with experiments

Steady shear flow of magnetic fiber suspensions: theory and comparison with experiments

attributed to the failure of the fiber network at a critical strain value. Another theoretical model for the calculations of the static yield stress and the storage modulus of magnetic fiber suspensions was proposed by de Vicente et al. (2009). These authors considered affine displacement of fibers under a strain applied, and the yield stress was calculated via the magnetic dipolar forces between fibers, which must be overcome in order to separate the particles. These theories predicted successfully the static yield stress at high magnetic fields but were not able to predict the flow curve of the suspension above the yield stress. The effect of the shear rate on the rheology of magnetic fiber suspensions was always modeled by a pure Bingham regime without consideration of an intermediate regime at low shear rate, which comes from viscous dissipation around elongated aggregates before their first rupture.
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Microfluidic Fabrication Solutions for Tailor-Designed Fiber Suspensions

Microfluidic Fabrication Solutions for Tailor-Designed Fiber Suspensions

Received: 3 October 2016; Accepted: 15 November 2016; Published: 30 November 2016 Abstract: Fibers are widely used in different industrial processes, for example in paper manufacturing or lost circulation problems in the oil industry. Recently, interest towards the use of fibers at the microscale has grown, driven by research in bio-medical applications or drug delivery systems. Microfluidic systems are not only directly relevant for lab-on-chip applications, but have also proven to be good model systems to tackle fundamental questions about the flow of fiber suspensions. It has therefore become necessary to provide fiber-like particles with an excellent control of their properties. We present here two complementary in situ methods to fabricate controlled micro-fibers allowing for an embedded fabrication and flow-on-a-chip platform. The first one, based on a photo-lithography principle, can be used to make isolated fibers and dilute fiber suspensions at specific locations of interest inside a microchannel. The self-assembly property of super-paramagnetic colloids is the principle of the second fabrication method, which enables the fabrication of concentrated suspensions of more flexible fibers. We propose a flow gallery with several examples of fiber flow illustrating the two methods’ capabilities and a range of recent laminar flow results.
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Microfluidic Fabrication Solutions for Tailor-Designed Fiber Suspensions

Microfluidic Fabrication Solutions for Tailor-Designed Fiber Suspensions

Received: 3 October 2016; Accepted: 15 November 2016; Published: 30 November 2016 Abstract: Fibers are widely used in different industrial processes, for example in paper manufacturing or lost circulation problems in the oil industry. Recently, interest towards the use of fibers at the microscale has grown, driven by research in bio-medical applications or drug delivery systems. Microfluidic systems are not only directly relevant for lab-on-chip applications, but have also proven to be good model systems to tackle fundamental questions about the flow of fiber suspensions. It has therefore become necessary to provide fiber-like particles with an excellent control of their properties. We present here two complementary in situ methods to fabricate controlled micro-fibers allowing for an embedded fabrication and flow-on-a-chip platform. The first one, based on a photo-lithography principle, can be used to make isolated fibers and dilute fiber suspensions at specific locations of interest inside a microchannel. The self-assembly property of super-paramagnetic colloids is the principle of the second fabrication method, which enables the fabrication of concentrated suspensions of more flexible fibers. We propose a flow gallery with several examples of fiber flow illustrating the two methods’ capabilities and a range of recent laminar flow results.
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Learning the macroscopic flow model of short fiber suspensions from fine-scale simulated data

Learning the macroscopic flow model of short fiber suspensions from fine-scale simulated data

When these calibrated nominal models are unable to reproduce experimental findings, some ad hoc terms are usually added, being then calibrated accordingly (e.g., diffusion term reflecting fiber interactions). Thus, data was traditionally employed for calibrating models derived from physical considerations. However, many times those models failed to address experimental findings even when they were finely calibrated. It is the case of models describing intense fiber interactions in semi-concentrated and concentrated fiber suspensions. This issue motivated numerous works referred to later, and remains even today not totally solved.
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Magnetorheology of fiber suspensions. II. Theory

Magnetorheology of fiber suspensions. II. Theory

of friction between them, their use in MR suspensions could enhance significantly the MR effect [López-López et al. (2007)]. Furthermore, an interesting rheological behavior is expected for such magnetic fiber suspensions, combining the behaviors observed in nonmagnetic fiber suspensions and in conventional MR suspensions. Some experimental rheological data on the rheology of elongated particle suspensions that support this statement are reported in the papers by López-López et al. (2007), Kuzhir et al. (2007), Bell et al. (2008) and Ngatu et al. (2008). A detailed experimental investigation on the shear rheology of magnetic fiber suspensions, including the concentration dependence of the yield stress and observations of the suspension structures under applied magnetic field, is presented in the companion paper. A two- to three-time increase in the yield stress of magnetic fiber suspensions compared to suspensions of spherical magnetic particles was found. However, no theoretical model explaining this increase has been reported. In the present paper, we introduce the first microstructural models for magnetic fiber suspensions and explain the enhanced magnetorheological response of these suspensions in terms of interfiber solid friction. Our theory covers the quasi-static regime of the shear deformation (before the flow onset) and combines the features of the point-wise interaction theory developed by Toll and Manson (1994) for classical fiber suspensions and the features of the column structure and zigzag structure models for classical MR suspensions [Bossis et al. (1997); Volkova (1998)].
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Investigation of hydrolysis of lignocellulosic fiber suspensions with in-situ and ex-situ multi-scale physical metrologies

Investigation of hydrolysis of lignocellulosic fiber suspensions with in-situ and ex-situ multi-scale physical metrologies

Eprints ID : 13541 To cite this version : Nguyen, Tien-Cuong and Anne-Archard, Dominique and Coma, Véronique and Pichavant, Frédérique and Cameleyre, Xavier and Lombard, Eric and To, Kim Anh and Fillaudeau, Luc Investigation of hydrolysis of lignocellulosic fiber suspensions with in-situ and ex-situ multi-scale physical

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Magnetorheology of fiber suspensions. I. Experimental

Magnetorheology of fiber suspensions. I. Experimental

at low fiber concentration (0.1 vol.%), each fiber seems to have at least a few contact points with the neighboring ones. It can also be observed that individual fibers are gathered together in aggregates, even though, as mentioned in section II, the suspensions were carefully dispersed right before the observations. Such aggregation in the absence of magnetic field

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In-situ Rheometry of Concentrated Cellulose Fiber Suspensions and Relation with Enzymatic Hydrolysis

In-situ Rheometry of Concentrated Cellulose Fiber Suspensions and Relation with Enzymatic Hydrolysis

School of Biotechnology and Food Technology, Hanoi University of Science and Technology, Hanoï, VIET-NAM 1. INTRODUCTION Lignocellulose biomass is one of the most abundant renewable resources and certainly one of the least expensive. It was considered as a glucose source for obtain energetic or chemical molecules by bioconversion. This enzymatic conversion was so complicate therefor a better scientific understanding and, ultimately, good technical control of these critical biocatalytic reactions, which involve complex matrices at high solid contents, is currently a major challenge if biorefining operations are to become commonplace. Amongst the main parameters to be studied, the rheological behaviour of the hydrolysis suspension and the fibre particle size, stand out as a major determinants of process efficiency and determine equipment to be used and the strategies applied. Rheological behaviour of fibre suspensions is usually described by an apparent yield stress, a shear viscosity (Hershel-Buckley or Bingham models) and elasticity. During biological hydrolysis, the apparent viscosity of suspensions decreases in parallel with a decrease of particle size (Nguyen et al., 2013). This study focuses on the characterisation of cellulose suspensions (Microcrystalline cellulose, Whatman paper and extruded paper pulp) during enzymatic hydrolysis using in-situ and ex-situ physical analysis. The complex relationships between fibre structure, degradation, chemical composition and rheological behaviour was scrutinised.
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Recirculating Flows Involving Short Fiber Suspensions: Numerical Difficulties and Efficient Advanced Micro-Macro Solvers

Recirculating Flows Involving Short Fiber Suspensions: Numerical Difficulties and Efficient Advanced Micro-Macro Solvers

• The driven cavity flow problem. Finally, we consider the complex flow generated in a driven cavity that involves a short fiber suspension characterized by the same material parameters that in the pre- vious tests. The velocity is prescribed on the domain boundary according to: v(x = 0, y) = 0, v(x = 1, y) = 0, v(x, y = 0) = 0 and v(x, y = 1) = (16v max x 2 (1 − x) 2 , 0). The velocity field is then solved by assuming a Newtonian behavior and by applying a standard mixed finite element formulation where the velocity and pressure approx- imations verify the LBB stability condition. The Fokker-Planck equation governing the evolution of the fiber orientation distribution function is then solved along some closed streamlines, where the periodicity condition of that distribution function was imposed as described in Ammar and Chinesta [3]. From the computed orientation distribution function, the characteristic modes are extracted by using the technique previously described based on the application of the Karhunen-Lo`eve decomposition, allowing to fit the empirical snapshot natural closure strategy previously introduced. Now, the evolution equation associated with the second order orientation tensor is solved by assuming different closure relations: linear, quadratic, hybrid, natural and the empirical natural one.
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Rheology of magnetic fiber suspensions

Rheology of magnetic fiber suspensions

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignemen[r]

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One and two-fiber orientation kinetic theories of fiber suspensions

One and two-fiber orientation kinetic theories of fiber suspensions

The passage from the governing equations of Stokesian dynamics of N fibers to kinetic equations consists of, first, writing the Liouville equation that corresponds to the time evolution [r]

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One and Two-Fiber Orientation Kinetic Theories of Fiber Suspensions

One and Two-Fiber Orientation Kinetic Theories of Fiber Suspensions

The passage from the governing equations of Stokesian dynamics of N fibers to kinetic equations consists of, first, writing the Liouville equation that corresponds to the time evolution eq[r]

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Extra stress tensor in fiber suspensions: Mechanics and thermodynamics

Extra stress tensor in fiber suspensions: Mechanics and thermodynamics

Results for fiber suspensions and two choices of microstructural state variables. Given a microstructural equation there are, in general, infinitely many corre- sponding to it formulas for the extra stress tensor that look very differently but are equivalent in the sense that they all imply the same predicted stresses. By a predicted stress we mean the stress obtained by evaluating the expression for the stress tensor at solutions to the microstructural equation. Predicted stresses are the stresses measured in rheological observations. The reason why there are, in general, infinitely many equivalent formulas for the extra stress tensor is that there are, in general, infinitely many partial solutions to the microstructural equation. Let us see the process of getting solution to the microstructural equa- tion as a gradual process in which the space in which the solutions are searched is being gradually restricted. Partial solutions are, in our terminology, the mi- crostructural state variables restricted to such submanifolds. From the physical point of view, the gradual process of restrictions leading to partial solutions can be interpreted as the process of a descend to more macroscopic (i.e. less detailed) levels of description. The submanifolds are also called closures (see more in Section 2.4 and in Grmela (2010)).
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Transfert de métaux entre eau et suspensions dans les estuaires

Transfert de métaux entre eau et suspensions dans les estuaires

Deux types de matière organique dissoute ont été utilisés, la première provient d'eau prélevée dans le fleuve du Rhône (R124, mission Mai 1992), et la seconde provient d'eau prélevée e[r]

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Utilisation de la spectroscopie acoustique pour la caractérisation de suspensions denses

Utilisation de la spectroscopie acoustique pour la caractérisation de suspensions denses

L’objet de cet article est de présenter les avantages et les limitations de la spectroscopie acoustique pour l’analyse de systèmes dispersés denses de particules de taille micronique ou colloïdale. L’étude expérimentale, menée sur deux matériaux différents, est basée sur la mesure de l’atténuation acoustique d’une onde ultrasonore à différentes fréquences à travers la suspension. Dans le cas de suspensions denses de calcite de quelques dizaines de microns, l’exploitation des spectres d’atténuation acoustique permet d’accéder aisément à la distribution de taille des particules en suspension. L’utilisation d’une plage de fréquence réduite peut être envisagée pour réduire le temps d’analyse sans limiter pour autant la précision des résultats. Au contraire, les interactions particulaires existant au sein de suspensions denses de silice colloidale perturbent la réponse acoustique du milieu dispersé et ne permettent pas une exploitation directe des spectres d’atténuation. Des méthodes de correction sont proposées pour parvenir à une détermination correcte des distributions de taille. Enfin, en se fondant sur des mesures effectuées avec des mélanges de produits, l’utilisation de la spectroscopie acoustique pour caractériser les propriétés de suspensions de particules polydisperses est discutée.
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Fluctuations in inertial dense homogeneous suspensions

Fluctuations in inertial dense homogeneous suspensions

moderate to high volume fractions ( φ > 0.1). This constitutes a significant improvement compared to current statistical models, either arising from two-fluid modeling or kinetic theory of granular media, and sheds light on the nature of pseudoturbulence in concentrated suspensions and in particular on the role of concentration fluctuations. Future work should focus on validating the model over different ranges of Reynolds number and density ratio. Furthermore, the modeling of diffusion mechanisms in such systems should be revisited in light of the proposed paradigm.

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Long fiber thermoplastic PET composites : fiber content and consolidation pressure

Long fiber thermoplastic PET composites : fiber content and consolidation pressure

/ La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur. Access and use of [r]

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