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  • 1.
    Alimadadi, Majid
    et al.
    Department of Natural Sciences, Mid Sweden University, Sweden.
    Lindström, Stefan B
    Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering.
    Kulachenko, Artem
    Department of Solid Mechanics, Royal Institute of Technology (KTH), Stockholm, Sweden.
    Role of microstructures in the compression response of three-dimensional foam-formed wood fiber networks2018In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 14, p. 8945-8955Article in journal (Refereed)
    Abstract [en]

    High-porosity, three-dimensional wood fiber networks made by foam forming present experimentally accessible instances of hierarchically structured, athermal fiber networks. We investigate the large deformation compression behavior of these networks using fiber-resolved finite element analyses to elucidate the role of microstructures in the mechanical response to compression. Three-dimensional network structures are acquired using micro-computed tomography and subsequent skeletonization into a Euclidean graph representation. By using a fitting procedure to the geometrical graph data, weare able to identify nine independent statistical parameters needed for the regeneration of artificial networks with the observed statistics. The compression response of these artificially generated networks and the physical network is then investigated using implicit finite element analysis. A direct comparison of the simulation results from the reconstructed and artificial network reveals remarkable differences already in the elastic region. These can neither be fully explained by density scaling, the size effect nor the boundary conditions. The only factor which provides the consistent explanation of the observed difference is the density and fiber orientation nonuniformities; these contribute to strain-localization so that the network becomes more compliant than expected for statistically uniform microstructures. We also demonstrate that the experimentally manifested strain-stiffening of such networks is due to development of new inter-fiber contacts during compression.

  • 2.
    Andric, J.
    et al.
    Chalmers, Sweden.
    Lindström, Stefan
    Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering.
    Sasic, S.
    Chalmers, Sweden.
    Nilsson, H.
    Chalmers, Sweden.
    Ballistic deflection of fibres in decelerating flow2016In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 85, p. 57-66Article in journal (Refereed)
    Abstract [en]

    We investigate the motion of inertial, rod-like fibres in the decelerating flow of a wedge-shaped channel with non-creeping fibre-flow interactions. We consider the trajectories of isolated fibres to identify the conditions for which these trajectories deflect from the streamlines of the flow as well as a rectilinear path. We carry out analytical and numerical studies under the assumption of an infinite fibre hydrodynamic resistance to transverse flow, and we expand the numerical study by taking into account a finite transverse hydrodynamic resistance. The analytical analysis identifies a longitudinal ballistic number Be and a transverse ballistic number B-t as two dimensionless parameters that govern the fibre dynamics. It is found that Be is the product of the Stokes number Ste(l) in the longitudinal direction of the fibre and the channel opening angle beta. As anticipated, a fibre moves along the streamlines in the viscosity-dominated regime (B-l amp;lt;amp;lt; 1, B-t amp;lt;amp;lt; 1), while it moves in a straight line without being rotated in the inertia-dominated regime (Bt amp;gt;amp;gt; 1). The focus of the present study is on the intermediate regime (B-l amp;gt;amp;gt; 1, B-t amp;lt; 1), in which we identify and analyse a fibre trajectory that significantly deviates from the streamlines of the flow. This behaviour is observed for both infinite and finite resistances to transverse flow, and it is referred to as ballistic deflection. We argue that ballistic deflection may increase the rate of collisions between suspended fibres, and thus potentially affects the rate of fibre aggregation in flowing suspensions. An order of magnitude estimate of the ballistic numbers identifies dry-forming of pulp mats, which includes an air-wood fibre flowing suspension, to operate in the regime of ballistic deflection. (C) 2016 Elsevier Ltd. All rights reserved.

  • 3.
    Andric, Jelena
    et al.
    Chalmers University of Technology.
    Fredriksson, Sam T.
    Chalmers University of Technology.
    Lindström, Stefan B
    Linköping University, Department of Management and Engineering, Mechanics. Linköping University, The Institute of Technology.
    Sasic, Srdjan
    Chalmers University of Technology.
    Nilsson, Håkan
    Chalmers University of Technology.
    A study of a flexible fiber model and its behavior in DNS of turbulent channel flow2013In: Acta Mechanica, ISSN 0001-5970, E-ISSN 1619-6937, Vol. 224, no 10, p. 2359-2374Article in journal (Refereed)
    Abstract [en]

    The dynamics of individual flexible fibers in a turbulent flow field have been analyzed, varying their initial position, density and length. A particle-level fiber model has been integrated into a general-purpose, open source computational fluid dynamics code. The fibers are modeled as chains of cylindrical segments connected by ball and socket joints. The equations of motion of the fibers contain the inertia of the segments, the contributions from hydrodynamic forces and torques, and the connectivity forces at the joints. Direct numerical simulation of the incompressible Navier-Stokes equations is used to describe the fluid flow in a plane channel, and a one-way coupling is considered between the fibers and the fluid phase. We investigate the translational motion of fibers by considering the mean square displacement of their trajectories. We find that the fiber motion is primarily governed by velocity correlations of the flow fluctuations. In addition, we show that there is a clear tendency of the thread-like fibers to evolve into complex geometrical configurations in a turbulent flow field, in fashion similar to random conformations of polymer strands subjected to thermal fluctuations in a suspension. Finally, we show that fiber inertia has a significant impact on reorientation timescales of fibers suspended in a turbulent flow field.

  • 4.
    Andric, Jelena
    et al.
    Chalmers, Sweden.
    Lindström, Stefan B
    Linköping University, Department of Management and Engineering, Solid Mechanics.
    Sasic, Srdjan
    Chalmers, Sweden.
    Nilsson, Hakan
    Chalmers, Sweden.
    Numerical investigation of fiber flocculation in the air flow of an asymmetrical diffuser2014In: Proceedings of the 12th International Conference on Nanochannels, Microchannels and Minichannels (ICNMM), AMER SOC MECHANICAL ENGINEERS , 2014, no V001T12A013, p. V001T12A013-Conference paper (Refereed)
    Abstract [en]

    A particle-level rigid fiber model is used to study flocculation in an asymmetric planar diffuser with a turbulent Newtonian fluid flow, resembling one stage in dry-forming process of pulp mats. The fibers are modeled as chains of rigid cylindrical segments. The equations of motion incorporate hydrodynamic forces and torques from the interaction with the fluid, and the fiber inertia is taken into account. The flow is governed by the Reynolds-averaged Navier-Stokes equations with the standard k - omega turbulence model. A one-way coupling between the fibers and the flow is considered. A stochastic model is employed for the flow fluctuations to capture the fiber dispersion. The fibers are assumed to interact through short-range attractive forces, causing them to interlock as the fiber-fiber contacts occur during the flow. It is found that the formation of fiber flocs is driven by both the turbulence-induced dispersion and the gradient of the averaged flow field.

  • 5.
    Andric, Jelena
    et al.
    Chalmers University of Technology.
    Lindström, Stefan B
    Linköping University, Department of Management and Engineering, Mechanics. Linköping University, The Institute of Technology.
    Sasic, Srdjan
    Chalmers University of Technology.
    Nilsson, Håkan
    Chalmers University of Technology.
    A particle-level fiber model, implemented in a general-purpose CFD code2013In: Svenska mekanikdagar, 2013, p. 113-Conference paper (Other academic)
  • 6.
    Andric, Jelena
    et al.
    Chalmers University of Technology.
    Lindström, Stefan B
    Linköping University, Department of Management and Engineering, Mechanics. Linköping University, The Institute of Technology.
    Sasic, Srdjan
    Chalmers University of Technology.
    Nilsson, Håkan
    Chalmers University of Technology.
    A particle-level fiber model, implemented in OpenFOAM(R)2013In: 8th Int. OpenFOAM(R) Workshop, 2013Conference paper (Other academic)
  • 7.
    Andric, Jelena
    et al.
    Chalmers University of Technology.
    Lindström, Stefan B
    Linköping University, Department of Management and Engineering, Mechanics. Linköping University, The Institute of Technology.
    Sasic, Srdjan
    Chalmers University of Technology.
    Nilsson, Håkan
    Chalmers University of Technology.
    A particle-level rigid fiber model for high-Reynolds number flow, implemented in a general-purpose CFD code2013In: 8th International Conference on Multiphase Flow ICMF 2013, Korea, 2013Conference paper (Refereed)
    Abstract [en]

    A particle-level rigid fiber model has been integrated into a general-purpose, open source computational fluid dynamics code to carry out detailed studies of fiber--flow interactions in realistic flow fields. The fibers are modeled as chains of cylindrical segments, and their translational and rotational degrees of freedom are considered. The equations of motion contain the contributions from hydrodynamic forces and torques, and the segment inertia is taken into account. The model is validated for the rotational motion of isolated fibers in simple shear flow, and the computed period of rotation is in good agreement with the one computed using Jeffery's equation for a prolate spheroid with an equivalent aspect ratio. The model is applied by suspending a number of fibers in the swirling flow of a conical diffuser, resembling one stage in the dry-forming of pulp mats. The Reynolds-averaged Navier--Stokes equations with an eddy-viscosity turbulence model are employed to describe the fluid motion, and a one-way coupling between the fibers and the fluid phase is included. The dependence of the fiber motion on initial position and density is analyzed.

  • 8.
    Andric, Jelena
    et al.
    Chalmers University of Technology.
    Lindström, Stefan B
    Linköping University, Department of Management and Engineering, Mechanics. Linköping University, The Institute of Technology.
    Sasic, Srdjan
    Chalmers University of Technology.
    Nilsson, Håkan
    Chalmers University of Technology.
    Description and validation of a flexible fiber model, implemented in a general purpose CFD code2013In: 8th Int. Conf. Multiphase Flow ICMF 2013, 2013Conference paper (Refereed)
    Abstract [en]

    A flexible fiber model has been implemented in a general purpose open-source Computational Fluid Dynamics code. The fibers are modeled as chains of cylindrical segments, and all the degrees of freedom necessary to realistically reproduce the dynamics of real fibers, are taken into account. Each segment is tracked individually and their equations of motion account for the hydrodynamic forces and torques from the interaction with the fluid, the elastic bending and twisting torques, and the connectivity forces and moments that ensure the fiber integrity. The segment inertia is taken into account and a one-way coupling with the fluid phase is considered. The model is applied to simulate the rotational motion of an isolated fiber in a low segment Reynolds number shear flow. In the case of a stiff fiber, the computed period of rotation is in good agreement with the one computed using Jeffery's equation for an equivalent spheroid aspect ratio. A qualitative comparison is made with experimental data for flexible fibers. Further, a generic test case is described and used to validate the energy conservation and the response time of the fiber model concept. These results show that the implemented model can reproduce the known dynamical behavior of rigid and flexible fibers successfully.

  • 9.
    Andric, Jelena
    et al.
    Chalmers University of Technology, Göteborg, Sweden.
    Lindström, Stefan B
    Linköping University, Department of Management and Engineering, Mechanics. Linköping University, The Institute of Technology.
    Sasic, Srdjan
    Chalmers University of Technology, Göteborg, Sweden.
    Nilsson, Håkan
    Chalmers University of Technology, Göteborg, Sweden.
    Rheological properties of dilute suspensions of rigid and flexible fibers2014In: Journal of Non-Newtonian Fluid Mechanics, ISSN 0377-0257, E-ISSN 1873-2631, Vol. 212, p. 36-46Article in journal (Refereed)
    Abstract [en]

    Particle-level simulations are used to study the rheology of monodispersed suspensions of rigid and flexible fibers in a creeping, simple shear flow of a Newtonian fluid. We also investigate the influence of different equilibrium shapes (straight and curved) of the fibers on the behavior of the suspension. A parametric study of the impacts of fiber flexural rigidity and morphology on rheology quantifies the effects of these realistic fiber features on the experimentally accessible rheological properties. A fiber is modeled as a chain of rigid cylindrical segments, interacting through a two-way coupling with the fluid described by the incompressible three-dimensional Navier–Stokes equations. The initial fiber configuration is in the flow–gradient plane. We show that, when the shear rate is increased, straight flexible fibers undergo a buckling transition, leading to the development of finite first and second normal stress differences and a reduction of the viscosity. These effects, triggered by shape fluctuations, are dissimilar to the effects induced by the curvature of stiff, curved fibers, for which the viscosity increases with the curvature of the fiber. An analysis of the orbital drift of fibers initially oriented at an angle to the flow–gradient plane provides an estimate for the time-scale within which the prediction of the rheological behavior is valid. The information obtained in this work can be used in the experimental characterization of fiber morphology and mechanics through rheology.

  • 10.
    Andric, Jelena S.
    et al.
    Chalmers University of Technology, Sweden.
    Lindström, Stefan B
    Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering.
    Sasic, Srdjan M.
    Chalmers University of Technology, Sweden.
    Nilsson, Håkan
    Chalmers University of Technology, Sweden.
    PARTICLE-LEVEL SIMULATIONS OF FLOCCULATION IN A FIBER SUSPENSION FLOWING THROUGH A DIFFUSER2017In: Thermal Science, ISSN 0354-9836, E-ISSN 2334-7163, Vol. 21, p. S573-S583Article in journal (Refereed)
    Abstract [en]

    We investigate flocculation in dilute suspensions of rigid, straight fibers in a decelerating flow field of a diffuser. We carry out numerical studies using a particle-level simulation technique that takes into account the fiber inertia and the non-creeping fiber-flow interactions. The fluid flow is governed by the Reynolds averaged Navier-Stokes equations with the standard k-omega eddy-viscosity turbulence model. A one-way coupling between the fibers and the flow is considered with a stochastic model for the fiber dispersion due to turbulence. The fibers interact through short-range attractive forces that cause them to aggregate into flocs when fiber-fiber collisions occur. We show that ballistic deflection of fibers greatly increases the flocculation in the diffilser. The inlet fiber kinematics and the fiber inertia are the main parameters that affect fiber flocculation in the predffuser region.

  • 11.
    Bakker, Henriëtte E
    et al.
    Wageningen University.
    Lindström, Stefan
    Linköping University, Department of Management and Engineering, Mechanics. Linköping University, The Institute of Technology.
    Sprakel, Joris
    Wageningen University.
    Geometry- and rate-dependent adhesive failure of micropatterned surfaces2012In: Journal of Physics: Condensed Matter, ISSN 0953-8984, Vol. 24, no 6, p. 065103-Article in journal (Refereed)
    Abstract [en]

    The dynamic nature of adhesive interface failure remains poorly understood, especially when the contact between the two surfaces is localized in microscopic points of adhesion. Here, we explore the dynamic failure of adhesive interfaces composed of a large number of micron-sized pillars against glass. Surprisingly, we find a large influence of the microcontact geometry; ordered arrays of these pillars exhibit significantly stronger adhesive properties than equivalent surfaces in which the pillars are disordered. This can be understood with a simple geometric argument that accounts for the number of adhesive bonds that needs to be broken simultaneously to propagate the crack front. Moreover, the adhesive strength in both cases depends largely on the velocity with which the surfaces are separated. This rate dependence is explained on the basis of a semi-phenomenological model that describes macroscopic failure as a consequence of microscopic bond-rupture events. Our results suggest that the dynamics of adhesive failure, in the limit explored here, is predominantly stress-driven and highly sensitive to local geometry effects.

  • 12.
    Carrick, Christopher
    et al.
    KTH Royal Institute of Technology, Stockholm, Sweden.
    Lindström, Stefan
    Linköping University, Department of Management and Engineering, Mechanics. Linköping University, The Institute of Technology.
    Larsson, Per Tomas
    KTH Royal Institute of Technology, Stockholm, Swede; Innventia AB, Stockholm Sweden .
    Wågberg, Lars
    KTH Royal Institute of Technology, Stockholm, Sweden .
    Lightweight, highly compressible, noncrystalline cellulose capsules2014In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 30, no 26, p. 7635-7644Article in journal (Refereed)
    Abstract [en]

    We demonstrate how to prepare extraordinarily deformable, gas-filled, spherical capsules from nonmodified cellulose. These capsules have a low nominal density, ranging from 7.6 to 14.2 kg/m(3), and can be deformed elastically to 70% deformation at 50% relative humidity. No compressive strain-at-break could be detected for these dry cellulose capsules, since they did not rupture even when compressed into a disk with pockets of highly compressed air. A quantitative constitutive model for the large deformation compression of these capsules is derived, including their high-frequency mechanical response and their low-frequency force relaxation, where the latter is governed by the gas barrier properties of the dry capsule. Mechanical testing corroborated these models with good accuracy. Force relaxation measurements at a constant compression rendered an estimate for the gas permeability of air through the capsule wall, calculated to 0.4 mL mu m/m(2) days kPa at 50% relative humidity. These properties taken together open up a large application area for the capsules, and they could most likely be used for applications in compressible, lightweight materials and also constitute excellent model materials for adsorption and adhesion studies.

  • 13.
    Denoyelle, Thibaud
    et al.
    Solid Mechanics, KTH.
    Kulachenko, Artem
    Solid Mechanics, KTH.
    Galland, Sylvain
    Wallenberg Wood Science Center, KTH.
    Lindström, Stefan B
    Linköping University, Department of Management and Engineering, Mechanics. Linköping University, The Institute of Technology.
    Elastic properties of cellulose nanopaper versus conventional paper2011In: / [ed] Ulrich Hirn, Graz, Austria: TU , 2011, p. 131-134Conference paper (Other academic)
  • 14.
    Fall, Andreas B.
    et al.
    Department of Fiber Technology, KTH.
    Lindström, Stefan B
    Department of Fiber Technology, KTH.
    Sundman, Ola
    Department of Forest Products Technology, Aalto University, Aalto, Finland.
    Ödberg, Lars
    Department of Fiber Technology, KTH.
    Wågberg, Lars
    Department of Fiber Technology, KTH.
    Colloidal stability of aqueous nanofibrillated cellulose dispersions2011In: Langmuir, ISSN 0743-7463, Vol. 27, no 18, p. 11332-11338Article in journal (Refereed)
    Abstract [en]

    Cellulose nanofibrils constitute an attractive raw material for carbon neutral, biodegradable, nanostructured materials. In a aqueous suspensions, these nanofibrils are stabilized by electrostatic repulsion arising from deprotonated carboxyl groups at the fibril surface. In the present work, a new model is developed for colloidal stability by considering the deprotonation and electrostatic screening. This model predicts the fibril-fibril interaction potential in a given pH and ionic strength environment. Experiments support the model predictions that aggregation is induced by decreasing pH, thus reducing the surface charge, or by increasing salt concentration. It is shown that the primary aggregation mechanism for salt addition is the surface charge reduction through specific interactions of counter-ions with the deprotonated carboxyl groups, while the screening effect of the salt is of secondary importance.

  • 15.
    Fall, Andreas
    et al.
    Department of Fiber Technology, KTH.
    Lindström, Stefan B
    Department of Fiber Technology, KTH.
    Sprakel, Joris
    Löfroth, J. -E
    Wågberg, Lars
    Department of Fiber Technology, KTH.
    Shear-stiffening cellulose nanofiber gels with tuneable mechanical characteristics2011In: 241St National Meeting And Exposition Of The American Chemical Society (Acs), 2011, Vol. 241, p. 131--Conference paper (Refereed)
    Abstract [en]

    Gels have been synthesized from the renewable, strong and low cost cellulose nanofibres; nanofibrillated cellulose (NFC). The gels are shown to exhibit pronounced shear-stiffening properties and large extensibility (above 100%). The stiffening is due to strain induced orientation of the nanofibers, which is enabled by the free rotation at the particle-particle joints. The gels are synthesized from low concn. aq. NFC solns. By decreasing the electrostatic double-layer repulsion between the NFC fibrils, aggregation is initiated and a fluid-gel transition occurs. This transition can be detected within a range of vol. fractions. We characterize the gel microstructures using dynamic light scattering and the mech. properties using a rheometer. The mech. properties of these gels are tuneable; significantly different properties are seen if gels are formed by reducing pH or by increasing ionic strength. It is also obsd. that the properties of the gels depend on the type of counter-ion.

  • 16.
    Fall, Andreas
    et al.
    Department of Fiber Technology, KTH.
    Lindström, Stefan B
    Linköping University, Department of Management and Engineering, Mechanics. Linköping University, The Institute of Technology.
    Sprakel, Joris
    Wågberg, Lars
    Department of Fiber Technology, KTH.
    A physical cross-linking process of cellulose nanofibril gels with shear-controlled fibril orientation2013In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 9, p. 1852-1863Article in journal (Refereed)
    Abstract [en]

    Cellulose nanofibrils constitute the smallest fibrous components of wood, with a width of approximately 4 nm and a length in the micrometer range. They consist of aligned linear cellulose chains with crystallinity exceeding 60%, rendering stiff, high-aspect-ratio rods. These properties are advantageous in the reinforcement components of composites. Cross-linked networks of fibrils can be used as templates into which a polymer enters. In the semi-concentrated regime (i.e. slightly above the overlap concentration), carboxy methylated fibrils dispersed in water have been physically cross-linked to form a volume-spanning network (a gel) by reducing the pH or adding salt, which diminishes the electrostatic repulsion between fibrils. By applying shear during or after this gelation process, we can orient the fibrils in a preferred direction within the gel, for the purpose of fully utilizing the high stiffness and strength of the fibrils as reinforcement components. Using these gels as templates enables precise control of the spatial distribution and orientation of the dispersed phase of the composites, optimizing the potentially very large reinforcement capacity of the nanofibrils.

  • 17.
    Gibaud, Thomas
    et al.
    Université de Lyon, Lyon, France.
    Perge, Christophe
    Université de Lyon, Lyon, France.
    Lindström, Stefan B
    Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering.
    Taberlet, Nicolas
    Université de Lyon, UFR de Physique, Université Claude Bernard Lyon I, Lyon, France .
    Manneville, Sebastien
    Laboratoire de Physique, CNRS/UMR 5672, Ecole Normale Supérieure de Lyon, Université de Lyon, 46 allée d'Italie, 69007 Lyon, France.
    Multiple yielding processes in a colloidal gel under large amplitude oscillatory stress2016In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 12, no 6, p. 1701-1712Article in journal (Refereed)
    Abstract [en]

    Fatigue refers to the changes in material properties caused by repeatedly applied loads. It has been widely studied for, e.g., construction materials, but much less has been done on soft materials. Here, we characterize the fatigue dynamics of a colloidal gel. Fatigue is induced by large amplitude oscillatory stress (LAOStress), and the local displacements of the gel are measured through high-frequency ultrasonic imaging. We show that fatigue eventually leads to rupture and fluidization. We evidence four successive steps associated with these dynamics: (i) the gel first remains solid, (ii) it then slides against the walls, (iii) the bulk of the sample becomes heterogeneous and displays solid-fluid coexistence, and (iv) it is finally fully fluidized. It is possible to homogeneously scale the duration of each step with respect to the stress oscillation amplitude sigma_0. The data are compatible with both exponential and power-law scalings with sigma_0, which hints at two possible interpretations of delayed yielding in terms of activated processes or of the Basquin law. Surprisingly, we find that the model parameters behave nonmonotonically as we change the oscillation frequency and/or the gel concentration.

  • 18.
    Hajian, Alireza
    Department of Fiber Technology, KTH.
    Pettersson, Torbjörn
    Department of Fiber Technology, KTH.
    Hamedi, Mahiar
    Department of Fiber Technology, KTH.
    Wågberg, Lars
    Department of Fiber Technology, KTH.
    Understanding the dispersive action of nanocellulose for carbon nanomaterials2017In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 17, p. 1439-1447Article in journal (Refereed)
    Abstract [en]

    This work aims at understanding the excellent ability of nanocelluloses to disperse carbon nanomaterials (CNs) in aqueous media to form long-term stable colloidal dispersions without the need for chemical functionalization of the CNs or the use of surfactant. These dispersions are useful for composites with high CN content when seeking water-based, efficient and green pathways for their preparation. To establish a comprehensive understanding of such dispersion mechanism, colloidal characterization of the dispersions has been combined with surface adhesion measurements using colloidal probe atomic force microscopy (AFM) in aqueous media. AFM results based on model surfaces of graphene and nanocellulose further suggest that there is an association between the nanocellulose and the CN. This association is caused by fluctuations of the counterions on the surface of the nanocellulose inducing dipoles in the sp2 carbon lattice surface of the CNs. Furthermore, the charges on the nanocellulose will induce an electrostatic stabilization of the nanocellulose-CN complexes that prevents aggregation. Based on this understanding, nanocelluloses with high surface charge density was used to disperse and stabilize carbon nanotubes (CNTs) and reduced graphene oxide in water and further increase in the dispersion limit of CNTs could be obtained. The dispersion limit reached the value of 75 wt% CNTs and resulted in high electrical conductivity (515 S/cm) and high modulus (14 GPa) of the CNT composite nanopapers.

  • 19.
    Hajian, Alireza
    et al.
    KTH Royal Institute Technology, Sweden.
    Lindström, Stefan
    Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering.
    Pettersson, Torbjörn
    KTH Royal Institute Technology, Sweden.
    Hamedi, Mahiar M.
    KTH Royal Institute Technology, Sweden.
    Wagberg, Lars
    KTH Royal Institute Technology, Sweden.
    Understanding the Dispersive Action of Nanocellulose for Carbon Nanomaterials2017In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 17, no 3, p. 1439-1447Article in journal (Refereed)
    Abstract [en]

    This work aims at understanding the excellent ability of nanocelluloses to disperse carbon nanomaterials (CNs) in aqueous media to form long-term stable colloidal dispersions without the need for chemical functionalization of the CNs or the use of surfactant. These dispersions are useful for composites with high CN content when seeking water based, efficient, and green pathways for their preparation. To establish a comprehensive understanding of such dispersion mechanism, colloidal characterization of the dispersions has been combined with surface adhesion measurements using colloidal probe atomic force microscopy (AFM) in aqueous media. AFM results based on model surfaces of graphene and nanocellulose further suggest that there is an association between the nanocellulose and the CN. This association is caused by fluctuations of the counterions on the surface of the nanocellulose inducing dipoles in the sp(2) carbon lattice surface of the CNs. Furthermore, the charges on the nanocellulose will induce an electrostatic stabilization of the nanocellulose-CN complexes that prevents aggregation. On the basis of this understanding, nanocelluloses with high surface charge density were used to disperse and stabilize carbon nanotubes (CNTs) and reduced graphene oxide particles in water, so that further increases in the dispersion limit of CNTs could be obtained. The dispersion limit reached the value of 75 wt % CNTs and resulted in high electrical conductivity (515 S/cm) and high modulus (14 GPa) of the CNT composite nanopapers.

  • 20.
    Holmvall, M.
    et al.
    Mid Sweden University.
    Lindström @FSCN, Stefan B
    Mid Sweden University.
    Uesaka, T.
    Mid Sweden University.
    Simulation of two-phase flow with moving immersed boundaries2011In: International Journal for Numerical Methods in Fluids, ISSN 0271-2091, Vol. 67, no 12, p. 2062-2080Article in journal (Refereed)
    Abstract [en]

    A two-dimensional multi-phase model for immiscible binary fluid flow including moving immersed objects is presented. The fluid motion is described by the incompressible Navier-Stokes equation coupled with a phase-field model based on van der Waals’ free energy density and the Cahn-Hilliard equation. A new phase-field boundary condition was implemented with minimization of the free energy in a direct way, to specifically improve the physical behavior of the contact line dynamics for moving immersed objects. Numerical stability and execution time were significantly improved by the use of the new boundary condition. Convergence toward the analytical solution was demonstrated for equilibrium contact angle, the Lucas-Washburn theory and Stefan’s problem. The proposed model may be used for multi-phase flow problems with moving boundaries of complex geometry, such as the penetration of fluid into a deformable, porous medium.

  • 21.
    Holmvall, M.
    et al.
    SCA R&D Centre, Sundsvall, Sweden.
    Uesaka, T.
    Fibre Science and Communication Network, Mid Sweden University.
    Drolet, F.
    FPInnovations-Paprican, St-Jean, Pointe-Claire, Québec, Canada.
    Lindström, Stefan B
    Department of Fibre and Polymer Technology, Stockholm, Sweden.
    Transfer of a microfluid to a stochastic fiber network2011In: Journal of Fluids and Structures, ISSN 0889-9746, Vol. 27, no 11, p. 937-946Article in journal (Refereed)
    Abstract [en]

    The transfer of a microscopic fluid droplet from a flat surface to a deformable stochastic fiber network is investigated. Fibre networks are generated with different levels of surface roughness, and a two-dimensional, two-phase fluid-structure model is used to simulate the fluid transfer. In simulations, the Navier-Stokes equations and the Cahn-Hilliard phase-field equations are coupled to explicitly include contact line dynamics and free surface dynamics. The compressing fiber network is modeled as moving immersed boundaries. The simulations show that the amount of transferred fluid is approximately proportional to the contact area between the fluid and the fiber network. However, areas where the fluid bridges and never actually makes contact with the substrate must be subtracted.

  • 22.
    Holmvall, Martin
    et al.
    FSCN, Mid Sweden University.
    Drolet, Francois
    FPInnovations.
    Uesaka, Tetsu
    FSCN, Mid Sweden University.
    Lindström, Stefan B
    Linköping University, Department of Management and Engineering, Mechanics. Linköping University, The Institute of Technology.
    Microfluidics in printing nip-liquid transfer on random fiber network surface2011In: / [ed] Ulrich Hirn, Graz, Austria: TU , 2011, p. 67-68Conference paper (Other academic)
  • 23.
    Hämäläinen, J.
    et al.
    University of Technology, P.O, Lappeenranta, Finland .
    Lindström, Stefan B
    Department of Fiber Technology, KTH.
    Hämäläinen, T.
    University of Eastern Finland, P.O., Kuopio, Finland .
    Niskanen, H.
    University of Eastern Finland, P.O, Kuopio, Finland .
    Papermaking fiber suspension flow simulations at multiple scales2011In: Journal of Engineering Mathematics, ISSN 0022-0833, Vol. 71, no 1, p. 55-79Article in journal (Refereed)
    Abstract [en]

    Papermaking flows are extremely challenging for modeling and simulation, if one accepts their full complexity. A wide range of particles, including fibers, fiber fragments (fines) and fillers (non-organic particles), flow and interact with each other in a nondilute suspension, a complex geometry and at a high flow rate. Different simulation approaches are reviewed from particle-level simulations, through mesoscale simulations to the full flow geometry of the papermaking line. Their application to papermaking and potential to provide fundamental understanding as well as direct process-optimization support are discussed.

  • 24.
    Kulachenko, Artem
    et al.
    Solid Mechanics, KTH, Stockholm.
    Denoyelle, Thibaud
    Solid Mechanics, KTH, Stockholm.
    Galland, Sylvain
    Wallenberg Wood Science Centre, KTH, Stockholm.
    Lindström, Stefan B
    Linköping University, Department of Management and Engineering, Mechanics. Linköping University, The Institute of Technology.
    Elastic properties of cellulose nanopaper2012In: Cellulose (London), ISSN 0969-0239, E-ISSN 1572-882X, Vol. 19, no 3, p. 793-807Article in journal (Refereed)
    Abstract [en]

    Nanopaper is a transparent film made of network-forming nanocellulose fibers. These fibers are several micrometers long with a diameter of 4–50 nm. The reported elastic modulus of nanopaper often falls short of even conservative theoretical predictions based on the modulus of crystalline cellulose, although such predictions usually perform well for other fiber composite materials. We investigate this inconsistency and suggest explanations by identifying the critical factors affecting the stiffness of nanopaper. A similar inconsistency is found when predicting the stiffness of conventional paper, and it is usually explained by the effects introduced during drying. We found that the effect of the drying cannot solely explain the relatively low elastic modulus of nanopaper. Among the factors that showed the most influence are the presence of non-crystalline regions along the length of the nanofibers, initial strains and the three-dimensional structure of individual bonds

  • 25.
    Kulachenko, Artem
    et al.
    Solid Mechanics, KTH.
    Lindström, Stefan B
    Mid Sweden University.
    Uesaka, Tetsu
    Mid Sweden University.
    Strength of wet fiber networks: Size scaling2009In: Papermaking Research Symposium, 2009Conference paper (Refereed)
    Abstract [en]

    In this work we investigate the strength scaling of wet fiber networks with the help of particle level network simulations. The objective is to identify the factors that control wet strength distributions and the way it changes with the size. This question is relevant for wet strength runnability. The simulations show that if the mean values of network properties are kept constant by process conditions, the disordered structure of the network produces only small scatters in strength for a sufficiently big network. The strength distribution at small scales does not follow weakest-link scaling with neither change of length nor width, presumably, due to the fact that "damage" clusters are too big compared to the size of considered networks.

  • 26.
    Kulachenko, Artem
    et al.
    Solid Mechanics, KTH.
    Uesaka, T.
    Mid Sweden University.
    Lindström, Stefan B
    Mid Sweden University.
    Reinventing mechanics of fiber networks2008In: Progress in Paper Physics Seminar, 2008, p. 185-187Conference paper (Other academic)
  • 27.
    Lavrykov, Sergiy
    et al.
    State University of New York, Dept of Paper and Bioprocess Engineering.
    Lindström, Stefan B
    Linköping University, Department of Management and Engineering, Mechanics. Linköping University, The Institute of Technology.
    Singh, K. M.
    International Paper Co..
    Ramarao, Bandaru
    State University of New York, Dept of Paper and Bioprocess Engineering.
    3D network simulations of paper structurewith fines and fillers2012In: Nordic Pulp and Paper Research Journal, ISSN 0283-2631, Vol. 27, no 2, p. 256-263Article in journal (Refereed)
    Abstract [en]

    The structure of paper influences its properties and simulations of it are necessary to understand the impact of fiber and papermaking conditions on the sheet properties. We show a method to develop a representative structure of paper by merging different simulation techniques for the forming section and the pressing operation. The simulation follows the bending and drape of fibers over one another in the final structure and allows estimation of sheet properties without recourse to arbitrary bending rules or experimental measurements of density and/or RBA. Fibers are first modeled as jointed beams following the fluid mechanics in the forming section. The sheet structure obtained from this is representative of the wet sheet from the couch. The pressing simulation discretizes fibers into a number of solid elements around the lumen. Bonding between fibers is simulated using spring elements. The resulting fiber network was analyzed to determine its elastic modulus and deformation under small strains. The influence of fiber dimensions, namely fiber lengths, widths and thicknesses as well as bond stiffnesses on the elasticity of the network are studied. A brief account of inclusion of fines, represented by individual cubical elements is also shown.

  • 28.
    Lindström, Stefan B
    Mittuniversitetet, Fakulteten för naturvetenskap, teknik och medier, Institutionen för naturvetenskap, teknik och matematik.
    Modeling and simulation of paper structure development2008Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    A numerical tool has been developed for particle-level simulations of fibre suspension flows, particularly forming of the fibre network structure of paper sheets in the paper machine. The model considers inert fibres of various equilibrium shapes, and finite stiffness, interacting with each other through normal, frictional, and lubrication forces, and with the surrounding fluid medium through hydrodynamic forces. Fibre–fluid interactions in the non-creeping flow regime are taken into account, and the two-way coupling between the solids and the fluid phases is included by enforcing momentum conservation between phases. The incompressible three-dimensional Navier–Stokes equations are employed tomodel themotion of the fluid medium.

    The validity of the model has been tested by comparing simulation results with experimental data from the literature. It was demonstrated that the model predicts well the motion of isolated fibres in shear flow over a wide range of fibre flexibilities. It was also shown that the model predicts details of the orientation distribution of multiple, straight, rigid fibres in a sheared suspension. Furthermore, model predictions of the shear viscosity and first normal stress difference were in fair agreement with experimental data found in the literature. Since the model is based solely on first principles physics, quantitative predictions could be made without any parameter fitting.

    Based on these validations, a series of simulations have been performed to investigate the basic mechanisms responsible for the development of the stress tensor components for monodispersed, non-Brownian fibres suspended in a Newtonian fluid in shear flow. The effects of fibre aspect ratio, concentration, and inter-particle friction, as well as the tendency of fibre agglomeration, were examined in the nonconcentrated regimes. For the case of well dispersed suspensions, semi-empirical relationships were found between the aforementioned fibre suspension properties, and the steady state apparent shear viscosity, and the first/second normal stress differences.

    Finally, simulations have been conducted for the development of paper structures in the forming section of the paper machine. The conditions used for the simulations were retrieved from pilot-scale forming trial data in the literature, and from real pulp fibre analyses. Dewatering was simulated by moving two forming fabrics toward each other through a fibre suspension. Effects of the jet-to-wire speed difference on the fibre orientation anisotropy, the mass density distribution, and three-dimensionality of the fibre network, were investigated. Simulation results showed that the model captures well the essential features of the forming effects on these paper structure parameters, and also posed newquestions on the conventional wisdom of the forming mechanics.

  • 29.
    Lindström, Stefan B
    Mittuniversitetet, Fakulteten för naturvetenskap, teknik och medier, Institutionen för naturvetenskap.
    Simulation of the dynamics of fiber suspension flows2007Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    A new model for simulating non-Brownian flexible fibres suspended in a Newtonian fluid has been developed. Special attention has been given to include realistic flow conditions found in the industrial papermaking process in the key features of the model; it is the intention of the author to employ the model in simulations of the forming section of the paper machine in future studies.

    The model considers inert fibres of various shapes and finite stiffness, interacting with each other through normal, frictional and lubrication forces, and with the surrounding fluid medium through hydrodynamic forces. Fibre-fluid interactions in the non-creeping flow regime are taken into account, and the two-way coupling between the solids and the fluid phase is included by enforcing momentum conservation between phases. The incompressible three-dimensional Navier-Stokes equations are employed to model the motion of the fluid medium.

    The validity of the model has been tested by comparing simulation results with experimental data from the literature. It was demonstrated that the model predicts the motion of isolated fibres in shear flow over a wide range of fibre flexibilities. It was also shown that the model predicts details of the orientation distribution of multiple straight, rigid fibres in a sheared suspension. Model predictions of the viscosity and first normal stress difference were in good agreement with experimental data found in the literature. Since the model is based solely on first-principles physics, quantitative predictions could be made without any parameter fitting.

  • 30.
    Lindström, Stefan B
    et al.
    Department of Fiber Technology, KTH.
    Cervin, N. T.
    Department of Fiber Technology, KTH.
    Wågberg, L.
    Department of Fiber Technology, KTH.
    Thermally activated capillary intrusion of water into cellulose fiber-based materials2011In: The proceedings of the Fundamental and Applied Pulp & Paper Modelling Symposium 2011: August 24-28 2011, Concordia University, Montréal, Canada / [ed] Roger Gaudreault; Sylvain Robert; M A Whitehead, PAPTAC , 2011, p. 13-26Chapter in book (Refereed)
    Abstract [en]

    The imbibition of water into cellulose fiber-based materials is studied with focus on the regime dominated by contact line dynamics rather than hydrodynamic drag of the bulk porous structure. Capillary rise is studied under the influence of gravity for different paper grades with a wide range of porosities. It is found that the capillary rise is logarithmic in time in the limit of long time-scales. This behavior is in excellent agreement with the Molecular Kinetics Theory (MKT) for contact line dynamics. For high-porosity paper grades, this previously neglected logarithmic regime starts already at about $5$\,cm of capillary intrusion, underscoring the critical importance of the contact line dynamics to the performance of cellulosic absorbents.

  • 31.
    Lindström, Stefan B
    et al.
    Linköping University, Department of Management and Engineering, Mechanics. Linköping University, The Institute of Technology.
    Johansson, Lars
    Linköping University, Department of Management and Engineering, Mechanics. Linköping University, The Institute of Technology.
    Karlsson, Nils R.
    Linköping University, Department of Management and Engineering, Mechanics. Linköping University, The Institute of Technology.
    Metastable states and activated dynamics in thin-film adhesion to patterned surfaces2014In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 89, p. 062401-1-062401-11Article in journal (Refereed)
    Abstract [en]

    We consider adhesion due to London–van der Waals attraction between a thin film and a patterned surface with nanometer asperities. Depending on the surface topography and the stiffness of the film, three regimes of adhesion are identified: complete contact adhesion, partial contact adhesion, and glassy adhesion. For complete contact adhesion, the film conforms to the undulations of the surface, whereas for partial contact and glassy adhesion, the adhesive interface breaks down into microscopic areas of contact. When a film in the glassy regime is peeled off the surface, metastable states develop at which the crack front becomes arrested, analogously to the frustrated motion of the three-phase contact line across a heterogeneous surface. For this glassy regime, we use transition state theory to model the thermally activated progression of the crack front. This theoretical treatment suggests that the rate of the adhesive failure increases exponentially with the applied force.

  • 32.
    Lindström, Stefan B
    et al.
    Linköping University, Department of Management and Engineering, Mechanics. Linköping University, The Institute of Technology.
    Karabulut, Erdem
    Department of Fiber Technology, KTH.
    Kulachenko, Artem
    Solid Mechanics, KTH.
    Sehaqui, Houssine
    Department of Fiber Technology, KTH.
    Wågberg, Lars
    Department of Fiber Technology, KTH.
    Mechanosorptive creep in nanocellulose materials2012In: Cellulose (London), ISSN 0969-0239, E-ISSN 1572-882X, Vol. 19, no 3, p. 809-819Article in journal (Refereed)
    Abstract [en]

    The creep behavior of nanocellulose films and aerogels are studied in a dynamic moisture environment, which is crucial to their performance in packaging applications. For these materials, the creep rate under cyclic humidity conditions exceeds any constant humidity creep rate within the cycling range, a phenomenon known as mechanosorptive creep. By varying the sample thickness and relative humidity ramp rate, it is shown that mechanosorptive creep is not significantly affected by the through-thickness moisture gradient. It is also shown that cellulose nanofibril aerogels with high porosity display the same accelerated creep as films. Microstructures larger than the fibril diameter thus appear to be of secondary importance to mechanosorptive creep in nanocellulose materials, suggesting that the governing mechanism is found between molecular scales and the length-scales of the fibril diameter.

  • 33.
    Lindström, Stefan B
    et al.
    Linköping University, Department of Management and Engineering, Mechanics. Linköping University, The Institute of Technology.
    Karabulut, Erdem
    Department of Fiber Technology, KTH.
    Kulachenko, Artem
    Solid Mechanics, KTH.
    Wågberg, Lars
    Department of Fiber Technology, KTH.
    Discriminating between different mechanosorptive creep hypotheses2011In: / [ed] Ulrich Hirn, Graz, Austria: TU , 2011, p. 121-126Conference paper (Other academic)
  • 34.
    Lindström, Stefan B
    et al.
    Mid Sweden University.
    Kulachenko, Artem
    Espoo, Finland.
    Uesaka, Tetsu
    Quebec, Canada.
    New insights in paper forming using particle-level process simulation2009In: Papermaking Research Symposium, 2009, p. 1-4Conference paper (Other academic)
    Abstract [en]

    A numerical model for simulating forming at particle-level is presented, and the procedure for simulating modern forming equipment is outlined. Simulations were conducted using fiber furnishes composed from different shares of softwood kraft, softwood TMP, and hardwood kraft pulps, and the simulated sheets were analysed for wet strength and stiffness.

  • 35.
    Lindström, Stefan B
    et al.
    Linköping University, Department of Management and Engineering, Mechanics. Linköping University, The Institute of Technology.
    Lavrykov, Sergiy
    State University of New York, Dept of Paper and Bioprocess Engineering.
    Singh, K. M.
    International Paper Co..
    Ramarao, Bandaru
    State University of New York, Dept of Paper and Bioprocess Engineering.
    Forming of paper sheets: A numerical simulation based on hydrodynamics of fibrous suspensions2012Conference paper (Other academic)
    Abstract [en]

    A fiber-level simulation of paper forming in a twin-wire roll former is conducted with a furnish typical of fine paper. A mix of bleached kraft softwood and hardwood pulps was chosen, each refined to different levels.  The resultant web structure obtained from the couch roll was determined from these numerical experiments. This wet web structure is used as input to a simulation of wet pressing, which produces a consolidated sheet structure. Results regarding how the change in fiber mix (e.g. HW/SW ratio) and changes in pulp refining affect the structure of the web are shown. Important features such as sheet fiber orientations, anisotropy, retention of fibers, fines and fillers and sheet exit moisture are all predicted by the simulation.

  • 36.
    Lindström, Stefan B
    et al.
    Linköping University, Department of Management and Engineering, Mechanics. Linköping University, The Institute of Technology.
    Sprakel, Joris
    Thermally-activated delayed failure in heterogeneous solids: An experimental model system2013In: Svenska mekanikdagar, 2013, p. 158-Conference paper (Other academic)
  • 37.
    Lindström, Stefan B
    et al.
    Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering.
    Thore, Carl-Johan
    Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering.
    10000 mechanics problems at the press of a button2015In: Proceedings of Svenska Mekanikdagar, Linköping University, 2015, p. 84-Conference paper (Other academic)
    Abstract [en]

    Problem solving is at the heart of the mechanics curriculum, and developing problem solving skills is an important learning objective in basic and advanced mechanics courses at the undergraduate level. In alignment with this tradition, written examinations are mainly designed to test problem solving capabilities. Despite the fact that students spend most of their mechanics studies solving mechanics problems, an alarming fraction of them fail the written examination. One possible explanation is that a problem solving infrastructure, e.g. answers to problems and opportunities for collaboration with fellow students, is provided during the study period of courses, but missing during the examination.

  • 38.
    Lindström, Stefan B
    et al.
    Mid Sweden University, Sundsvall, Sweden.
    Uesaka, T.
    Mid Sweden University, Sundsvall, Sweden.
    Paper structure modeling with particle-level process simulation2006In: Progress in Paper Physics Seminar, 2006Conference paper (Other academic)
    Abstract [en]

    Print quality is highly dependent on the interactions of paper, ink and printing plate at the length scales of half-tone dot size and floc size. Knowledge of the details of the micro-structure is needed to predict these interactions, but it is difficult to obtain experimentally. One way to recreate the micro-structures of paper and to gain understanding of the mechanisms behind their formation is to systematically model each unit process of papermaking at particle level. The output of such simulations is numerical descriptions of the three-dimensional paper structure, from which any structural data can be accessed. Consequently, the simulations relate process parameters and particle properties directly to the micro-structures of paper. Many characteristics of the fiber network structure and the distribution of fillers and fines are determined in the headbox and during drainage in the forming section. A model for fiber suspensions, which couples the Navier-Stokes equations to a mechanical model of discretely represented flexible fibers, has been developed. Its performance allows for the study of large particle systems at very high Reynolds numbers (20000 particles at Re=40000), which is enough to model the conditions prevalent in the forming section of a modern paper machine.

  • 39.
    Lindström, Stefan B
    et al.
    Mid Sweden University.
    Uesaka, T.
    Mid Sweden University.
    Particle-level simulation of forming of the fiber network in papermaking2008In: International Journal of Engineering Science, ISSN 0020-7225, Vol. 46, no 9, p. 858-876Article in journal (Refereed)
    Abstract [en]

    A model for particle-level simulation of fiber suspensions has been used to simulate paper sheet forming on a roll-blade former. The fibers were modeled as chains of fiber segments, flowing and interacting with the medium and with each other. The incompressible three-dimensional Navier-Stokes equations were used to describe the fluid motion. Real pulps were analyzed to provide raw material data for the simulations. Dewatering was simulated by moving two model forming fabrics toward each other through a fiber suspension. Close examination of the dewatering process revealed that no large concentration gradients develop through the thickness of the pulp suspension. In this sense, twin-wire dewatering does not resemble a filtration process. The effects of the jet-to-wire speed difference on the network structure of the paper were investigated. The structural features of interest were fiber orientation anisotropy, mass density distribution and three-dimensionality of the fiber network. It was demonstrated that these simulated structural features were at least in qualitative agreement with experimental data found in the literature.

  • 40.
    Lindström, Stefan B
    et al.
    Mid Sweden University, Sundsvall, Sweden .
    Uesaka, T.
    Mid Sweden University,Sundsvall, Sweden .
    Simulation of the motion of flexible fibers in viscous fluid flow2007In: Phys. Fluids, Vol. 19, no 11, p. 113307-1-113307-16Article in journal (Refereed)
    Abstract [en]

    A model for flexible fibers in viscous fluid flow is proposed, and its predictions compared with experiments found in the literature. The incompressible three-dimensional Navier-Stokes equations are employed to describe the fluid motion, while fibers are modeled as chains of fiber segments, interacting with the fluid through viscous and dynamic drag forces. Fiber segments, from the same or from different fibers, interact with each other through normal, frictional, and lubrication forces. Momentum conservation is enforced on the system to capture the two-way coupling between phases. Quantitative predictions could be made, and showed good agreement with experimental data, for the period of Jeffery orbits in shear flow, as well as for the amount of bending of flexible fibers in shear flow. Simulations, using the proposed model, also successfully reproduced the different regimes of motion for threadlike particles, ranging from rigid fiber motion to complicated orbiting behavior, including coiling and self-entanglement.

  • 41.
    Lindström, Stefan B
    et al.
    Mid Sweden University, Sundsvall, Sweden.
    Uesaka, T.
    Mid Sweden University, Sundsvall, Sweden.
    Suspensions of flexible fibers in flows of decaying turbulence2006In: Euromech, 2006Conference paper (Refereed)
  • 42.
    Lindström, Stefan B
    et al.
    Mid Sweden University.
    Uesaka, Tetsu
    A model for flexible fibers in viscous and inert fluid2007In: International Paper Physics Conference, 2007, p. 23-28Conference paper (Refereed)
    Abstract [en]

    A model is proposed for simulating the motion of flexible fibers in fluid flow. Care has been taken to include typical papermaking conditions into the validity range of the model. Fibres are modeled as chains of fiber segments, whose motion is governed by Newton’s second law. The fluid motion is calculated from the three-dimensional incompressible Navier-Stokes equations. By enforcing momentum conservation, the two-way coupling between the solids and fluid phase is taken into account. Fiber–fiber interactions as well as self-interactions include normal, frictional and lubrication forces. Furthermore, the model considers nonlaminar fiber–fluid interactions and particle inertia. Simulation results were compared with experimental data found in the literature. The model predicts very well the orbit period of rigid fiber motion in shear flow. Quantitative predictions were made for the amount of bending of flexible fibers in shear flow. It was also possible to reproduce the different regimes of motion of flexible fibers in shear flow, ranging from rigid motion to coiled motion and self-entanglement.

  • 43.
    Lindström, Stefan B
    et al.
    Mid Sweden University.
    Uesaka, Tetsu
    Quebec, Canada.
    A numerical investigation of the rheology of fiber suspensions2009In: Physis of Fluids, ISSN 1070-6631, Vol. 21, no 8, p. 083301-1-083301-18Article in journal (Refereed)
    Abstract [en]

    A model for flexible fibers in viscous fluid flow is proposed, and its predictions compared with experiments found in the literature. The incompressible three-dimensional Navier-Stokes equations are employed to describe the fluid motion, while fibers are modeled as chains of fiber segments, interacting with the fluid through viscous and dynamic drag forces. Fiber segments, from the same or from different fibers, interact with each other through normal, frictional, and lubrication forces. Momentum conservation is enforced on the system to capture the two-way coupling between phases. Quantitative predictions could be made, and showed good agreement with experimental data, for the period of Jeffery orbits in shear flow, as well as for the amount of bending of flexible fibers in shear flow. Simulations, using the proposed model, also successfully reproduced the different regimes of motion for threadlike particles, ranging from rigid fiber motion to complicated orbiting behavior, including coiling and self-entanglement.

  • 44.
    Lindström, Stefan B
    et al.
    Mid Sweden University.
    Uesaka, Tetsu
    Mid Sweden University.
    Effects of interparticle friction on the rheology of fiber suspensions2008In: 5th Euro. Congress Comput. Meth. Sci. Eng., 2008Conference paper (Other academic)
    Abstract [en]

    Controlling the rheology of fibre suspensions is of critical importance in papermaking and composites processing; the spatial distribution and orientation distribution of fibres are retained in the final structure of the produced materials, significantly affecting their performance in end-use.

  • 45.
    Lindström, Stefan B
    et al.
    Mid Sweden University.
    Uesaka, Tetsu
    Mid Sweden University.
    Simulation of paper structure development in a roll-blade former2008In: Progress in Paper Physics Seminar, 2008, p. 139-141Conference paper (Other academic)
    Abstract [en]

    Print quality is largely dependent on the variations of paper properties on the length scales of halftone dot size and floc size. By simulating forming of the paper sheet at a particle level, it is possible to extract detailed information about the fibre network at microand mesoscales. Moreover, simulations yield equally detailed information about the development of the fibre network structure, and may thus give new insights in the mechanisms governing its formation.

  • 46.
    Lindström, Stefan B
    et al.
    Mid Sweden University.
    Uesaka, Tetsu
    Mid Sweden University.
    Simulation of semidilute suspensions of non-Brownian fibers in shear flow2008In: the Journal of chemical Physics, Vol. 128, no 2, p. 024901-1-024901-14Article in journal (Refereed)
    Abstract [en]

    Particle-level simulations are performed to study semidilute suspensions of monodispersed non-Brownian fibers in shear flow, with a Newtonian fluid medium. The incompressible three-dimensional Navier-Stokes equations are used to describe the motion of the medium, while fibers are modeled as chains of fiber segments, interacting with the fluid through viscous drag forces. The two-way coupling between the solids and the fluid phase is taken into account by enforcing momentum conservation. The model includes long-range and short-range hydrodynamic fiber-fiber interactions, as well as mechanical interactions. The simulations rendered the time-dependent fiber orientation distribution, whose time average was found to agree with experimental data in the literature. The viscosity and first normal stress difference was calculated from the orientation distribution using the slender body theory of Batchelor [J. Fluid Mech. 46, 813 (1971)] , with corrections for the finite fiber aspect ratios. The viscosity was also obtained from direct computation of the shear stresses of the suspension for comparison. These two types of predictions compared well in the semidilute regime. At higher concentrations, however, a discrepancy was seen, most likely due to mechanical interactions, which are only accounted for in the direct computation method. The simulated viscosity determined directly from shear stresses was in fair agreement with experimental data found in the literature. The first normal stress difference was found to be proportional to the square of the volume concentration of fibers in the semidilute regime. As concentrations approached the concentrated regime, the first normal stress difference became proportional to volume concentration. It was also found that the coefficient of friction has a strong influence on the tendency for flocculation as well as the apparent viscosity of the suspension in the semidilute regime.

  • 47.
    Lindström, Stefan B
    et al.
    Mid Sweden University.
    Uesaka, Tetsu
    Mid Sweden University.
    Stochastic modeling of paper structure: Effects of forming section2005In: 5th Biennial Johan Gullichsen Colloquium, 2005, p. 25-35Conference paper (Other academic)
    Abstract [en]

    A new model for particle-level simulation of forming was developed. A set of process parameters and a statistical description of the stock were provided as inputs to the model and a three-dimensional model paper was produced. This allowed the analyses of the relations between process parameters and structural properties of the formed sheet. Numerical experiments were set up to study the effects of the yarns of the forming fabric on the surface distributions of fillers and fines and to investigate the fiber orientation anisotropy and specific formation as functions of the jet-to-wire speed ratio.

  • 48.
    Lindström, Stefan B
    et al.
    Mid Sweden University.
    Uesaka, Tetsu
    Mid Sweden University.
    Hirn, U.
    Graz, Austria.
    Evolution of the paper structure along the length of a twin-wire former2009In: 14th Fund. Res. Symp. / [ed] I’Anson, S. J., The Pulp and Paper Fundamental Research Society , 2009, Vol. 1, p. 207-245Chapter in book (Refereed)
    Abstract [en]

    A particle-level numerical model is used to simulate forming with a twin-wire former configuration. The development of the paper structure along the length of the former is observed to explain the effects of the dewatering elements on the paper structure at different jet-to-wire speed ratios, consistencies, and target basis weights. The simulations indicate that most of the structure development takes place in the initial part of forming (forming roll) and, in some instances, at the drop to atmospheric pressure after the forming roll. Dramatic effects on the through-thickness fibre orientation anisotropy are observed when the consistency is varied by changing the jet thickness, while changes in basis weight had less impact. The through-thickness concentration gradient was almost uniform throughout the forming process, except in the lower range of typical papermaking consistencies. This indicates that the dewatering mechanism is normally thickening, rather than filtration.

  • 49.
    Lindström, Stefan B
    et al.
    Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering.
    Uhlin, Fredrik
    Linköping University, Department of Medical and Health Sciences, Division of Drug Research. Linköping University, Faculty of Medicine and Health Sciences. Region Östergötland, Heart and Medicine Center, Department of Nephrology. Department of Biomedical Engineering, Technomedicum, Tallinn University of Technology, Tallinn, Estonia.
    Bjarnegård, Niclas
    Linköping University, Department of Medical and Health Sciences, Division of Cardiovascular Medicine. Linköping University, Faculty of Medicine and Health Sciences. Region Östergötland, Heart and Medicine Center, Department of Clinical Physiology in Linköping.
    Gylling, Micael
    Linköping University, Department of Medical and Health Sciences. Linköping University, Faculty of Medicine and Health Sciences. Region Östergötland, Heart and Medicine Center, Department of Nephrology.
    Nilsson, Kamilla
    Linköping University, Department of Medical and Health Sciences. Linköping University, Faculty of Medicine and Health Sciences. Region Östergötland, Heart and Medicine Center, Department of Clinical Physiology in Linköping.
    Svensson, Christina
    Linköping University, Department of Medical and Health Sciences, Division of Cardiovascular Medicine. Linköping University, Faculty of Medicine and Health Sciences. Region Östergötland, Heart and Medicine Center, Department of Clinical Physiology in Linköping.
    Yngman-Uhlin, Pia
    Linköping University, Department of Medical and Health Sciences, Division of Nursing Science. Linköping University, Faculty of Medicine and Health Sciences. Region Östergötland, Local Health Care Services in West Östergötland, Research & Development Unit in Local Health Care.
    Länne, Toste
    Linköping University, Department of Medical and Health Sciences, Division of Cardiovascular Medicine. Linköping University, Faculty of Medicine and Health Sciences. Region Östergötland, Heart and Medicine Center, Department of Thoracic and Vascular Surgery.
    Computer-Aided Evaluation of Blood Vessel Geometry From Acoustic Images2018In: Journal of ultrasound in medicine, ISSN 0278-4297, E-ISSN 1550-9613, Vol. 37, no 4, p. 1025-1031Article in journal (Refereed)
    Abstract [en]

    A method for computer-aided assessment of blood vessel geometries based on shape-fitting algorithms from metric vision was evaluated. Acoustic images of cross sections of the radial artery and cephalic vein were acquired, and medical practitioners used a computer application to measure the wall thickness and nominal diameter of these blood vessels with a caliper method and the shape-fitting method. The methods performed equally well for wall thickness measurements. The shape-fitting method was preferable for measuring the diameter, since it reduced systematic errors by up to 63% in the case of the cephalic vein because of its eccentricity.

  • 50.
    Lindström, Stefan B
    et al.
    Royal Institute of Technology, Stockholm, Sweden.
    Vader, D. A.
    Cambridge, USA.
    Kulachenko, Artem
    Royal Institute of Technology,Stockholm, Sweden.
    Weitz, D. A.
    Cambridge, USA.
    Biopolymer network geometries: Characterization, regeneration, and elastic properties2010In: Physical Review E, Vol. 82, no 5, p. 051905-Article in journal (Refereed)
    Abstract [en]

    We study the geometry of biopolymer networks and effects of the geometry on bulk mechanical properties. It is shown numerically that the physical network geometry can be quantified statistically and regenerated from its statistical description, so that the regenerated network exhibits the same network mechanics as the physical network in the elastic regime. A collagen-I biopolymer network is used for validation. The method enables parametric studies of the network geometry, whose parameters are often difficult to vary independently in experiments.

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