19.02.2018

Research and collaboration

Contact area measurements of cellulose fibre bonds using X-ray nano-CT imaging

Cellulose fibres are the main constituents of paper which consists of a network of interconnected fibres that bond to each other. The bond strength and bond area have an important effect both on the mechanical and the optical properties of paper. Bond area, in contrast to bond strength, is a difficult research topic, since fibre bonding happens in molecular distances that cannot be resolved with conventional optical methods. The advancements in X-ray optics have enabled imaging devices to reach resolutions in the nanometre scale, allowing access to more accurate fibre bond research.

X-ray nanotomography has been used successfully to image 26 bleached kraft softwood fibre bond samplesThree different bond types were studied: spring-to-summerwood, summer-to-summerwood and spring-to-springwood fibre bonds. The obtained results showed that there was no significant difference between the relative contact area (ratio of contact area to total fibre intersection area) of the different bond types. The average was found to be 58 %. As such, it seems that the well-established strength differences between bond types are not due to differences between relative or absolute contact areas, as can be resolved with the imaging system.

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Shear rheology of two-dimensional wet foam

Shear rheology of two-dimensional wet foam was investigated with the DySMaL simulation model which features dynamically deformable bubbles in a wet foam context. It was found that the rheological behavior of the model at low shear rates is subject to a scaling law involving the bubble size, surface tension and the viscosity of the carrier fluid. This is consistent with experimental data and soft glassy rheological models for such systems. At high shear rates the model predicts a dynamic phase transition to a turbulent flow pattern in which the effective viscosity of the foam changes rapidly. This phase transition can be linked to changes in the average bubble deformation and nematic order of the system.

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Selected Publications:

Friction of shear-fracture zones

Friction behavior between shearing brittle surfaces was studied using discrete element method simulations and an experimental device manufactured for this specific purpose. It was demonstrated that there is self-lubrication by fragmentation within shear-fracture zones if there is a mechanism for stress relaxation at the microscopic scale. In the numerical model used here, this mechanism is either low-friction sliding or rotation. If neither of these mechanisms are present, no self lubrication is observed in the investigated model. In the generic case, there are many possible additional relaxation mechanisms such as partial melting, plastic deformations, creep, vibrations, etc.

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 Selected Publications:

Time-resolved X-ray microtomographic measurement of water transport in wood-fibre reinforced composite material

Many materials are prone to absorb moisture from the environment. Moisture absorption may lead to degradation of the properties of the material. We have developed a method for direct non-intrusive measurement of local moisture content inside a material sample. The method is based on X-ray microtomography, digital image correlation and image analysis, and it has been applied to study axial transport of water in a cylindrical polylactic acid/birch pulp composite material sample with one end exposed to water. The method seems to give plausible estimates of water content profiles inside the cylindrical sample. The results may be used, e.g., in developing and validating models of moisture transport in biocomposites.assembly.png

Selected Publications:

  • A. Miettinen, T. Harjupatana, M. Kataja, S. Fortino, K. Immonen, Time-resolved X-ray microtomographic measurement of water transport in wood fibre reinforced composite material, IOP Conference Series: Materials Science and Engineering 139 (2016), doi:10.1088/1757-899X/139/1/012037.

Discrete-element model for glacier dynamics 

Global warming is one of the greatest threats that mankind has to face in the beginning of this century. Related to this is the incontrovertible mass loss of glacier ice in polar regions. Understanding the behaviour of large ice masses is thus crucial for estimating, and perhaps hindering the propagation of mass loss. In order to avoid shortcomings of continuum models, traditionally used to study glacier dynamics, a new discrete element model (DEM) for viscoelastic materials with fracture is developed [1]. In DEM models, material is comprised of a large number of particles that are bound together with interaction potentials e.g. springs or beams. Fracture and fragmentation is introduced through a breaking threshold of an interaction. With proper setting of properties of the particles themselves and the interaction potentials between them, the material can be made to realise a wide variety of elastic, plastic and viscoelastic behaviour.

Selected Publications:

  • T. Riikilä, Discrete Element Model for Viscoelastic Materials with Brittle Fracture: Applications on Glacier Dynamics, Department of Physics, University of Jyväskylä, Research Report 2/2017 (Doctoral thesis)

Multiscale modelling of heterogeneous complex fluids

We develop a multiscale modeling scheme for numerical studies of complex fluids composed of immiscible phases, and characterized by distinct spatial and temporal scales associated with the observable macroscopic flow behaviour and the mesoscopic phenomena related to underlying heterogeneities. Practical examples of this type heterogenous complex fluids include e.g. liquid-particle suspensions, colloids, aerosols and bubbly flows. The multiscale approach is based on concurrent coupling of a macroscale continuum model with mesoscale quasicontinuum simulations used to find the macroscale stress tensor. In particular, the mesoscale simulations either replace completely the rheological macroscale stress tensor modeling or are used to determine locally the material parameters of the assumed rheological model. The approach also aims in reducing the simulation time and simplifying the mesoscale simulation set-up. These are obtained by affecting the model coupling by choosing suitable frame of references for the mesoscale simulations and by applying sparse sampling simulation grids and interpolation of the material parameters whenever possible. The feasibility of the approach has been studied by solving flow of a wet foam in one-dimensional and in two-dimensional channel flows, by utilising DySMaL foam model in the mesoscale simulations. The method in general is potentially useful for solving flows of complex fluids for which the observable macroscopic properties may be strongly affected by their heterogeneous mesoscopic scale structure.

Swelling of bentonite clay

Compacted bentonite clay is planned to be used as a buffer material between high level radioactive waste canisters and the bedrock in many deep geological nuclear waste repository concepts. High swelling capability and low hydraulic permeability of the bentonite are meant to protect the canisters against rock movements and groundwater seepage. However, material modeling is needed for long-term safety evaluations of the buffer and the repository concepts. The hydromechanical behavior of the bentonite is still not fully understood, and hence detailed experimental data is needed to develop and validate the models. To this end, non-destructive methods based on X-ray imaging and tomography have been developed and successfully used to measure the deformation and water transport in bentonite samples during wetting experiments.

Recently, the X-ray imaging method has been developed further and used for constant volume wetting experiments where small-scale bentonite samples were wetted from one end in a sample holder tube. Sequential X-ray imaging of the wetting samples and image analysis techniques were used to figure out the solid and water content and the deformation in the samples as a function of time. The swelling stress at both ends was also monitored using force sensors. A similar sample holder and method have also been used to study how the initial density differences in bentonite samples are decreased during the wetting process.

Fibre staining methods for contrast enhancement in X-ray nano-CT imaging

X-ray nanotomography has been utilized in the study of pulp fibre bonds due to its non-invasiveness and high spatial resolution in samples of magnitudes in the micrometer range. Cellulose fibres have very low X-ray absorption owing to their lack of heavy elements and low density. This poses a challenge for the imaging process, since X-ray transmittance of the fibre bond sample is very close to that of the background, resulting in long exposure times and consequently extending the total imaging time to days in order to distinguish details in the bond area. Several staining materials and methods were examined in order to increase the contrast between the sample and the background. Phosphotungstic acid (PTA) provided the highest contrast, increasing the absorption of the samples up to 40 % in highly concentrated solutions. As a result, imaging time can be considerably reduced without impairing the resolution in the process.

Lattice-Boltzmann flow simulation

Development of the lattice-Boltzmann method as well as its application to fluid flow phenomena in complex porous media including rocks, foam-deposited pulp sheets, and random fiber networks were instabilities.pngcontinued. The work on high-performance implementations of the lattice-Boltzmann method has also been actively pursued.

Selected publications:

  • Mattila, K., Hegele, L. A., & Philippi, P. C. Investigation of an entropic stabilizer for the lattice-Boltzmann method. Physical Review E, 91 (6), 063010 (2015). doi:10.1103/PhysRevE.91.063010