04.02.2019

Research and collaboration

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.

The X-ray imaging method has been used to measure water transport and deformation in bentonite samples held in a constant volume while wetted from one end (32 days). The sample holder used in these experiments was also equipped with force sensors and strain gages in order to monitor the swelling stress during the experiments. Altogether 12 samples, varying initial dry density and water content (with two repetitions for each case), were measured. The results have been collected into a databank and used by material modelers. Recently, the method has also been used to study how well initial density differences in bentonite samples are reduced during the wetting process.

bentonite.png 

X-ray tomography and 3D-printing

X-ray tomography imaging combined with 3D printing allows making enlarged 3D copies of real 3D samples. Opposite of the miniature, the enlarged model provides possibility to study properties of natural materials, containing micrometer details, in much larger scales. This can have relevance in material design as e.g. mechanical or flow properties of the samples can be investigated and estimated in more easily controllable environment. To demonstrate the method, an X-ray microtomography digital image of capacitor was uploaded to 3D-printer, where it was upscaled and then printed. While the capacitor plates where printed separately the capacitor outer shell and the insulator material were printed simultaneously using different materials.

In addition to making objects with desired 3D structure, when supplemented with suitable chemically active printing material, a new functional objects can be made using 3D printer. To demonstrate the possibilities of this method e.g. a functional ion scavanger filter for electronic waste and flexible carbon electrode were produced using selective laser sintering 3D printer. Porosity of these objects were investigated using X-ray microtomography and image analysis. 

capacitor.jpgcapacitor.png

Selected Publications:

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.

Selected Publications:

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:

 

 

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.

microscope_image.JPGopening_thickness.png