Simulations of the structure and rheology of wet webs
Julkaistu sarjassa
Research report / Department of Physics, University of JyväskyläTekijät
Päivämäärä
2009Oppiaine
FysiikkaIn this Thesis we modified a recently introduced model for fiber suspensions to be
applicable to wet fiber networks so as to analyze by computer simulations their
structural and rheological properties. These properties were compared, under tensile
loading in particular, to those of wet and dry paper. The model used operated at the
fiber level, where the dynamics of fiber motion were determined by fiber stiffness and
fiber-fiber interactions such as friction and adhesive forces. Water surface tension,
inter-fiber contact area, and moisture content all contributed to the latter force.
The tensile strength of wet fiber networks could be described in terms of a very
simple function of adhesion-force magnitude, number of inter-fiber contacts, friction
coefficient, and network grammage. Relaxation of the tensile force as simulated for
model networks was found to compare well with experimental results for wet paper,
and the force was found to decrease proportional to logarithmic time. Relaxation rate
in the model networks and in wet paper was found to be higher than in dry paper for
which previous results are available.
In the simulations the permanent deformation that appeared after relaxation was,
however, clearly higher than what was measured in wet paper. We suggest that this
difference arises because, in the model networks, all the contact points between fibers
were frictional, while in real wet paper there may also appear some chemical bonding
between fibers.
Results of analytical models and computer simulations for the number of fiber-fiber
contacts in compressed and stretched fiber networks were compared. When the
majority of fibers lay parallel to the xy plane, the analytical and numerical predictions
for the number of contacts were in good agreement both for compressed and stretched
networks. However, in flocculated and stretched networks the strain was mainly
concentrated between flocs that remained largely intact during straining. The nonuniform
strain in this case means that mean-field approach does not work for wet
flocculated networks.
Simulation of tri-axial deformation of model networks showed that the lateral
network-contraction ratio (Poisson ratio) was nearly constant for very small strains as
expected for a linear regime. There after it increased with increasing applied strain
and sample length. For very large strains it leveled off to a constant value depending
on the sample length. Simulations also captured the experimentally observed behavior
for paper thickness during straining: The thickness of the network decreased or
increased during stretching depending on fiber stiffness.
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Julkaisija
University of JyväskyläISBN
978-951-39-3750-8ISSN Hae Julkaisufoorumista
0075-465XJulkaisuun sisältyy osajulkaisuja
- Artikkeli I: Miettinen, P. P. J., Ketoja, J. A., and Klingenberg, D. J. (2007) Simulated Strength of Wet Fibre Networks,. Journal of Pulp and Paper Science, 33(4), 199-205.
- Artikkeli II: Miettinen, P. P. J., Ketoja, J. A. and Hjelt, T. (2007). Simulated structure of wet fiber networks. Nordic Pulp & Paper Research Journal, 22(4), 516-522. DOI: https://doi.org/10.3183/npprj-2007-22-04-p516-522
- Artikkeli III: Miettinen, P. P. J. and Ketoja, J. A. (2008). Simulation of triaxial deformation of wet fiber networks. Nordic Pulp and Paper Research Journal, 23(3), 264-271. DOI: 10.3183/npprj-2008-23-03-p264-271
- Artikkeli IV: Miettinen, P. P. J., Kekko, P., Kouko, J. (2009) Relaxation of wet paper by simulations and laboratory-scale experiments. Nordi Pulp & Paper Research Journal, 24, 381-387. DOI: 10.3183/npprj-2009-24-04-p381-387
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