Simulations of the structure and rheology of wet webs

Abstract

In 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.