UBC Theses and Dissertations
The formation and properties of coherent flocs in fibre suspensions Soszyński, Robert Marian
Fibres in concentrated suspensions are in continuous contact with other fibres and may interlock through elastic bending to form coherent networks. Such interlocking is termed Type-C cohesion. The process by which Type-C cohesion forms among fibres and the resulting structure and tensile strength of individual floes of fibres have been examined in experimental study in which relatively straight, smooth nylon (6-6) fibres of aspect ratios from 65 to 189 were suspended in aqueous-sugar solutions. The fibres were in most cases neutrally buoyant. The suspensions were caused to flow in a partially filled, inclined-to-the-horizontal or horizontally-oriented cylinder rotated about its principal axis to produce a recirculating and moderately unsteady flow. At a well-defined and reproducible "threshold concentration" Type-C coherent floes formed. The floes were verified to be of Type-C by heat treatment. The heat treatment caused stress relaxation in elastically bent fibres resulting in reduced floe strength. Visual observations of floe formation and velocity measurements with Laser Doppler Anemometer indicated that the floes originated in the zone in which flow decelerated. In this zone floes formed by compaction of crowded fibres. The threshold concentration depended on fibre geometry and viscosity of the suspending liquid. Below an aspect ratio of approximately 50 and above a suspending liquid viscosity of approximately 0.013 Pa‧s, Type-C coherent floes did not form at any concentration of fibres. Under the test conditions of this work, the threshold concentration was unaffected by the cylinder rotational speed, cylinder diameter, and angle of incline to the horizontal, provided that sufficient shear was induced in the cylinder to create recirculating flow. The structure and strength of Type-C coherent floes were examined. The number of contact points per fibre was less than values estimated from theoretical, statistical models in the literature. The tensile strength of individual Type-C floes measured in a tester with a unique comb support showed values larger than strengths reported in the literature for either man-made or wood-pulp fibre networks. A mathematical model developed to describe tensile strength based on frictional fibre-to-fibre interaction accounted for only a part of the total floe strength.
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