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Title: Sedimentation of circular and elliptical objects in a two-dimensional foam
Author: Davies, Ioan Tudur
ISNI:       0000 0004 2683 5698
Awarding Body: Aberystwyth University
Current Institution: Aberystwyth University
Date of Award: 2009
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The rheology of a two-dimensional dry foam is probed with quasi-static bubblescale simulations of the sedimentation of circular discs and elliptical objects. The sedimenting objects move in response to a combination of their weight and the forces exerted on them by the network of soap films and the pressures in the bubbles. Viewed macroscopically, the plasticity and elasticity of the foam combine to determine the rate of descent of a circular disc. A critical disc weight is found that determines whether the disc is supported by the foam or not. This critical weight increases linearly with disc diameter and decreases with the liquid fraction of the foam with a power-law relation. Similarly, the drag force exerted on a disc increases linearly with its diameter and decreases with the liquid fraction of the foam with a power-law relation. An attractive force between a disc and a nearby wall is seen when the disc is further than two bubble diameters from the wall. Such wall effects are minimal when the disc sediments from a central position in a channel of sufficient width. The interaction between two sedimenting discs is quantified by placing them in one of two configurations: one in which the discs are placed side by side and the other in which the discs are initially one above the other. The discs descend through the foam and move towards a stable orientation in which they are positioned directly above one another with a constant separation of one or two bubbles. Above a critical initial separation of the order of 5 bubble diameters, the discs do not interact. The existence of the critical separation is shown to be a result of the discrete nature of a dry foam. The descent and rotational motion of an ellipse of similar size and weight to one of the circular discs is then considered. An ellipse rotates towards a stable orientation in which its major axis becomes parallel to gravity, driven by the local structure of the foam. This rotational motion is much slower than the downward motion.
Supervisor: Cox, Simon ; Brown, Daniel Stephen Sponsor: EPSRC (EP/D071127)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available