Flow of Elastic Liquids

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Summary of Grant Proposal Polymers are sometimes added to liquids to make the material more viscous – paint and ink are examples – but the polymer also makes the liquid elastic, and this additional fluid property can affect a flow. In inkjet printing, for example, the ink’s elasticity can cause unwanted satellite droplets. Sometimes the polymer is added solely for the improvement provided by elasticity. Two well-developed applications are: to agricultural spraying, for elasticity improves surface deposition of the coating; and to turbulent pipe flow, because elastic stresses reduce the driving pressure. Often the amount of added polymer is minute but the effect can be dramatic. In turbulent pipe flow, for instance, the addition of 10 parts per million of polymer can reduce the required pressure by 50%. While viscosity is an obvious fluid property - liquids are often referred to as thick or thin, for example - elasticity in a liquid is generally less evident. The most visible manifestation is stringiness, as seen with saliva and egg whites. This grant application focusses on four flows in which the effect of elasticity is significant. Two of these are new uses of added polymer - to water jet micromachining for finer resolution, and to concentrated oil sand tailings for better flow characteristics. Tailings are normally watery, but when some water is removed to create a liquidy paste, the material can be deposited on the surface around a mine and not into a pond, which is the normal but environmentally dangerous practice. With flow behaviour modified by elasticity, tailings deposited on the ground require less area. The other two flows are also experimental: flow through an array of rods and flow past a single cylinder. These two are more fundamental but are practically relevant because the former pertains to fibre-reinforced plastics and the latter to the swimming of micro-organisms using flagella. The overall objective of the four experiments is to understand the role of elasticity in flows, that is, to identify when the property is operative, to quantify its effect, and to determine how the stresses caused by elasticity alter a flow. The latter two experiments above will be conducted with special viscoelastic fluids – liquids which are elastic but have constant viscosities – that will enable elastic effects to be clearly separated from viscous effects. Initial results with the rod arrays, for instance, have indicated that the resistance to flow caused by elasticity can be several times that caused by viscosity, demonstrating the importance of elasticity in some flows.