Lubricating Channel and Tube Flows - Fluid Sheathing using Textured Walls

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It is difficult to imagine a day going by without us benefiting from liquids transported by tubes or pipes. In the morning we turn on the taps in the bathroom to wash. At breakfast we use milk kept fresh in a fridge that uses an intricate network of cooling tubes. To go to work we use cars or buses whose engines rely on fuel pumped from a tank. The fuel itself is often transported in its crude form over vast distances, in pipelines. We do not transport just one type of liquid. It is hardly surprising that resistance to flow is a major industrial cost, and that finding ways to improve the flow rate could lead to increased efficiencies and new applications in a wide range of industries. When water or other liquids flow down a channel or tube the liquid in the middle equidistant from each wall flows most easily. This is because of the frictional resistance that occurs at the boundary wall. This project uses insights from our research in fluid mechanics and materials science to reduce this resistance and therefore increase flow rate by sheathing the flowing liquid in a recirculating fluid of lower viscosity.In recent work we showed that when a solid sphere is encased in a gas bubble the motion of the sphere through a liquid is lubricated due to a 'recirculating' flow in the bubble, and this can lead to lower frictional resistance. In a subsequent model, we showed that this idea could apply to the flow of a core liquid through a tube or channel, with a thin sheathing boundary layer having a recirculating flow. This reduces frictional resistance at the boundary and eases the flow of the core liquid (as highlighted by a recent Journal of Fluid Mechanics "Focus on Fluids" article, vol. 736 (2013) pp. 1-4).In this project we suggest two ways of experimentally implementing flow in channels and tubes with recirculating boundary layer conditions as demonstrated theoretically in the models above. In both cases, we use boundary walls having a solid texture and the ability to self-repair and which should therefore be robust. In the first case, suitable for transport of water (and similar liquids), we use a thin vapour layer initially caused by a hydrophobic textured surface, but which can self-heal if the layer collapses locally. This self-healing can be achieved in two ways: i) by using a localised electrolysis technique to generate and refresh the layer of gas; ii) using instantaneous thin film boiling, also known as the 'Leidenfrost' effect. In the second case, suitable for transport of oils (and similar liquids), we use a thin (immiscible) liquid layer retained in an oleophilic surface texture; by designing the texture as a porous connected space the infused liquid can redistribute to self-heal. The work of this project is multidisciplinary across fluids, materials science and engineering. A range of materials innovations will be used from lithographically produced structured channels to meshes and wrapped membranes. Channels and tubes will have embedded electrodes and heaters, or be infused with liquids to enable them to self-repair their boundaries. The project will provide the understanding needed to allow future development of novel containment walls to reduce frictional resistance to the flow of liquids. This will provide benefits in a range of industrial and domestic contexts, and may benefit applications we cannot yet imagine, but ones that our industrial partners may have the vision to imagine.