Lubricating Channel and Tube Flows - Fluid Sheathing using Textured Walls

很难想象没有我们从管道或管道输送的液体中受益的一天。早上我们打开浴室的水龙头洗澡。早餐时，我们用冰箱里的新鲜牛奶，冰箱里有错综复杂的冷却管网络。我们乘坐汽车或公共汽车去上班，它们的引擎依靠从油箱里泵出的燃料。燃料本身通常以原油形式通过管道远距离运输。我们不只运输一种液体。毫无疑问，流体阻力是主要的工业成本，寻找提高流速的方法可以提高效率，并在广泛的行业中获得新的应用。当水或其他液体沿着通道或管道流动时，中间距离各壁等距离的液体最容易流动。这是因为在边界壁上产生了摩擦阻力。该项目利用我们在流体力学和材料科学方面的研究成果来减少这种阻力，从而通过将流动液体包裹在低粘度的再循环流体中来提高流速。在最近的工作中，我们表明，当一个固体球体被包裹在一个气泡中时，由于气泡中的“再循环”流动，球体通过液体的运动得到了润滑，这可以导致更低的摩擦阻力。在随后的模型中，我们证明了这一想法可以应用于核心液体通过管或通道的流动，具有再循环流动的薄鞘边界层。这减少了边界处的摩擦阻力，并缓解了核心液体的流动（正如最近流体力学杂志“聚焦流体”的文章所强调的那样，第736卷（2013）第1-4页）。在这个项目中，我们提出了两种实验方法来实现通道和管道中流动的再循环边界层条件，如上面的理论模型所示。在这两种情况下，我们使用具有坚固纹理和自我修复能力的边界墙，因此应该是坚固的。在第一种情况下，适合运输水（和类似的液体），我们使用一层薄薄的蒸汽层，最初由疏水性纹理表面引起，但如果层局部坍塌，它可以自我修复。这种自我修复可以通过两种方式实现：i)通过使用局部电解技术来产生和刷新气体层；ii)利用薄膜瞬时沸腾，也称为“莱顿弗罗斯特”效应。在第二种情况下，适合运输油（和类似的液体），我们使用薄的（不混溶的）液体层保留在亲油的表面纹理；通过将纹理设计成多孔的连接空间，注入的液体可以重新分配以自我修复。该项目的工作涉及流体科学、材料科学和工程等多学科。一系列的材料创新将被使用，从光刻生产的结构通道到网格和包裹膜。管道和管道将嵌入电极和加热器，或者注入液体，使它们能够自我修复边界。该项目将提供所需的理解，以允许未来开发新的安全壳壁，以减少液体流动的摩擦阻力。这将在一系列工业和家庭环境中带来好处，并可能使我们无法想象的应用受益，但我们的工业合作伙伴可能有远见。

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.