Flow of Elastic Liquids

*该提案涉及含有溶解聚合物的液体，这通常会使液体既有弹性又有粘性。聚合物通常被添加到液体中，以增加材料的粘性--油漆和油墨就是一例--但聚合物也会使液体具有弹性，这种额外的流体特性可能会影响流动。例如，在喷墨打印中，油墨的弹性可能会导致不需要的卫星液滴。有时，添加聚合物完全是为了改善弹性。两个成熟的应用是：农业喷洒，因为弹性改善了涂层的表面沉积；以及湍流管道流动，因为弹性应力降低了驱动压力。聚合物的加入量通常很小，但效果可能是戏剧性的。例如，在湍动的管流中，添加百万分之十就可以将所需压力降低50%。粘度是液体的一个明显性质--液体被称为稠度或稀度--但弹性通常不那么明显。最明显的表现可能是粘稠，如唾液和蛋清。*本拨款申请集中于弹性效应显著的四个资金流。对于其中的一种，添加聚合物是为了特定的好处：即，用于水射流微加工以提高分辨率或精度。对于其他三种流动，聚合物是测试流体的固有成分，因此实验液体既有弹性又有粘性。其中一个实验是流过圆柱体，在微观条件下进行，这样结果就可以帮助解释微生物如何利用它们的鞭毛在粘液中游泳。另一项实验是流过一系列杆，该阵列是纤维多孔介质的模型，应用于纤维增强塑料。三是滞留点流。由于它是所有其他流动的组成部分，研究结果将有助于解释在这些流动中观察到的弹性效应。*该计划的主要目标是了解弹性在这些实验流动中的作用，即确定属性何时起作用，量化其影响，并确定由弹性引起的应力如何改变流动。后三个实验将使用特殊的粘弹性液体进行，这种液体具有弹性，但具有恒定的粘度，这将使弹性效应与粘性效应完全分开，从而可以评估弹性的作用。例如，杆阵列的初步结果表明，仅由弹性引起的流动阻力可能是由粘性引起的阻力的几倍，这表明了弹性的重要性。

***The proposal concerns liquids containing a dissolved polymer, which generally makes the liquid elastic as well as viscous. Polymers are often 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 because elasticity improves surface deposition of the coating; and to turbulent pipe flow because elastic stresses reduce the driving pressure. The amount of added polymer is often minute but the effect can be dramatic. In turbulent pipe flow, for instance, the addition of 10 parts per million can reduce the required pressure by 50%. Viscosity is an obvious property of a fluid - liquids are referred to as thick or thin - but elasticity is generally much less evident. The most visible manifestation is probably stringiness, as seen with saliva and egg whites. ******This grant application focusses on four flows in which the effect of elasticity is significant. For one of these, polymer is added for a specific benefit: namely, to water jet micromachining to improve resolution or precision. For the other three flows, polymer is an inherent constituent of the test fluids, and so the experimental liquids are elastic as well as viscous. One experiment is flow past a cylinder, carried out under microscopic conditions so that results can help explain how micro-organisms use their flagella to swim in mucus. Another experiment is flow through an array of rods, the array being a model of a fibrous porous medium, with application to fibre-reinforced plastics. The third is stagnation point flow. Because it is component of all the other flows, results from the study will help in explaining the elastic effects observed in those flows.******The primary objective of the program is to understand the role of elasticity in these experimental 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 three experiments will be conducted with special viscoelastic fluids - liquids which are elastic but have constant viscosities - that will enable elastic effects to be cleanly separated from viscous effects so that the role of elasticity can be assessed. Initial results with the rod arrays, for instance, have indicated that the resistance to flow caused by elasticity alone can be several times the resistance caused by viscosity, demonstrating the importance of elasticity.