Mixing viscoelastic fluid flows at the micro scale

诸如提高油气采收率、涂层和封装等应用涉及多相流，其中流变行为和界面动力学对结果起着重要作用。许多这些工艺需要添加聚合物和表面活性剂，以减少摩擦，促进乳化或改善润湿等。这些添加剂，即使添加极少量，也会对流体特性产生重大影响，特别是在粘弹性和表面张力方面，从而影响流动行为。对于牛顿流体，低雷诺数（Re）下的流动主要由扩散控制，并倾向于保持层流。因此，混合两种不混溶的流体，例如形成乳化液或取代其中一相（例如提高采收率），在非常粘滞的体系或小规模流动中，本质上是难以有效实现的。然而，当少量高分子量聚合物加入水中（或任何牛顿溶剂），使溶液具有粘弹性时，在任意小的Re值下流动的流体表现出不稳定性和“类似湍流”的特征，湍流更有效地混合物质。研究表明，在小的长度尺度上，表面效应得到加强，使弹性的作用更加突出，其程度远远超过传统尺度上的典型作用。本项目的目的是从根本上研究这种“弹性”湍流在单相和多相系统中的混合效果，特别是在不混相的两种液体系统中。我们计划利用典型流动和简单的微流体流动，它们提供了剪切和延伸的组合，以了解在良好控制的条件下，在更复杂的流动中产生的一系列现象，并解决许多工程应用：通过多孔介质的流动，石油/采收应用中神经节的去除，在不利条件下形成乳剂。

Applications such as enhanced oil/gas recovery, coatings and encapsulation involve multiphase flows where rheological behaviour and interfacial dynamics plays a major part on the outcomes. Many of these processes require the addition of polymers and surfactants, to reduce friction, promote emulsification or improve wetting, among others. These additives, even when added in extremely small quantities, have a significant impact on the fluid properties, particularly in terms of its viscoelasticity and surface tension, with consequent impact on the flow behaviour.For Newtonian fluids, flows at low Reynolds numbers (Re) are dominated by diffusion and tend to remain laminar. As a consequence mixing of two immiscible fluids, e.g. to create an emulsion or to displace one of the phases such as in enhanced oil recovery, is inherently difficult to achieve efficiently for very viscous systems or flows at small scales. However, when a small amount of high molecular-weight polymer is added to water (or any Newtonian solvent), making the solution viscoelastic, then fluid flows at arbitrarily small values of Re have been shown to exhibit instabilities and "turbulent-like" characteristics and turbulent flows mix things much more efficiently. It has been shown that small length-scales, in which surface effects are enhanced, accentuate the role of elasticity to levels far beyond those typical at conventional scales.The purpose of this project is to fundamentally investigate the mixing effectiveness of this "elastic" turbulence for single and multiphase systems and, in particular, for immiscible two liquid systems.We plan to make use of canonical flows and simple microfluidic flows, which provide a combination of shear and extension, to understand under well-controlled conditions a range of phenomena arising in much more complicated flows and as such address a number of engineering applications: flow through porous media, removal of ganglia in oil/recovery applications, creation of emulsions under adverse conditions.