Mixing viscoelastic fluid flows at the micro scale

提高石油/天然气采收率、涂层和封装等应用涉及多相流，其中流变性和界面动力学对结果有很大影响。其中许多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性等。即使加入极少量的添加剂，这些添加剂也会对流体的性质产生重大影响，特别是在粘弹性和表面张力方面，从而影响流体的流动行为。对于牛顿流体，低雷诺数(Re)的流动以扩散为主，倾向于保持层流。因此，对于非常粘稠的体系或小规模流动来说，混合两种不相容的流体，例如创建乳状液或驱替其中一相，如在提高采油效率方面，固有地难以有效实现。然而，当向水(或任何牛顿溶剂)中加入少量的高分子量聚合物，使溶液具有粘弹性时，流体在任意较小的Re值下流动被证明表现出不稳定和类似湍流的特征，而湍流混合的效率要高得多。研究表明，在小尺度下，表面效应增强，弹性的作用大大增强，达到远超常规尺度的水平。本项目的目的是从根本上研究单相和多相系统，特别是不相容的两个液体系统中这种弹性湍流的混合效果。我们计划利用提供剪切和拉伸相结合的正则流和简单的微流体流，来理解在良好控制的条件下，在更复杂的流动中出现的一系列现象，并以此解决一些工程应用：流过多孔介质，去除石油/采油应用中的神经节，在不利条件下产生乳状液。

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.