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Rheology of Complex Fluids in Microscopic Flows: Quantitative Characterisation from Molecular Dynamics to Fluid Flows

微观流动中复杂流体的流变学：从分子动力学到流体流动的定量表征

基本信息

批准号：
EP/E033091/1
负责人：
Jeffrey Odell
金额：
$ 40.22万
依托单位：
University of Bristol
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2007
资助国家：
英国
起止时间：
2007 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=EP%2FE033091%2F1
关键词：
Rheology Complex Fluids Microscopic Flows

项目摘要

Recently experimental evidence on the intriguing flow phenomena of complex fluids in microfluidic systems has attracted enormous attention. The small scale of microfluidics makes flow of large deformation rate easily accessible. Hence even a low-viscosity polymer solution with a short relaxation time can reach the high Weissenberg (We) number flow regime, in which elastic forces dominate over viscous forces, and so exhibit strong viscoelastic effects. Flow-induced phase separation of polymer solutions can be much more pronounced in this regime. There are also examples of turbulence-like instabilities in the flows of polymer solutions at low Reynolds (Re) number but high We number regimes. Such elastic turbulence could be harnessed in microfluidic devices to act as powerful and efficient mixers and as dynamic valves, and even to construct functional memory and control devices which are insensitive to electromagnetic noise. Thus progress in microfluidics technology gives rise to new opportunities in understanding the fundamental physics of complex fluid flows, while innovation and optimisation of the technology itself can also greatly benefit from the new knowledge generated from a fundamental study.There are few quantitative results available concerning complex fluid flow in the characteristic flow dimension less than 200 micron. The industrial sectors involving rheology of complex fluids in microscopic flow, such as ink-jet printing/direct-writing and enhanced oil recovery in porous media, encounter a major difficulty as experimental data produced in macroscopic flows (characteristic dimension larger than 500 micron) by conventional rheometric techniques are of little relevance to behaviour in microscopic flow environments at typical deformation rates of 10^6 s-1 or higher. In such a fast flow regime, the addition of even small amounts of polymer to a formulation results in a profound perturbation of complex fluid flows, for example in the formation of long-lived ligaments connecting the ejected droplet with the nozzle of the printer. The length and lifetime of the ligaments is strongly dependent upon the molecular weight, functionality and concentration of the polymer. Above certain concentrations of polymer the capillary force of the ejected droplet is not able to break the ligament and the elastic ligament retracts the ejected droplet back into the nozzle. This can be related to the timescale of the coil-stretch transition and the subsequent relaxation compared to the timescale of the inkjet drop ejection event. A better understanding of the relationship between the molecular physics of complex fluids and their flow behaviour in microfluidics through quantitative characterisation is a prerequisite for a novel technology breakthrough of this area, especially for establishing design principles for ink formulation and printheads.The essential physics of complex fluids in microfluidics lies in the constitutive relationship which forms a bridge between flow behaviour and microstructure evolution in flow. We propose to study quantitatively aqueous solutions of poly(ethylene oxide) (PEO) and self-assembing PEO-based block copolymers under benchmark and rheometric microscopic flows. A state-of-the-art flow characterisation platform will be developed for measurement of velocity, stress fields and concentration fluctuations across the flow geometry. The experimental data for various flow configurations will be used to validate the constitutive model and its parameters by comparison with calculated results. The effects of shrinking the flow geometry, from a characteristic length scale of 600 micron to one of 3 micron, on complex fluid flows will be carefully investigated. Such a quantitative approach promises to extract total constitutive information for any given complex fluid in microscopic flow. This systematic and integrated approach will be the first of its kind to be applied to microfluidic technology.

近年来，复杂流体在微流控系统中的流动现象引起了人们的极大关注。微流体的小尺度使得大变形率的流动更容易实现。因此，即使是具有短弛豫时间的低粘度聚合物溶液也可以达到高Weissenberg（We）数流动状态，其中弹性力在粘性力上占主导地位，并且因此表现出强粘弹性效应。在这种情况下，聚合物溶液的流动诱导相分离可能更加明显。在低雷诺数（Re）但高We数的情况下，聚合物溶液的流动中也存在类似于湍流的不稳定性的例子。这种弹性湍流可以在微流体装置中被利用，作为强大而有效的混合器和动态阀，甚至可以构建对电磁噪声不敏感的功能性存储器和控制装置。因此，微流体技术的进步为理解复杂流体流动的基础物理提供了新的机会，而技术本身的创新和优化也可以大大受益于基础研究产生的新知识。关于特征流动尺寸小于200微米的复杂流体流动，几乎没有定量结果。涉及微观流动中复杂流体流变学的工业部门，如喷墨打印/直写和多孔介质中的提高采收率，遇到一个主要的困难，因为实验数据产生的宏观流动（特征尺寸大于500微米）与微观流动环境中典型变形速率为10^6 s-1时的行为关系不大。1或更高。在这样的快速流动状态下，即使向制剂中添加少量的聚合物也会导致复杂流体流动的深刻扰动，例如形成将喷射的液滴与打印机的喷嘴连接的长寿命韧带。韧带的长度和寿命强烈依赖于聚合物的分子量、官能度和浓度。在聚合物的某些浓度以上，喷射的液滴的毛细力不能破坏韧带，并且弹性韧带将喷射的液滴缩回到喷嘴中。这可能与线圈拉伸转变的时间尺度和与喷墨液滴喷射事件的时间尺度相比的随后松弛有关。通过定量表征更好地理解复杂流体的分子物理学与其在微流体中的流动行为之间的关系是该领域新技术突破的先决条件，微流体中复杂流体的基本物理特性在于本构关系，本构关系在流动行为和微结构演化之间形成桥梁在流动中。我们建议定量研究水溶液的聚（环氧乙烷）（PEO）和自溶胀PEO为基础的嵌段共聚物在基准和流变微观流动。将开发一个最先进的流动特性平台，用于测量整个流动几何形状的速度、应力场和浓度波动。通过与计算结果的比较，将使用各种流动配置的实验数据来验证本构模型及其参数。收缩的流动几何形状的影响，从600微米的特征长度尺度为3微米之一，对复杂的流体流动将仔细研究。这种定量的方法有望提取微观流动中任何给定的复杂流体的总的本构信息。这种系统和综合的方法将是第一个应用于微流体技术的方法。