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

近年来，关于复杂流体在微流体系统中有趣的流动现象的实验证据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很容易实现。因此，即使是弛豫时间较短的低粘度聚合物溶液也可以达到高Weissenberg （We）数流动状态，在该状态下，弹性力优于粘性力，因此表现出很强的粘弹性效应。在这种情况下，流动引起的聚合物溶液相分离更为明显。在低雷诺数但高We数的情况下，聚合物溶液的流动也存在类似湍流的不稳定性。这种弹性湍流可以在微流控装置中用作强大而高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记忆和控制装置。因此，微流体技术的进步为理解复杂流体流动的基本物理提供了新的机会，而技术本身的创新和优化也可以从基础研究中产生的新知识中受益匪浅。对于特征流动尺寸小于200微米的复杂流体流动，目前的定量结果还很少。涉及微观流动中复杂流体流变学的工业部门，如喷墨印刷/直接写字和多孔介质中提高采收率，遇到了一个主要困难，因为传统流变学技术在宏观流动（特征尺寸大于500微米）中产生的实验数据与典型变形率为10^6 s-1或更高的微观流动环境中的行为几乎没有关系。在这种快速流动状态下，即使在配方中加入少量聚合物，也会对复杂的流体流动产生深远的扰动，例如，在连接喷射液滴和打印机喷嘴的长寿命韧带的形成中。韧带的长度和寿命在很大程度上取决于聚合物的分子量、功能和浓度。在聚合物的一定浓度以上，喷出的液滴的毛细力不能破坏韧带，而弹性韧带会将喷出的液滴缩回喷嘴。这可能与线圈拉伸过渡的时间尺度以及随后与喷墨滴喷射事件的时间尺度相比较的松弛有关。通过定量表征，更好地理解复杂流体的分子物理及其在微流体中的流动行为之间的关系，是该领域取得新技术突破的先决条件，特别是建立墨水配方和打印头的设计原则。复杂流体在微流体中的本质物理在于本构关系，本构关系在流动行为和微观结构演化之间架起了一座桥梁。我们提出在基准和流变微观流动下定量研究聚环氧乙烷（PEO）和自组装PEO基嵌段共聚物的水溶液。将开发一个最先进的流动表征平台，用于测量流动几何形状的速度、应力场和浓度波动。不同流态的实验数据将通过与计算结果的对比来验证本构模型及其参数。将仔细研究将流动几何形状从600微米的特征长度尺度缩小到3微米的特征长度尺度对复杂流体流动的影响。这种定量方法有望在微观流动中提取任何给定复杂流体的总本构信息。这种系统和综合的方法将首次应用于微流体技术。

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-assembling 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.