基于稳定性理论的高性能分离膜结构优化和制备方法研究

Technology of membrane separations has been playing an important role in many fields. Enhancing the efficiency of membrane separations can be achieved by optimizing the substructures of the membrane, which entails deeper insights into the mechanisms accounting for the formation of various substructures and their roles during the filtration processes. The proposed project advocates the application of stability theory to establishing the relationships between the fabrication conditions and the evolution of the substructures during the polymer-network formation. Specifically, the linear stability analysis could reveal the criteria that can be used to determine different ways of the evolution of infinitesimal disturbances, i.e., the formation of various porous structures with periodic characteristics during the membrane fabrication. It is therefore desirable to develop novel methods to tailor the substructures in a more accurate way. On the other hand, the combination of the linear stability analysis and numerical simulations would offer a powerful tool for investigating the subtle interactions between the flowing fluid and the substructures on the membrane surface, thereby interpreting the appearance of interfacial secondary flows during a filtration process using the crossflow mode. This would be of great value in exploiting localized effects to mitigate the concentration polarization and membrane fouling. As a complementary study, in-situ characterization methods based on advanced physical techniques (e.g., optical coherence tomography and others) will be developed to capture the dynamic processes in real time, which might motivate or verify the development of mathematical models describing more interesting details. The proposed project will open a new paradigm for designing and optimizing high performance membranes via stability analysis.

膜分离技术已经在很多领域扮演着重要的角色。其效率的提高可以通过优化分离膜的次级结构来实现，而该优化需要理解次级结构的形成机理以及它们在膜分离过程中的作用。本申请项目提出将稳定性理论应用于建立分离膜制备条件与次级结构在聚合物网络形成过程中演化的联系。具体地说，线性稳定性分析能够确定极小扰动不同的演化方式（即在膜制备过程中具有周期性特征结构的形成）的标准，以期开发出更精确调控次级结构的新型方法。另一方面，线性分析与数值模拟的联合可以研究流体与膜表面次级结构微妙的相互作用，进而阐释在过滤过程中出现界面次生流动的机理；这对利用局域化效应减弱浓差极化和膜污染具有重要意义。作为互补性研究，该申请项目还将开发基于高端物理技术（例如光学相干断层成像仪等）的原位表征方法，以用于实时捕捉所研究的动态过程，以此启发或者验证数学模型的建立。该申请项目将开启通过稳定性分析设计和优化高性能分离膜的新模式。

膜分离技术的发展离不开深入的探讨分离膜中各种次级结构形成的机理和厘清这些次级结构如何在过滤过程中扮演实质性角色。以此为契机，本研究提出在稳定性理论的框架下关联分离膜的制备和过滤性能；将光学相干断层成像（OCT）技术应用于分离膜成膜和膜分离过程的原位表征。一方面，该研究首次通过基于OCT方法解析成膜动力学，以此解读不同的机理如何贡献于聚合物传递中的失稳现象以及随之发生的斑图形成现象。相应的表征结果有力地表明：（i）界面聚合形成薄膜的非平面生长有利于在深度方向上分析非稳定现象诱发的结构演化；（ii）壳聚糖在水溶相分离过程中的凝胶会发生聚合物链段固化与扩散的周期性竞争（即Liesegang现象）。另一方面，该研究开发了一系列数值算法，在与分离膜中形成的各种次级结构有关的膜污染现象中，用于减弱界面漂移对基于OCT表征分析的影响。分析结果揭示了（i）正弦型流道会诱发Dean涡流（即离心诱发的非稳定现象），并影响污染颗粒沉积的时空分布；（ii）条痕现象（即渗透流诱发的非稳定现象）会在渗透压驱动过程（其边界层中水力学压力对流体流动的影响相对弱）和膜蒸馏过程（其边界层中传质和传热发生耦合）中发生。该研究开启了辨析非稳定现象在成膜或膜分离过程中扮演关键角色的新范式，在建立更为稳固的构效关系的意义上，推动膜分离技术的发展。

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