Department of Chemical and Biological Engineering, University of Colorado Boulder , 596 UCB, Boulder, Colorado 80309-0596, United States.
ACS Appl Mater Interfaces. 2017 Dec 13;9(49):43258-43266. doi: 10.1021/acsami.7b15335. Epub 2017 Dec 1.
As particles flow through porous media, they follow complex pathways and experience heterogeneous environments that are challenging to characterize. Tortuosity is often used as a parameter to characterize the complexity of pathways in porous materials and is useful in understanding hindered mass transport in industrial filtration and mass separation processes. However, conventional calculations of tortuosity provide only average values under static conditions; they are insensitive to the intrinsic heterogeneity of porous media and do not account for potential effects of operating conditions. Here, we employ a high-throughput nanoparticle tracking method which enables the observation of actual particle trajectories in polymer membranes under relevant operating conditions. Our results indicate that tortuosity is not simply a structural material property but is instead a functional property that depends on flow rate and particle size. We also resolved the spatiotemporal heterogeneity of flowing particles in these porous media. The distributions of tortuosity and of local residence/retention times were surprisingly broad, exhibiting heavy tails representing a population of highly tortuous trajectories and local regions with anomalously long residence times. Interestingly, local tortuosity and residence times were directly correlated, suggesting the presence of highly confining regions that cause more meandering trajectories and longer retention times. The comprehensive information about tortuosity and spatiotemporal heterogeneity provided by these methods will advance the understanding of complex mass transport and assist rational design and synthesis of porous materials.
当粒子流经多孔介质时，它们会沿着复杂的路径行进，并经历具有挑战性的非均相环境，难以进行特征描述。迂曲度常被用作描述多孔材料中路径复杂性的参数，对于理解工业过滤和质量分离过程中受阻的质量传递很有用。然而，传统的迂曲度计算仅在静态条件下提供平均值；它们对多孔介质的固有非均质性不敏感，也不考虑操作条件的潜在影响。在这里，我们采用了高通量纳米颗粒跟踪方法，可以在相关操作条件下观察聚合物膜中实际的颗粒轨迹。我们的结果表明，迂曲度不是简单的结构材料特性，而是依赖于流速和颗粒尺寸的功能特性。我们还解析了这些多孔介质中流动颗粒的时空非均质性。迂曲度和局部停留/保留时间的分布出人意料地广泛，表现出长尾分布，代表了高度迂曲轨迹和局部区域具有异常长停留时间的种群。有趣的是，局部迂曲度和停留时间直接相关，这表明存在高度约束的区域，导致更多的曲折轨迹和更长的保留时间。这些方法提供的迂曲度和时空非均质性的综合信息将推进对复杂质量传递的理解，并有助于合理设计和合成多孔材料。
