Ecole Supérieure de Physique et de Chimie Industrielles, Laboratoire de Physique et Mécanique des Milieux Hétérogènes (PMMH - UMR 7636 CNRS - ESPCI-ParisTech - Université Pierre et Marie Curie 6 - Université Paris Diderot), 10 rue Vauquelin, 75231 Paris Cedex 05, France.
J Phys Chem A. 2012 Jun 28;116(25):6540-51. doi: 10.1021/jp212414e. Epub 2012 May 30.
A simple theoretical model for the size selectivity, S(d), in the lift mode of retention in field-flow fractionation (FFF) is developed on the basis of the near-wall lift force expression. S(d) is made up of two contributions: the flow contribution, S(d,f), arising from the variation of the flow velocity at center of particle due to a change in particle position with particle size, and a slip contribution, S(d,s), arising from the concomitant change in the extent of retardation due to the presence of a nearby channel wall. The slip contribution is minor, but not negligible, and amounts to 10-20% of the overall size selectivity. It contributes to reduce S(d) in sedimentation FFF but to enhance it in flow FFF. S(d) would steadily increase with particle size if the flow profile was linear (Couette flow). Because of the curvature of the flow profile encountered in the classical Poiseuille flow, S(d) exhibits a maximum at some specific particle size. The model predicts a significant difference in S(d) between sedimentation FFF and flow FFF, arising from the different functional dependences of the field force with particle size between these two methods. The predictions are in good agreement with the various S(d) values reported in the literature in both sedimentation and flow FFF. On the basis of the model, guidelines are given for adjusting the operating parameters (carrier flow rate and field strength) to optimize the size selectivity. Finally, it is found that S(d) generally decreases with decreasing channel thickness.
基于近壁升力表达式，建立了场流分离（FFF）中提升模式保留的尺寸选择性 S(d) 的简单理论模型。S(d) 由两个贡献组成：由于颗粒位置随粒径变化导致颗粒中心流速度的变化而产生的流动贡献 S(d,f)，以及由于附近通道壁的存在导致滞后程度的相应变化而产生的滑移贡献 S(d,s)。滑移贡献虽然较小，但并非可以忽略不计，占总体尺寸选择性的 10-20%。它有助于降低沉降 FFF 中的 S(d)，但增强流动 FFF 中的 S(d)。如果流型是线性的（Couette 流），则 S(d)会随着颗粒尺寸的增加而稳定增加。由于在经典泊肃叶流中遇到的流型曲率，S(d)在某个特定的颗粒尺寸处表现出最大值。该模型预测了沉降 FFF 和流动 FFF 之间 S(d) 的显著差异，这是由于这两种方法中场力与颗粒尺寸之间的函数依赖性不同所致。预测结果与文献中报道的沉降和流动 FFF 中的各种 S(d) 值非常吻合。基于该模型，给出了调整操作参数（载体流速和场强）以优化尺寸选择性的指南。最后，发现 S(d)通常随通道厚度的减小而减小。
