Department of Physics, University of Surrey, Guildford GU2 7XH, United Kingdom.
Unilever R&D Port Sunlight, Quarry Road East, Bebington, Wirral CH63 3JW, United Kingdom.
Phys Rev E. 2017 Dec;96(6-1):062602. doi: 10.1103/PhysRevE.96.062602. Epub 2017 Dec 6.
A colloidal particle placed in an inhomogeneous solution of smaller nonadsorbing polymers will move towards regions of lower polymer concentration in order to reduce the free energy of the interface between the surface of the particle and the solution. This phenomenon is known as diffusiophoresis. Treating the polymer as penetrable hard spheres, as in the Asakura-Oosawa model, a simple analytic expression for the diffusiophoretic drift velocity can be obtained. In the context of drying films we show that diffusiophoresis by this mechanism can lead to stratification under easily accessible experimental conditions. By stratification we mean spontaneous formation of a layer of polymer on top of a layer of the colloid. Transposed to the case of binary colloidal mixtures, this offers an explanation for the stratification observed recently in these systems [A. Fortini et al., Phys. Rev. Lett. 116, 118301 (2016)PRLTAO0031-900710.1103/PhysRevLett.116.118301]. Our results emphasize the importance of treating solvent dynamics explicitly in these problems and caution against the neglect of hydrodynamic interactions or the use of implicit solvent models in which the absence of solvent backflow results in an unbalanced osmotic force that gives rise to large but unphysical effects.
将胶体颗粒置于较小的非吸附聚合物的不均匀溶液中，颗粒将朝着聚合物浓度较低的区域移动，以降低颗粒表面与溶液之间的界面自由能。这种现象称为扩散泳。在 Asakura-Oosawa 模型中，将聚合物视为可穿透的硬球，可获得扩散泳动速度的简单解析表达式。在干燥膜的情况下，我们表明，通过这种机制的扩散泳动可以在易于获得的实验条件下导致分层。通过分层，我们是指在胶体层之上自发形成聚合物层。对于二元胶体混合物，这为最近在这些系统中观察到的分层现象提供了解释[A. Fortini 等人，Phys. Rev. Lett. 116, 118301 (2016)PRLTAO0031-900710.1103/PhysRevLett.116.118301]。我们的结果强调了在这些问题中明确处理溶剂动力学的重要性，并告诫人们不要忽略流体动力学相互作用，或者使用隐式溶剂模型，其中缺乏溶剂回流会导致不平衡的渗透压，从而产生大但不合理的影响。
