When films are deposited from mixtures of colloidal particles of two different sizes, a diverse range of functional structures can result. One structure of particular interest is a stratified film in which the top surface layer has a composition different than in the interior. Here, we explore the conditions under which a stratified layer of small particles develops spontaneously in a colloidal film that is cast from a binary mixture of small and large polymer particles that are suspended in water. A recent model, which considers the cross-interaction between the large and small particles (Zhou et al., Phys. Rev. Lett., 2017, 118, 108002), predicts that stratification will develop from dilute binary mixtures when the particle size ratio (α), initial volume fraction of small particles (ϕ_S), and Péclet number are high. In experiments and Langevin dynamics simulations, we systematically vary α and ϕ_S in both dilute and concentrated suspensions. We find that stratified films develop when ϕ_S is increased, which is in agreement with the model. In dilute suspensions, there is reasonable agreement between the experiments and the Zhou et al. model. In concentrated suspensions, stratification occurs in experiments only for the higher size ratio α = 7. Simulations using a high Péclet number, additionally find stratification with α = 2, when ϕ_S is high enough. Our results provide a quantitative understanding of the conditions under which stratified colloidal films assemble. Our research has relevance for the design of coatings with targeted optical and mechanical properties at their surface.

中文翻译：

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二元胶体聚合物薄膜的分层：实验和模拟

当从两种不同尺寸的胶体颗粒的混合物中沉积薄膜时，会产生各种各样的功能结构。特别关注的一种结构是分层膜，其中顶表面层具有与内部不同的组成。在这里，我们探索在胶体膜中自发形成小颗粒分层层的条件，胶体膜由悬浮在水中的小和大聚合物颗粒的二元混合物流延而成。最近的模型，它考虑到大，小颗粒之间的交叉相互作用（周等人，物理评论快报。，2017年，118，108002），预测，分层将从稀二元混合物开发当粒径比（α），小粒子（初始体积分数φ_小号），和Peclet数是高的。在实验和朗之万动力学模拟，我们系统地改变α和φ_小号在两个稀释并浓缩悬浮液。我们发现，随着ϕ _S的增加，分层薄膜会发展，这与模型相符。在稀悬液中，实验与Zhou等人之间有合理的共识。模型。在浓缩的悬浮液中，仅在较大的尺寸比α = 7的情况下才发生分层。使用高佩克利数进行模拟，另外，当ϕ时，发现α = 2的分层。_S足够高。我们的结果提供了对分层胶体膜组装条件的定量理解。我们的研究与在表面具有目标光学和机械性能的涂料的设计有关。