Hypothesis

Stratification or self-segregation of multicomponent particle mixtures during drying is an important phenomenon to understand for the development of single-step deposition processes for complex coatings. We hypothesize that varying the ratio of particle Peclet numbers will lead to different types of stratification behavior.

Experiments

Binary colloidal films of polystyrene and silica were prepared by evaporative film formation, and stratification of nanoparticles of different size ratio (7.7–1.2) was studied using microbeam small-angle X-ray scattering (SAXS).

Findings

SAXS spectra showed noticeable variations at different film depths, consistent with stratification. These results are quantified to obtain vertical composition profiles. We observe “sandwich”-type layered structures at different nanoparticle size ratios, which to our knowledge have not been previously observed experimentally or predicted by theory. For example, for films of larger particle size ratios (7.7–4.8), large particles are enriched at the film top and bottom, leading to a large-small-large or “LSL” behavior; while within films of smaller particle size ratio (2.2–1.2), small particles are enriched at the top and bottom of the film (small-large-small or “SLS” structures). The enrichment of particles at the top persists over several hundred particle layers and is not just a single monolayer pinned to the upper surface.

中文翻译：

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二元纳米颗粒薄膜中的夹心分层和尺寸比对分层行为的影响

假设

干燥过程中多组分颗粒混合物的分层或自离析是了解复杂涂层单步沉积工艺发展的重要现象。我们假设改变粒子Peclet数的比率将导致不同类型的分层行为。

实验

通过蒸发膜形成聚苯乙烯和二氧化硅的二元胶体膜，并使用微束小角X射线散射（SAXS）研究了不同尺寸比（7.7–1.2）的纳米颗粒的分层。

发现

SAXS光谱显示在不同的薄膜深度有明显的变化，与分层一致。量化这些结果以获得垂直的组成轮廓。我们在不同的纳米颗粒尺寸比率下观察到“三明治”型层状结构，据我们所知，这是以前从未通过实验或理论预测得到的。例如，对于较大粒径比（7.7-4.8）的膜，大颗粒在膜的顶部和底部富集，从而导致大-小-大或“ LSL”行为。而在较小粒径比（2.2–1.2）的薄膜中，小颗粒在薄膜的顶部和底部富集（小-大-小或“ SLS”结构）。顶部的粒子富集持续了数百个粒子层，而不仅仅是固定在上表面的单个单层。