Sandwich layering in binary nanoparticle films and effect of size ratio on stratification behavior

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.

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).

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.