Evidence of Stratification in Binary Colloidal Films from Microbeam X‑ray Scattering: Toward Optimizing the Evaporative Assembly Processes for Coatings

Colloidal films have many important applications where a layered configuration is desirable, including flexible electronics, antireflective coatings, and antimicrobial paints. We report stratification during evaporative film formation in binary colloidal dispersions, probed using a novel microbeam small-angle X-ray scattering (SAXS) technique. To our knowledge, SAXS approaches have not been used to experimentally obtain quantitative data of concentration profiles in multicomponent colloidal films. We measured the local scattering of a film at different vertical locations using a microfocused X-ray beam and determined particle concentrations at different film depths using a linear combination analysis of the mixed film and pure film scattering data. Using small particle size ratios ranging from 2.6 to 1.2, we experimentally observed and quantify three distinct stratification configurations: inverted small-on-top, large-on-top, and no stratification. Our results show some agreement with a previously proposed stratification state diagram, although there are some limitations. Experimental verification of these stratification phenomena is critical to fully understanding the physics of particle movement and structure development during film formation, which is crucial for optimizing evaporative assembly processes for coatings.