Experimental and Numerical Investigation of Stratification Regimes in Drying Polystyrene and Silica Nanoparticle Films: Implications for Evaporative Self-Assembly of Colloidal Particles

Controlled evaporative self-assembly of colloidal particles based on particle size is desirable when designing efficient, single-step deposition coating processes such as those encountered in specialized antireflective silica coatings. Particle movement during the drying process is complex. We experimentally and numerically investigate particle concentration profiles in binary colloidal films as a function of the small particle volume fraction and particle size ratio. Using a chemical potential expression that incorporates dilute interactions and also accounts for different particle size ratios, initial concentrations, and Peclet values, we calculate small and big particle concentration profiles. By characterizing the full film profile, we observe complex sandwich structures that were previously not predicted by theoretical models of film stratification and are relevant to specific coating applications. We employ microbeam X-ray scattering to characterize dried film profiles as a function of the film depth. These concentration data also show complex sandwich structures and highlight the necessity of full film characterization as opposed to only examining the surface. The presented numerical model does not correctly predict the measured film configurations, implying that additional particle interactions may need to be considered.