Optimizing Stratification in Binary Colloidal Supraparticles

Stratified binary supraparticles enable functional architectures for applications such as photonics, pharmaceutics, and additive manufacturing. However, the extent of stratification under predefined particle size ratios and compositions remains unclear. Here, stratification in spray-dried supraparticles from charge-stabilised binary mixtures of colloidal primary particles under such boundary conditions is quantified via SEM image analysis and confocal microscopy. These experiments reveal the existence of optimal combinations of size ratios and volume fractions that produce maximal stratification, contrasting the intuitive picture that stratification increases with increasing size ratios. For low volume fractions of small particles, stratification is maximal with size ratios of ∼3-5 and shifts to lower size ratios with increasing relative volume fractions. CFD-DEM simulations reproduce this trend and provide a contour plot of the extent of stratification as a function of particle size ratio and volume fraction. The simulations further suggest that stratification is driven by migration of small particles through interstitial channels between large particles at lower small particle concentrations, while colloidal diffusiophoresis becomes prominent at higher small particle contents. The understanding of stratification under experimental boundary conditions provides a framework for the predictive design of hierarchical supraparticle architectures with tailored internal structure.