Numerical study of particle segregation during spray drying of binary suspension droplets

Spray drying bidisperse suspensions typically produces segregated microstructures, with small particles concentrated on the surface and larger particles inside. These supraparticles hold potential as functional materials in fields such as catalysis, drug delivery, and ceramics. Yet, the underlying structure formation mechanisms remain poorly understood. A coupled discrete element method (DEM) and computational fluid dynamics (CFD) model was employed to simulate structure formation during spray drying at the single-droplet scale. The effects of particle size ratio and particle concentration were systematically analysed to evaluate their impact on particle segregation. Segregation intensity was strongly influenced by particle size ratio, with larger ratios enhancing segregation, whereas higher particle concentrations reduced this effect. Consistent with theories on stratification in drying films, the particle concentration gradient was identified as the primary cause for segregation. The resulting differences in radial forces result in higher migration velocities towards the droplet centre for larger particles, leading to their preferential localisation inside the aggregate. At higher particle concentrations, the rapid formation of a jammed layer of small particles restricts the mobility of the particles, reducing the intensity of this effect. This study advances the understanding of particle segregation during spray drying by providing the first systematic investigation of the factors influencing this phenomenon. The findings deepen the mechanistic understanding of segregation and provide a basis for the development of predictive models for the design of tailored microstructures in spray-dried particles.