Diffusion controlled formation of microparticles

A new hybrid analytical/numerical model that describes the formation of structured microparticles from evaporating microdroplets is presented. It is applicable when diffusion and surface recession are the main mechanisms of mass transport in the droplets. The model accounts for the transient nature of the solute concentration profiles during the evaporation process and thereby extends previously published models to particle formation processes in which the droplet surface recession rate is much faster than the diffusion in the droplets, i.e., in cases with large Péclet numbers. Furthermore, the model is useful when the initial concentration of solutes in the droplets is close to saturation. The model also predicts the dry particle diameter, assuming a spherical particle, particle density, and aerodynamic diameter. For hollow particles formed at large Péclet numbers the shell thickness can be approximated. The model is capable of predicting the radial distribution of the components in the final dry particle in the case of multi-component formulations. The results of the model were recast in a simple analytical form, which can be used in particle design without the need for numerical tools. Predictions of the model were found to be in good agreement with numerical and experimental results in the literature.