Evaporation-induced Self-assembly of Uniform Silica Microparticles with Mesoscopic Structures via Spray Drying

The properties of particles formed during spray drying are largely determined by the properties of the precursors and the drying conditions. The thermal and mass transfer histories significantly affect the structure formation (and subsequent functionality) and the processes are indeed very complex, particularly when dealing with polydisperse droplets as in conventional spray driers. Here a more systematic study is done using a microfluidic spray drying approach capable of generating monodisperse particles. We observed the formation of uniform silica microparticles with unique mesoscopic structures from colloidal silica nanoparticles and a hydrophilic polymer (sodium alginate) via microfluidic spray drying. A buckling-driven shape transition of the initial spherical droplets took place in all the observed cases during the evaporation induced self-assembly. The mesoscopic architectures and surface features of the microparticles were shown to be strongly dependent on the mass fraction of the polymer incorporated in the initial precursors, but were independent of the mass fraction of silica nanoparticles (2% to 10% range) in the droplets. The formation of sub-micron aggregates was noticeable with increasing ratio of added polymer in the droplets. However, the phenomena of shape deformation and buckling were found to be less significant at a lower drying rate.