Controllable Inducing Preparation of Polymorphous Hundred-Micron Alumina Microspheres via Droplet-Based Two-Stage Microfluidic Solvent Extraction Technology

The sol–gel process has attracted extensive attention for the preparation of advanced inorganic materials. Notably, solvent extraction significantly affects the properties of the final products. However, the poor controllability of the extraction process generally limits the use of solvent extraction technology in regulating the morphologies and structures of inorganic materials. In this study, we successfully constructed a two-stage microfluidic chip, which decoupled the sol droplet generation and extraction solidification processes and realized highly controllable extraction. The influence of flow field parameters on the diffusion of the extractant was investigated via in situ observations, particle image velocimetry, and computational fluid dynamics simulations, which could induce changes in the microsphere morphologies by regulating the extraction diffusion rate. Rapid diffusion of the extractant was found to be beneficial for maintaining sphericity, whereas slow diffusion induced directional deformation in the microspheres. In addition, we revealed the transformation process of the internal microstructure. 1-Nonanol was found to be conducive to the formation of gel networks and long fibers, whereas n-butanol was found to be conducive to the directional ordering of fibers under droplet circulation. Polymorphous hundred-micron alumina microspheres (such as spherical, ravine ellipsoid, erythrocyte, and valve types) were successfully prepared using microfluidic-assisted solvent diffusion and extraction processes.