Preparation of solid, hollow, hole-shell and asymmetric silica microspheres by microfluidic-assisted solvent extraction process

Present work demonstrated the facile preparation of silica microspheres with various structures (solid, hollow, hollow with a hole and filbert-like solid). These were prepared by first forming monodisperse silica sol droplets in a simple microfluidic device, followed by extracting the solvent from the droplets in an extractant or at the interface between the extractant and liquid paraffin at different conditions. The effect of different extractants and extracting temperature was investigated. The products were characterized by optical microscope and scanning electron microscope. Extraction in fatty acid methyl ester (FAME) at room temperature led to formation of solid silica microspheres, while extraction at the interface between FAME and liquid paraffin at 60. °C resulted in formation of hollow silica microspheres. Use of mixture of castor oil (CO) and dimethyl carbonate (DMC) as extractant resulted in formation of hollow silica microspheres with a hole on the surface, whereas increase in the DMC content in extracting medium led to formation of filbert-like silica solid microspheres. Change in size of cavity and hole was studied by changing the extracting temperature. The formation process and mechanism of these silica microspheres are proposed based on the diffusion rate. The relationship between the size of the microspheres and the state of the droplet at the interface is correlated.