Chromium hydroxide microsphere synthesis using droplet microfluidics

Abstract

In this work, a capillary microfluidic platform based on microbore tubes is demonstrated to continuously synthesize chromium hydroxide microspheres with a narrow particle size distribution using sol-gel process. Chromium hydroxide microspheres are later calcined to obtain chromium oxide microspheres. An in-situ mixing technique is utilized to mix the chromium precursor with the gelling agent. The effects of different design (microfluidic junction configuration, size of the microbore tube) and operating variables (continuous to dispersed phase flow rate ratio, flow velocity, residence time) on the average diameter and polydispersity index of microspheres are studied systematically. Microspheres of an average size of 574 μm with polydispersity index of 6.6% could be prepared using a double T-junction at an organic to aqueous (O/A) flow rate ratio of 60 in an 800 µm diameter microbore tube. The detailed analysis of the flow conditions reveals that the regime of drop (of aqueous phase) formation varies between transition and dripping. Finally, a correlation is proposed to predict average diameter of the microspheres based on values of capillary number (Ca) and organic to aqueous (O/A) flow rate ratio.