Experimental and computational analysis of mixing inside droplets for microfluidic fabrication of gold nanoparticles

Emulsions formed by mixing reactant streams inside microdroplets are efficient micro-scale reactors for synthesis of nano/micro- particles/crystals due to small quantities of reagents confined within each droplet and separation of droplet contents from the reactor walls. In this work, the synthesis of size-tuneable gold nanoparticles (AuNPs) within emulsion droplets generated in a three-phase glass capillary microfluidic device was investigated experimentally and numerically using computational fluid dynamics (CFD). AuNPs were produced by micromixing two aqueous streams, 1.15 mM HAuCl₄ containing 1% polyvinylpyrrolidone capping agent and 20 mM ascorbic acid solutions, inside monodispersed droplets created by 3D counter-current flow focusing in a medium-chain triglyceride. The mean particle size of AuNPs was tuneable in the range between 26 and 56 nm and depended on the degree of premixing of the reactant streams shortly before droplet generation, and the mixing efficiency within droplets, which was controlled by hydrodynamic conditions within the microfluidic device. The CFD results were compared and validated against experimental observations and revealed the presence of a recirculation zone near the outer wall of the injection capillary tip. The mixing efficiency was higher at smaller droplet size causing a reduction in the particle size of the AuNPs.