Tunable passive chaotic mixing in droplet microfluidics for controlled nanomaterial synthesis

Mixing in droplet-based microfluidic systems is crucial for nanomaterial synthesis and process optimization, where precise control over reaction kinetics and product quality is essential. However, achieving efficient and rapid mixing within droplets remains challenging, particularly when handling viscous liquids. In this study, we present an easy-to-implement, passive approach that allows on-the-fly control of mixing within microdroplets through adjusting the flow rate ratio between two continuous-phase streams in a flow-focusing geometry. Increasing flow asymmetry enhances shear at the droplet interface and induces immediate swirling within the droplet after break off. This disrupts internal recirculation patterns progressively intensifying chaotic advection, thereby improving mixing. Our method remains effective across a wide viscosity range, enabling efficient mixing within milliseconds, even for fluids with viscosities exceeding 100 cP. Our results demonstrate that mixing time and efficiency can be precisely controlled on demand by modifying the asymmetry of the continuous flow, and efficient mixing can be achieved in less than 2.5 ms, more than 30-fold improvement over the conventional configuration. We utilize computational fluid dynamics and micro-PIV analysis to elucidate the underlying mixing mechanism. The method's practical utility in nanomaterial synthesis is demonstrated through the synthesis of lipid nanoparticles, silver nanoparticles, and calcium carbonates, all of which exhibit reduced particle sizes and improved uniformity. This approach can be easily applied to other systems where enhanced polydispersity control is crucial, with promising implications in pharmaceutics and materials science.