Rapid in-droplet tri-fluid micromixing and concentration gradient generation for nanoparticle synthesis

Multiple micromixing is a pivotal process for complex chemical and biochemical assays involving multicomponent reactions or particle synthesis. This study introduces a unique in-droplet micromixing strategy that leverages periodically spaced expansion-contraction flow within droplets to achieve rapid mixing among parallel segment fluids. We have designed an in-droplet micromixer, integrating a standard flow-focusing droplet generator with a periodically wide-narrow serpentine microchannel, which exploits naturally recirculating vortex flow pairs to induce micromixing and uses regular channel width variations to enhance mixing performance. Sufficient tri-fluid micromixing can be achieved within a mixing length of submillimeter scale that is much smaller than other methods. Furthermore, the platform's capability to generate droplets with multiple concentration gradients showcases its flexibility in manipulating fluidic samples. The strategy is successfully applied in the nanoprecipitation of proteins and the chemical synthesis of Ni−Fe Prussian blue analogue nanoparticles, yielding nanoparticles with nondeposition on channel wall, controllable size and high dispersion. This in-droplet micromixing concept, which facilitates precise control over multicomponent reactions and complex nanoparticle synthesis, represents a significant advancement in addressing the chemical and biochemical reaction challenges that have been prevalent in the field.