(350e) Nanoemulsion Meets Droplet Microfluidics: Controlled Mass Transport and Applications in Micro- and Nanoparticle Preparation

We present a collection of studies on a novel hierarchical emulsion system consisting of both nanoemulsions and microfluidic droplets with a focus on mass transport mechanism and material preparation applications.

First, we successfully applies nanoemulsions as carriers to deliver reactants into microfluidic droplets quasi-continuously due to the dramatic size difference between nanodroplets and microdroplets. With different physiochemical stimuli, the dosage and rate of chemical injection can be precisely controlled to control the reaction kinetics. Among these stimuli, electronic signals offer a sensitive, non-invasive, and digital control with a simple non-touching equipment setup. While we use gold and gold-palladium core-shell nanoparticle preparation as application demonstrations, this method is well-generalizable to a broad range of chemical and biological applications that require controlled chemical delivery into microfluidic droplets.

Second, microdroplets could also act as templates for microparticle preparation. Thanks to their high monodispersity, uniformly sized alginate hydrogel microbeads can be prepared by solidifying microdroplets containing alginate precursor solution with a gelation reagent, CaCl₂, supplied from nanoemulsions. With similar thermal or electronic stimuli, gelation can be triggered at the desired moment to “freeze” particle internal compartmentation. The dosage of CaCl₂ can be well-controlled to yield microbeads of different swelling properties.

Third, by treating the nanodroplets as reaction vessels, microdroplets can be used as “micro-reservoirs” to supply chemicals steadily into nanodroplets. We demonstrate an example in the continuous preparation of superparamagnetic nanoparticles by supplying ammonium hydroxide from microdroplets into nanodroplets containing a Fe²⁺/Fe³⁺ precursor solution. With enhanced heat transfer, nanoparticles can be easily prepared by thermally decomposing the precipitated hydroxides within minutes, and the inherent oxygen-free environment in fluoropolymer channels ensures no oxidation during decomposition.