Nanostructured Microsphere Production by Osmotic Extraction of Microfluidic Emulsion Templates

Microscale emulsion droplets are versatile soft templates for assembling nanoparticle powders and forming secondary microparticles. When combined with microfluidics, nanostructured microparticles can be produced with precise size and uniformity. However, assembly requires removal of the droplet solvent, which is particularly slow for water-in-oil emulsions. This has been a longstanding challenge, preventing the deployment of emulsion-structured nanomaterials at scale. Here, an osmotic pressure-driven method is presented that achieves controlled water extraction from emulsion droplets. This is a faster, more cost-effective, and sustainable alternative to prolonged heating; particle solidification is achieved by introducing a second emulsion containing a high solute concentration. The effect of system composition and droplet size on the rate of water extraction, emulsion stability, and nanoparticle assembly is explored, generating an empirical model for the solidification of 100–1000 μm diameter template droplets. By combining this extraction method with microfluidic emulsification, batches of spherical microparticles were formed composed entirely of nanoparticles, in this case, carbon nanotubes as a model system. Particle solidification was up to 5 times faster than evaporation while maintaining control over morphology and size distribution. Additionally, this processing method was demonstrated on other nanoparticle systems, confirming a broad material applicability.