Reconfigurable Microfluidic Droplets Stabilized by Nanoparticle Surfactants.

Interfacial assemblies of nanoparticles can stabilize liquid-liquid interfaces. Due to the interactions between functional groups on nanoparticles dispersed in one liquid and polymers having complementary end-functionality dissolved in a second immiscible fluid, the anchoring of a well-defined number of polymer chains onto the nanoparticles leads to the formation of NP-surfactants that assemble at the interface and reduce the interfacial energy. We have developed droplet interfaces covered with elastic, responsive monolayers of NP-surfactants. Due to the presence of an elastic layer at the interface, the droplets offer a greater resistance to coalescence and can prevent the exchange of materials across interfaces. Our results show the successful encapsulation of nanoparticles, dyes, and proteins with diameters in the 2.4-30 nm range. Further, we show that stable water-in-oil droplets can be generated for various combinations of polymer ligands and nanoparticles bearing complementary functionalities. These NP-surfactant-stabilized microfluidic emulsions would enable applications requiring liquid-liquid interfaces that can adapt and respond to external stimuli and whose mechanical properties can be easily tailored.