Large Scale Flow-Mediated Formation and Potential Applications of Surface Nanodroplets

Microscopic droplets located on a solid substrate in contact with an immiscible liquid promise a broad range of applications in miniaturized analytical techniques, fabrication of antireflective coatings, high-resolution near-field imaging techniques, and many others. A simple method of producing oil nanodroplets with desirable morphology is a bottom-up approach called solvent exchange, where nanodroplets nucleate and grow, as a good solvent of oil is displaced by a poor solvent. In this work, we have achieved the production of surface nanodroplets over a large surface area on planar or curved surfaces, guided by the principles of the solvent exchange. The droplet size is uniform over the entire surface of a planar or curved substrate and tunable. The production rate is extremely high at 106 nanodroplets per second. This advance in the nanodroplet production provides a general platform for droplet-based applications. Here we demonstrate that the application of surface nanodroplets in microextraction of hydrophobic solute (dye) from its highly diluted aqueous solution and in situ detection of the dye in a simple process, and in fabrication of highly ordered array of microlens arrays and polymer-capped microstructures by simple processes.