(436e) Highly Controlled Material Transfer into Microfluidic Droplets from an Active Colloidal Continuous Phase

Droplet microfluidics enables the use of monodisperse micro-droplets as reaction vessels with very precise control over the reaction conditions, such as reagent concentration and rapid mixing. However, once the droplets are generated in a microfluidic device, it is usually difficult to add new material into the droplets for subsequent operations. Fusion, via both electrical and hydrodynamic means, of droplets of similar sizes has been the most common strategy to achieve this objective. We have developed a novel and versatile method to transfer materials into the microfluidic droplets using a nano- or mini-emulsion as the continuous phase. We conduct a detailed study on the droplet generation mechanism, and three different strategies for controlled mass transfer: addition of co-surfactant, heat treatment, and electro-coalescence. In particular, electro-coalescence is a non-invasive and highly responsive method for extremely sensitive control over the amount of material to be added and, equally importantly, the rate of addition of these materials. The relationship between the applied electrical field and the coalescence between nano- and micro-droplets is studied. Finally, we demonstrate an application of this method in the controlled nucleation and growth of nanocrystals. Our method expands the toolbox of droplet microfluidics, enabling reaction engineering with precise mass transfer control at the sub-micron level using an active colloidal continuous phase carrying chemical payloads.