Electrically initiated upstream coalescence cascade of droplets in a microfluidic flow

Two phase microfluidic systems creating size-controlled microdroplets have recently emerged as powerful tools to achieve liquid compartmentalization for high throughput chemical and biological assays. Emulsion electrocoalescence is a destabilization process that can be used in droplet-based platforms for water phase separation to enable lab-on-a-chip applications in biotechnology, including particle or cell recovery. In this paper, we report upon a series of phenomena associated with electrocoalescence of water microdroplet-in-oil populations in microfluidics. In our experiments, we formed microdroplets whose size and dispersion in the channel were varied according to the ratio of the flow rates of the two phases. Different types of electrocoalescence between droplets were obtained. For low applied voltages, drops merged in pairs over the electrode region; for higher values of the applied voltage, a cascade of droplet coalescence was produced against the flow direction, for a range of droplet sizes, lateral distributions of droplets in the channel and localized electric fields.