Electro-Coflow: A Novel Approach to Controlled Droplet Generation Using Microfluidics

Low Reynolds numbers involved in microfluidic devices allow great control over the generation of emulsions in terms of the droplet size and the monodispersity. Here we use a novel approach, called Electro-Coflow, wherein electric and hydrodynamic forces are coupled in a glass-based microfluidic device with a coflow geometry to generate micron and sub-micron droplets. In these experiments, we use three immiscible liquids, an outer dielectric liquid, an inner conducting liquid, and another conducting liquid (collector liquid) which acts as a counter-electrode. Once a stable conical meniscus is formed due to applied external field, the device mainly operates in two modes: Cone-jet and whipping modes. We investigate the transition from one mode to the other with the external field and the inner flow rate. The droplet size in both modes is a strong function of the applied voltage, thereby allowing an additional tuning parameter, a feature that is typically absent in classical electrospray. We also investigate the growth rate of the whipping instability as a function of system parameters such as applied voltage and outer fluid viscosity.