(307b) Electric Fields in Coflowing Liquids as a Route to Fine Emulsions

Liquid-liquid emulsion droplets in the micron and sub-micron size range find applications in several fields such as cosmetics, drug delivery, and food industry. Microfluidics offers a promising route to produce these droplets by decoupling the droplet size from its composition. Here we propose a novel method for producing emulsion droplets in the sub-micron size range in a microfluidic device that couples electric and hydrodynamic forces. Electric fields are applied to a conducting liquid coflowing with an outer insulating liquid in a glass-based coflow geometry. We investigate the drop formation mechanism as a function of operating parameters such as the applied electric field strength, flow rates of the inner and outer liquids, and device geometry. Two modes of operation for the stable generation of emulsion droplets are observed: Cone-Jet mode and Whipping mode. Both the modes operate in a steady-state manner over long periods of time, allowing continuous generation of monodisperse emulsion droplets. For a given conductivity of the inner liquid, the smallest possible droplet diameter is obtained for small inner flow rates, high outer flow rates and high applied voltages. Measured currents from the experiments indicate a strong dependence on applied voltage and a weak dependence on inner flow rate indicating the charge transport mechanism to be predominantly conductive. We envisage this observation to be a consequence of the small size ratio of capillary tip to the jet diameter which prevents the establishment of convective regime. This result is in stark contrast with what is expected in an electrospray where the outer medium is air rather than a coflowing insulating liquid.