Visualization study of emulsion droplet formation in a coflowing microchannel

• Droplet formation dynamics in a coflowing microchannel is experimentally studied.
• Dripping, jetting and wavy regimes as well as morphologies are observed visually.
• Emulsification regimes and morphologies are characterized by a phase diagram.
• Droplet diameter is insensitive to capillary number for thinning jetting.
• Dripping and thinning jetting are favorable way to generate monodispersed droplet.

An experimental visualization study is conducted to investigate the hydrodynamic characteristics of emulsion droplet formation in a coflowing microchannel. Both monodisperse and polydisperse patterns of drop formation are observed, including dripping regime, jetting regime (widening jetting and narrowing jetting). Especially, two dripping-to-jetting transition regimes and wavy regime with no individual droplet produced are captured and analyzed. A corresponding phase diagram is provided to characterize the transitions between different emulsification patterns through the control of flow rate of continuous phase. In addition, the dependence of generated droplet size on the Capillary number of the continuous phase (Ca) and the Weber number of the dispersed phase (We) is presented. It is indicated that, when Ca is below 3, the generated droplet size is sensitive to the viscous force and the drop formation regime is widening jetting and dripping. However, when Ca exceeds 3, the generated droplet size is approximately independent of Ca, and the droplet formation regime is thinning jetting.