Controllable preparation of double emulsion droplets in a dual-coaxial microfluidic device

This article investigates the formation process of double emulsions, controlling their sizes, and manipulating the encapsulation process in a three nested microcapillary tube configuration. In the proposed geometry, as a result of the interaction of liquids using dripping instability, double emulsions are generated. Governing equations are discretized using the finite-element method utilizing COMSOL Multiphysics 5.4 software, employing axisymmetric assumption, and the interface position is tracked by solving additional transport equations of the phase-field method. Influence of phase flow rates in the form of inner phase Weber number, We(in), intermediate phase capillary number, Ca-int, and outer phase capillary number, Ca-out, are studied on the flow regimes and characteristics of double emulsions. Results exhibit that the outer phase flow rate has the most influence among other phase flow rates. Additionally, three distinct regimes, named single emulsion, decussate, and double emulsion, are observed by changing intermediate phase flow rare. Furthermore, satellite droplets are detected at relatively high flow rates of the outer phase due to the second instability induced by the continuous phase to the intermediate phase jet. A comparison between the formation frequency of the present study with some similar works, including both capillary-based and channel-based devices, unveils that the existing configuration has a higher throughput of double emulsion generation in a broad range of flow rates.