Controlled preparation of compound droplets in a double rectangular co-flowing microfluidic device

Abstract

This paper presents a 3D numerical simulation on a microfluidic device for creating double emulsions, controlling their sizes, and also manipulating the encapsulation process using a two-sequential co-flowing microfluidic configuration. In the proposed geometry, double emulsions can be generated as a result of the liquid’s interaction using dripping instability in a highly controllable manner. Dimensionless numbers, including Weber and capillary of the inner phase (Wein and Cain), Weber and capillary of the middle phase (Wemid and Camid), and capillary number of the outer phase (Caout) are considered to peruse effects of the phase flow rates on compound droplets characteristics. The mentioned dimensionless parameters can individually affect the formation process of the compound droplet, size of the inner and the outer droplets, and formation frequency. A three-dimensional numerical model based on the Volume of Fluid-Continuum Surface Force (VOF-CSF) with adaptive mesh refinement was utilized to perform parametric analysis on the double emulsion process. The model predicted the process of emulsification successfully in both dripping and jetting regimes and was used to predict the effects of phase flow rates on the size, morphology, and frequency of compound droplets. As the inner phase flow rate is increased, both inner and outer droplet sizes increase, while increasing the outer phase flow rate results in a reduction in compound droplet size. However, the middle phase flow rate increment eventuates to an increase and decrease in detached droplet size of the outer and inner phases, respectively. Also, it was observed that the number of dripping instabilities increases by increasing both middle and outer phases flow rates, which also lead to the higher compound jet filament length. The comparison of results with previous works reveals that the present geometry has high throughput in a broad range of flow rates while it has the capability of generating relatively spherical droplets even at high production rates. This is while the fabrication of this device is straightforward and precise in comparison with capillary-based devices.