Controllable generation of core-shell composite droplets in a conical inlet double-focus microchannel

In this paper, a double-focused conical inlet axisymmetric structure with double acceleration is proposed and the evolution of composite core-shell droplets is calculated using the volume of fluid (VOF) method, focusing on the size and droplet formation frequency of composite core-shell droplets as well as the quality control method. The results show that dripping and jetting regimes occur in the channel, and the generation period and flow distribution under the two mechanisms are investigated accordingly. The effect of local geometry on the evolution of composite droplets in the core-shell is investigated and analyzed using the composite droplet size, frequency, maximum extended length, and core droplet relative offset rate at the initial stage. It is found that changing the entrance angle can achieve the regulation of droplet size to a certain extent. With the increase of dimensionless width of the collection channel, the core-shell composite droplets size shows an overall increasing trend and is inversely proportional to the outer phase capillary number. Finally, a quality control method for composite droplet formation based on the maximum relative offset rate of core droplets is proposed for the purpose of controlling the integrity of composite droplets.