Release characteristics of core droplet from a double-emulsion droplet

Double-emulsion droplets have been applied in biological, chemical, and medical fields due to their excellent encapsulation and release properties of active substances. However, their application and popularization are limited by a poor understanding of the double-interface characteristics. In this work, the core release characteristics from double-emulsion droplets were studied by numerical simulations. The coupled level set and volume of fluid method was used to track the interface motion, and the continuous surface tension was applied to describe the interfacial tension force. It was found that the initial core release may be interrupted by the retraction of the middle phase when the core droplet diameter is large, the interfacial tension is high, or the external droplet diameter is small. The retraction is induced by the pressure increase behind the droplet in the middle phase. Four core release types were found: release at one time (Type I), release at two times on the left side (Type II), partial release (Type III), and release on both sides at different times (Type IV). Due to rising interfacial energy and reduction of the middle-phase resistance, the release type will gradually transform from Type I to Type II, III, or IV with increasing core diameter, increasing interfacial tension, and decreasing external diameter. Finally, a phase diagram summarizing the four release types of the core release was obtained. The results of this study have profound guiding significance for the application and understanding of the release properties of double-emulsion droplets.