Broad tunability of core size and number in core-shell compound droplets via tip positioning in coaxial capillary

Core-shell compound droplets play a crucial role in various fields such as materials science, biomedicine, and food technology, where their size and structure are pivotal for functionality. This study investigates the formation of core-shell compound droplets using a coaxial capillary device. By adjusting the inner channel tip from a retracted to an extended position relative to the outer tip, we trigger a transition of droplet formation to the unconfined state. This enables precise control over an ultra-broad range of inner droplet sizes and numbers (radius ∼0.3–2 mm (channel size ∼1 mm), core number ∼1–46), overcoming dimensional limitations imposed by the channel of a conventional device to the droplet size. Force analysis and model predictions reveal the roles of the forces involved in the droplet formation processes and the regime transitions. As the relative positions of the inner and outer channel tips is varied, dimensionless number analysis and particle image velocimetry experiments reveal a fundamental transition in the inner droplet formation mechanism from a shear-dominated pinch-off to a gravity-driven detachment. Especially when utilizing the mismatch between the core formation, the controlled deviation between the core size within one compound droplet is achieved (diameter ratio ranging from 1.2 to 1.7). This work provides novel insights into manipulating compound droplet structures across applications.