Creating Tunable Low-Surface-Tension-Liquid Capsules via Impact-Driven Liquid-Liquid Encapsulation.

The encapsulation of droplets with low surface tension constitutes a critical area of investigation due to its extensive application across pharmaceuticals, environmental engineering, and advanced thermal management. A facile and effective approach─impact-driven liquid-liquid encapsulation (LLE)─has gained attention recently. This approach involves a descending core liquid impacting an immiscible liquid film (shell layer) that floats atop a host bath, subsequently being encapsulated by the liquid film. Despite its promise, the controlled encapsulation targeting low-surface-tension droplets via such an impact-driven mechanism has never been fundamentally explored. In this study, we successfully advanced the ultrafast impact-driven LLE technique to enable the encapsulation of low-surface-tension droplets. We experimentally demonstrated that the FC-40 droplets of varying impact diameters from 1.79 to 1.26 mm, with surface tension as low as 16 mN/m, were able to be encapsulated reliably within tens of milliseconds using an ultrathin silicone oil interfacial (shell) layer. Three regimes of encapsulation, including interfacial trapping, penetration with an air bubble, and penetration without an air bubble, were identified. Both the morphology and the size distribution of capsules were able to be precisely regulated by adjusting the impact kinetic energy and interfacial layer thickness. Higher volumetric encapsulation efficiency was attained for a larger impact diameter under lower impact kinetic energy, although a loss in volume was observed after encapsulation due to droplet pinch-off. A thicker interfacial layer was beneficial for achieving air-bubble-free, low-surface-tension capsules. This work contributes to the fundamental understanding of liquid-liquid encapsulation and offers valuable perspectives for the efficient and cost-effective production of functional capsules in practical applications.