Capillary-Wave-Induced ‘Endocytosis’ Enables Programmable Core-Shell Capsules Formation for Controlled Release

Understanding droplet-liquid pool interactions in immiscible systems is critical for advancing fluidic encapsulation strategies. Here, we uncover a previously unreported ‘endocytosis’ phenomenon in which an aqueous droplet impacts a deep immiscible pool and spontaneously engulfs the pool liquid, forming a stable double-layered liquid structure. This process origins from the focusing of capillary waves at the droplet-pool interface and followed by a pinch-off of a central oil column via Rayleigh-Plateau instability. We propose a dimensionless criterion for central column formation, determined by whether viscous dissipation of the droplet's kinetic energy. By introducing a UV-crosslinkable precursor, gelatin methacryloyl (GelMA) into the droplet, we succeed in encapsulating pool liquid into hydrogel capsules with tunable core-shell ratios by simply adjusting the impact velocity. This enables precise control of release mechanism: thick-shell capsules exhibit non-Fickian sustained release, while thin-shell capsules display delayed burst Case II release upon swelling. The proposed ‘endocytosis’ phenomena offer a surfactant-free, single-parameter controlled strategy compared to conventional liquid encapsulation methods, which offers a new opportunity for on-demand release in drug delivery and responsive materials.