Gravity-driven liquid-liquid encapsulation: A new paradigm in encapsulation for low-surface-tension drops.

HYPOTHESIS: The encapsulation of low-surface-tension (core) liquids in an immiscible continuous phase (shell) is of fundamental and practical importance in interfacial engineering because of its broad applications. Existing impact-driven liquid-liquid encapsulation (LLE) relies on impact inertia to overcome the interfacial energy barrier. However, the huge interfacial deformation frequently causes air entrapment, core liquid loss, and evaporation of volatile liquids. We hypothesize that replacing impulsive impact with gradual gravitational loading of core liquid very close to the interfacial shell layer can fundamentally alter the encapsulation pathway and minimize interfacial disturbance. Such a gravity-driven mechanism is expected to provide a more efficient route for encapsulating low-surface-tension liquids. EXPERIMENTS: We first develop a gravity-driven LLE strategy in which a core drop composed of 3 M Fluorinert™ FC-40 (FC-40) penetrates a floating silicone-oil interfacial layer kept in a host water bath under gravity and subsequently undergoes gravity-driven pinch-off. Furthermore, a loop-assisted interfacial confinement strategy is introduced to tailor the interfacial layer geometry and interfacial evolution. High-speed visualization and theoretical analysis are combined to investigate the hydrodynamics and encapsulation sizes, and establish regime maps for encapsulation. FINDINGS: Gravity-driven LLE enables stable, air-bubble-free encapsulation without impact-induced core liquid loss. The encapsulation process is governed primarily by interfacial-layer geometry, with both experiments and scaling analysis demonstrating a decrease in encapsulation size with increasing interfacial-layer thickness. Loop-assisted confinement enables effective regulation of encapsulation size over the millimeter range (∼3-6 mm), while regime maps define the operational boundaries for successful encapsulation. The strategy provides a facile and scalable approach for tunable low-surface-tension encapsulation and offers new insights into LLE for diverse applications.