Deuterated Water Accelerates Phase-Separated Droplet Formation and Enables Directional Motion

The spatiotemporal coordination of compartment formation and directed transport is fundamental to the cellular organization. However, replicating these coupled behaviors in fully aqueous synthetic systems remains challenging. We report an isotopic solvent signaling strategy that leverages the physicochemical differences between deuterated water (D₂O) and light water (H₂O) to control the liquid-liquid phase separation (LLPS) and motility of dynamic covalent droplets. Our system utilizes the in situ generation of cationic imine surfactants that complex with anionic macrocycles to form coacervate droplets. We demonstrate that D₂O significantly accelerates droplet formation compared to that of H₂O by promoting early association and amplifying the hydrophobic interactions associated with imine surfactants. Furthermore, by establishing a spatial H₂O/D₂O gradient, we trigger a surface-tension imbalance that drives the directional transport of droplets from D₂O-rich to H₂O-rich regions via Marangoni flow. These motile droplets can move faster than that without an isotope gradient and perform complex functions using fluorescent dyes as a demonstration. These functions include autonomous cargo transport and chemical exchange with their surroundings during migration. This work establishes isotopic substitution as a powerful and noninvasive trigger for governing supramolecular assembly and motility. It offers a new dimension for engineering adaptive, lifelike soft matter.