Thermal Marangoni natural convection enables directional transport across immiscible liquids.

The ability to control molecular transport across immiscible liquid interfaces is critical for applications in manufacturing, biotechnology, and space research. Such transport mechanisms have garnered attention for their intriguing behavior and transformative potential in technology and science for a long time. Here, we demonstrate that unsteady temperature fields drive directional transport of water molecules across the oil-water interface, where weakened hydrogen bonding orients hydroxyl groups toward the oil phase. Through the Marangoni natural convection, water molecules migrate into the oil phase, forming stable nanoscale dispersions. We provide evidence that this transport is tunable via temperature gradients, offering a controllable pathway for mass transfer. This approach offers a potential, scalable, surfactant-free method for nanoemulsions production, with applications in drug delivery and advanced manufacturing. With surface tension as one of the driving forces, this phenomenon is equally valid in microgravity or zero-gravity environments. These findings advance the understanding of Marangoni natural convection and provide new insights into microscale fluid dynamics and spontaneous molecular migration at the oil-water interface.