Adjustable high-speed and directional diffusion of water nanodroplets confined by graphene sheets.

Diffusion of confined water is important in nanofluidic and other water transport systems. In this study, the diffusion of water nanodroplets confined by graphene sheets is investigated based on molecular dynamics simulations. We find that the confined water nanodroplets can achieve a high-speed and directional motion. The impact of the size of water nanodroplets and distance of graphene sheets on diffusion is studied. The results show that the diffusion of confined water nanodroplets is adjustable and the speed is about 3 orders of magnitude faster than that of the self-diffusing water molecules in liquid water. Subsequently, the most suitable morphology of confined nanodroplets for rapid movement is found. We also find that the direction of diffusion of confined water nanodroplets is affected by the thermal vibrations of carbon atoms. Finally, the interaction energy and friction coefficient between confined nanodroplets and graphene sheets are analyzed to give an insight into the fast and directional diffusion behaviors of water nanodroplets. Our results reveal that a variation in the structure of interfacial water molecules with the distance of graphene sheets is the key to the rapid movement of confined water nanodroplets. The phenomena reported here can enrich the knowledge of molecular mechanisms for nanoconfined water systems, and may stimulate more ideas for the rapid removal of confined water.