Ultralow friction of ion-containing water nanodroplets

We reveal the ultralow friction or superlubricity of water nanodroplets containing cations and anions on graphene substrates at high ion concentration by molecular dynamics simulations. When the ion concentration is higher than 7 wt.% and the nanodroplet diameter is larger than 10 nm, the friction coefficients of water nanodroplets are lower than 10⁻², and can decrease to the order of 10⁻³ with increasing the ion concentration further. At a certain ion concentration, the optimal nanodroplet diameter of 17–20 nm exists at which the friction coefficient is the lowest. The ultralow friction behaviors of water nanodroplets containing cations and anions are mainly attributed to the opposite variation trends between the interfacial adhesion energy and surface energy of water nanodroplet with ion concentration, and the interfacial hydrophobicity sustained by high ion concentration. These results unveil the essential role of ions in achieving the superlubricity of water nanodroplets. [Figure not available: see fulltext.]