Understanding modulation of liquid molecule slippage along graphene surfaces is crucial for many promising applications of two-dimensional materials, such as in sensors, nanofluidic devices, and biological systems. Here, we use force measurements by atomic force microscopy (AFM) to directly measure hydrodynamic, solvation, and frictional forces along the graphene plane in seven liquids. The results show that the greater slip lengths correlate with the interfacial ordering of the liquid molecules, which suggests that the ordering of the liquid forming multiple layers promotes slip. This phenomenon appears to be more relevant than solely the wetting behavior of graphene or the solid-liquid interaction energy, as traditionally assumed. Furthermore, the slip boundary condition of the liquids along the graphene plane is sensitive to the substrate underneath graphene, indicating that the underlying substrate affects graphene's interaction with the liquid molecules. Because interfacial slip can have prominent consequences on the pressure drop, on electrical and diffusive transport through nanochannels, and on lubrication, this work can inspire innovation in many applications through the modulation of the substrate underneath graphene and of the interfacial ordering of the liquid.
Keywords: friction; graphene; interfacial structure; slip; substrate-induced doping; transition state.