Translucency and negative temperature-dependence for the slip length of water on graphene

Carbonous materials, such as graphene and carbon nanotubes, have attracted tremendous attention in the fields of nanofluidics due to the slip at the interface between solid and liquid. The dependence of slip length for water on the types of supporting substrates and thickness of the carbonous layer, which is critical for applications such as sustainable cooling of electronic devices, remains unknown. In this paper, using colloidal probe atomic force microscopy, we measured the slip length l_s of water on graphene supported by hydrophilic and hydrophobic substrates, i.e., SiO₂ and octadecyltrimethoxysilane (OTS). The l_s on single-layer graphene supported by SiO₂ is found to be 1.6 ± 1.9 nm, and that of OTS is 8.5 ± 0.9 nm. When the thickness of few-layer graphene increases to 3-4 layers, both l_s values gradually converge to the value of graphite (4.3 ± 3.5 nm). Such a thickness dependence is termed slip length translucency. Further, l_s is found to decrease by about 70% when temperature increases from 300 K to 350 K for 2-layer graphene supported by SiO₂. These observations are explained by analysis based on the Green-Kubo relation and McLachlan theory. Our results provide the first set of reference values for the slip length of water on supported few-layer graphene. They can not only serve as a direct experimental reference for solid-liquid interaction, but also provide a guideline for the design of nanofluidics-based devices, for example thermo-mechanical nanofluidic devices.