  Carbonous materials, such as graphene and carbon nanotube, 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 carbonous layer, which is
critical for applications such as sustainable cooling of electronic devices,
remains unknown. In this paper, using colloidal probe atomic force microscope,
we measured the slip length of water on graphene ls supported by hydrophilic
and hydrophobic substrates, i.e., SiO2 and octadecyltrimethoxysilane (OTS). The
ls on single-layer graphene supported by SiO2 is found to be 1.6~1.9 nm, and by
OTS is 8.5~0.9 nm. With the thickness of few-layer graphene increases to 3~4
layers, both ls gradually converge to the value of graphite (4.3~3.5 nm). Such
thickness dependence is termed slip length translucency. Further, ls is found
to decrease by about 70% with the temperature increases from 300 K to 350 K for
2-layer graphene supported by SiO2. These observations are explained by
analysis based on 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 guideline for the design of
nanofluidics-based devices, for example the thermo-mechanical nanofluidic
devices.
