  The wetting properties of graphene have proven controversial and difficult to
assess. The presence of a graphene layer on top of a substrate does not
significantly change the wetting properties of the solid substrate, suggesting
that a single graphene layer does not affect the adhesion between the wetting
phase and the substrate. However, wetting experiments of water on graphene show
contact angles that imply a large amount of adhesion. Here, we investigate the
wetting of graphene by measuring the mass of water vapor adsorbing to graphene
flakes of different thickness at different relative humidities. Our experiments
unambiguously show that the thinnest of graphene flakes do not adsorb water,
from which it follows that the contact angle of water on these flakes is ~180o.
Thicker flakes of graphene nanopowder, on the other hand, do adsorb water. A
calculation of the van der Waals (vdW) interactions that dominate the
adsorption in this system confirms that the adhesive interactions between a
single atomic layer of graphene and water are so weak that graphene is
superhydrophobic. The observations are confirmed in an independent experiment
on graphene-coated water droplets that shows that it is impossible to make
liquid 'marbles' with molecularly thin graphene.
