  We demonstrate the temperature-mediated and completely reversible process of
desorption-readsorption of water on a few layers of epitaxial graphene on a
4H-SiC(0001) substrate. We show that under ambient conditions water forms solid
structures on top of the second and third layers of graphene. In the case of
strained or highly defective graphene domains, these features produce strongly
correlated and reproducible patterns, implying importance of the underlying
defects for the initial stages of water adsorption. Hydrophobicity increases
with number of graphene layers. Evolution of the water layer as a function of
temperature is accompanied by a significant (two-fold) change of the absolute
surface potential difference between one and two layers of graphene. In situ
observation of water evolution during heating also potentially provides a
direct method for measurement of the heat adsorption on the nanoscale.
