  A series of poly(methyl methacrylate) (PMMA) surfaces decorated by Cu
nanoparticles (NP) with gradually varied morphology were prepared by
high-pressure CO2 treatment at various time spans. Combining the
characterizations of transmission electron microscopy (TEM) and atomic force
microscopy (AFM), an accurate 3-dimensional view of the morphology of the
surfaces was presented. Subsequently, the wettability of the surfaces decreases
near-linearly with the increase of the apparent height of the decorating NPs in
both static (static contact angle) and dynamic (contact angle hysteresis)
aspects. The observed tendency contradicts to the Wenzel or Cassie-Baxter model
and is explained by the contribution of nanomeniscus formed between the
decorating NP and the flat substrate. The capillary pressure from this meniscus
is negative and results in the increase of the contact angle with the apparent
height (H_N) of the Cu NPs decorating the PMMA surface. In addition, the effect
of the coverage (C_N) by NPs on the wettability can be explained on the same
basis. Our experiment demonstrates the important influence of the nanomeniscus
on the wettability, which is usually not taken into account. The results in
this work provide a comprehensive understanding of how nanostructure affects
the wettability of the decorated surfaces and shed light on how to obtain
certain wettability through nanostructuring of the surface morphology.
