Wetting-transparent graphene films hydrophobic water-harvesting surfaces

Graphene coatings can be wetting-transparent, even for roughened copper surfaces, which are strongly hydrophobic and significantly less wettable than graphene alone.

Article  |  Spring 2015

Over the last two years there has been a vigorous debate in the science community regarding the wetting properties of free-standing graphene as well as graphene supported on substrates. The terms “wetting-transparent” and “wetting-opaque” mean that, when wet, a substance is transparent and opaque, respectively. The current understanding of the wetting of graphene-coated surfaces is that graphene is wetting-transparent, provided that the underlying surface is significantly more wettable than graphene, while it is wetting-opaque for surfaces that are less wettable. This wetting-opacity of graphene on strongly hydrophobic surfaces presents a major problem for practical applications.

Rough or textured surfaces with entrapped air pockets are strongly hydrophobic. Such textured surfaces have tremendous applications in surface science, from water harvesting, anti-fouling, self-cleaning, lab-on-chip devices to low friction and drag coatings, as well as in two-phase heat transfer applications such as pool-boiling and condensation. Coating such devices with graphene could dramatically enhance their corrosion resistance and chemical stability. However, since graphene is wetting-opaque for such surfaces, coating the surface with graphene will incur a significant loss in water-repellency from the super-hydrophobic range down to the graphene value. This wetting-opacity severely limits the use of graphene coatings for super-hydrophobic surfaces. Super-hydrophobic surfaces represent one of the most intellectually rich and practically significant areas of surface science. Hence, it is critical to establish whether or not graphene coatings can be used to retain the extreme water repellency of hydrophobic metal surfaces.

Prof. Il-Kwon Oh’s research group in the division of ocean system engineering at KAIST shows that, contrary to current understanding, graphene coatings can be wetting-transparent even for roughened copper surfaces, which by virtue of their entrapped air-pockets are strongly hydrophobic and significantly less wettable than graphene itself. The graphene film in this case is grown in situ by chemical-vapor-deposition on the roughened copper surface. The graphene coating, therefore, conforms perfectly to the roughness features of the copper substrate as opposed to previous studies with transferred graphene films that simply sat on top of the roughness features. The authors show that such wetting-transparent graphene films offer excellent resistance to copper corrosion while maintaining the intrinsic hydrophobicity of the surface, enabling superior performance for water-harvesting applications. Such graphene-coated hydrophobic surfaces enable enhanced droplet mobility, which is required in a variety of applications, such as heat exchangers, seawater desalination, dehumidifiers, anti-corrosive coatings, and drag reduction coatings.

References

Gun-Tae Kim, Su-Ji Gim, Seung-Min Cho, Nikil Koratkar* and Il-Kwon Oh*, “Wetting-Transparent Graphene Films for Hydrophobic Water-Harvesting Surfaces”, Advanced Materials (IF=15.409), Vol. 26, No. 30, 2014.08, pp. 5166-5172
Additional links for more information
http://onlinelibrary.wiley.com/doi/10.1002/adma.201401149/abstract
http://sdss.kaist.ac.kr/