title: Exploring the Role of Adsorption and Surface State on the Hydrophobicity of Rare Earth Oxides.
abstract: Rare earth oxides (REOs) are attracting attention for use as cost-effective, high-performance dropwise condensers because of their favorable thermal properties and robust nature. However, to engineer a suitable surface for industrial applications, the mechanism governing wetting must be first fully elucidated. Recent studies exploring the water-wetting state of REOs have suggested that these oxides are intrinsically hydrophobic owing to the unique electronic structure of the lanthanide series. These claims have been countered with evidence that they are inherently hydrophilic and that adsorption of contaminants from the environment is responsible for the apparent hydrophobic nature of these surfaces. Here, using X-ray photoelectron spectroscopy and dynamic water contact angle measurements, we provide further evidence to show that REOs are intrinsically hydrophilic, with ceria demonstrating advancing water contact angles of ≈6° in a clean surface state and similar surface energies to two transition metal oxides (≳72 mJ/m2). Using two model volatile species, it is shown that an adsorption mechanism is responsible for the apparent hydrophobic property observed in REOs as well as in transition metal oxides and silica. This is correlated with the screening of the polar surface energy contribution of the underlying oxide with apparent surface energies reduced to <40 mJ/m2for the case of nonane adsorption. Moreover, we show that the degree of surface hydroxylation plays an important role in the observed contact angle hysteresis with the receding contact angle of ceria increasing from ∼10° to 45° following thermal annealing in an inert atmosphere. Our findings suggest that high atomic number metal oxides capable of strongly adsorbing volatile species may represent a viable paradigm toward realizing robust surface coating for industrial condensers if certain challenges can be overcome.
authors.affiliation: Thermal Management Research Group, Efficient Energy Transfer (ηET) Department, Bell Labs Ireland, Nokia , Blanchardstown Business & Technology Park, Snugborough Rd, Dublin 15, Ireland.|Stokes Laboratories, University of Limerick , County Limerick V94 T9PX, Ireland.
authors.forename: Ross
authors.identifier: http://orcid.org/0000-0002-1329-8614
authors.initials: R
authors.lastname: Lundy
authors.affiliation: School of Physical Sciences, Dublin City University , Glasnevin, Dublin 9, Ireland.
authors.forename: Conor
authors.initials: C
authors.lastname: Byrne
authors.affiliation: School of Physical Sciences, Dublin City University , Glasnevin, Dublin 9, Ireland.
authors.forename: Justin
authors.initials: J
authors.lastname: Bogan
authors.affiliation: Thermal Management Research Group, Efficient Energy Transfer (ηET) Department, Bell Labs Ireland, Nokia , Blanchardstown Business & Technology Park, Snugborough Rd, Dublin 15, Ireland.
authors.forename: Kevin
authors.initials: K
authors.lastname: Nolan
authors.affiliation: Stokes Laboratories, University of Limerick , County Limerick V94 T9PX, Ireland.
authors.forename: Maurice N
authors.initials: MN
authors.lastname: Collins
authors.affiliation: Stokes Laboratories, University of Limerick , County Limerick V94 T9PX, Ireland.
authors.forename: Eric
authors.initials: E
authors.lastname: Dalton
authors.affiliation: Thermal Management Research Group, Efficient Energy Transfer (ηET) Department, Bell Labs Ireland, Nokia , Blanchardstown Business & Technology Park, Snugborough Rd, Dublin 15, Ireland.
authors.forename: Ryan
authors.initials: R
authors.lastname: Enright
keywords: adsorption; dropwise promoter; heat transfer; hydrophobic; metal oxide; rare earth oxide; water contact angle; wetting hysteresis