Effect of hydrocarbon adsorption on the wetting of rare earth oxides

Water vapor condensation is a familiar phenomenon in everyday life and is routinely used in industry as an effective means of transferring heat. In typical industrial systems, condensed water forms a thin liquid film due to the high surface energy associated with many industrial materials. If condensation occurs on a low surface energy surface, the condensate tends to form discrete liquid droplets which shed at radii approaching the capillary length (≈2 mm for water) and refresh the surface for re-nucleation, resulting in an improvement in heat transfer performance of 5 - 7× compared to filmwise condensation. Current state-of-the-art dropwise technology relies on functional hydrophobic coatings, for example long chain fatty acids or polymers, which are often not robust and therefore undesirable in industrial conditions. However, since the 1960s, natural surface contamination due to hydrocarbon adsorption, particularly on noble metals (gold and silver), has been explored as an alternative to create dropwise condensing surfaces, with one paper demonstrating continuous dropwise condensation on gold for over five years. Unfortunately, the high price of noble metals prohibits this approach. The recent discovery of robust rare earth oxide (REO) hydrophobicity has generated interest for dropwise condensation applications due to material costs approaching 1% of gold. However, the underlying mechanism of REO hydrophobicity remains under debate. In this work, we show through experiments that REO hydrophobicity occurs due to the same hydrocarbon adsorption mechanism seen previously on noble metals. To investigate adsorption dynamics, we studied ceria (CeO₂) and holmia (Ho₂O₃) along with control samples of gold and silica via X-ray photoelectron spectroscopy and dynamic contact angle measurements as a function of time. The contact angle measurements started at ≈0 degrees on the argon plasma cleaned samples and increased asymptotically over time for every sample, with the rare earth oxides displaying hydrophobic (< 90 degrees) behavior at long times (< 4 days). The surface carbon atomic percent increased from ≈0% immediately after cleaning to an asymptotic value for each sample following the same trend as the contact angle measurements. This work not only indicates that the hydrophobicity of REO materials is due to hydrocarbon adsorption, but also provides insight into a new material group that could be used to promote stable dropwise condensation and improve heat transfer performance in industrial environments.