From lotus effect to petal effect: Tuning the water adhesion of non-wetting rare earth oxide coatings

A facile post-deposition plasma treatment was developed to tune the wetting behavior of Yb₂O₃ coatings deposited via the solution precursor plasma spray process. The as-deposited coatings exhibited the lotus effect after vacuum treatment, with a water contact angle of ～162° and a roll-off angle of ～5°, due to the hierarchical columnar microstructure. When the plasma torch was scanned over the coatings as a post-deposition treatment, the rough top surfaces of the columns present in the as-deposited coatings became smooth, and the coatings then exhibited the petal effect, with a water contact angle of ～150° to ～130° and high, tunable adhesion to water. Analysis of the surface compositions showed that after vacuum treatment hydrocarbon adsorption on the as-deposited coatings and plasma-treated coatings was very similar. A mathematical model correlating surface structures with wetting behaviors was developed to elucidate the transition from the lotus effect to the petal effect.