Study on the influencing factors of spontaneous wettability transition behaviour on metallic-based surfaces

Hydrophobic surfaces and coatings have attracted more and more interest in recent years due to their broad applications in diverse areas. In many cases, a metallic surface would demonstrate a reversible wettability from hydrophilic state to hydrophobic state spontaneously. However, the wetting behavior and hydrophobic mechanism of metallic surfaces are far from clear. In this work, 304 stainless steel (SS) surface and Ni–Cu–P coatings have been chosen to investigate the intrinsic mechanism of the surface wettability transition with the help of X-ray photoelectron spectroscopy (XPS) and ambient air/vacuum storage treatment. The surface topographies, wetting behaviors and surface chemistry were studied systemically. The results showed that water contact angle (WCA) variation of the as-prepared surfaces changing from hydrophilic state to hydrophobic state is largely associated with the specific surface area. The recovery of water repellency is related to a time constant which mainly depends on micro-scale surface convolution or surface roughness. Rougher surface tends to accelerate the surface adsorption of airborne hydrocarbon species in the ambient atmosphere, leading to a more intensive WCA increase. The study provides a generic theoretical justification for relevant time-dependent wettability studies.