Aging effect on the wettability of stainless steel

Commercial metals have recently been reconsidered as useful materials for functionalized surfaces with controlled wettability, yet the intrinsic wettability of various metal surfaces remain uncertain. In this study, we report that the wettability of a stainless steel (Type 304) surface exhibits a time-dependent transition, initially from a hydrophilic state to a relatively hydrophobic state. The static contact angle of a water droplet was monitored on polished stainless steel surfaces every 24 h, and a rapid increase in the contact angle, from 44 to 81, was found during the first three days. The contact angle reached 98 in 40 days. To clarify the main factor causing the phenomenon, we compared the contact angles of water stored in four different environments for five days, where the oxygen and moisture levels were controlled. From the difference in contact angles depending on the amount of oxygen, we reached the conclusion that oxygen is likely the main cause of the aforementioned phenomenon. Finally, we demonstrated time-dependent wettability on a micro-grooved stainless steel surface whose initial super-hydrophilicity became superhydrophobic after 30 days. (C) 2016 Published by Elsevier B.V.