Objective The wettable functional surface in nature continues to be a great impetus to application of functional surfaces, which attracted widespread attention. Inspired by the porous surface of plants such as moss, many studies on the surface of superhydrophilic micro-nano structure have been carried out, which demonstrate that it has the ability to anti-fog and heat transfer enhancement. To further confirm that the surface wettability is unstable which will change with storage time in dark. Therefore, maintaining the stability of surface wetting has been challenging for the surface application of hydrophilic micro-nano structures. The current studies of wettability transition mechanism are built on different material surfaces and different storage environments, no systematic study concerning the influence of different wetting transition mechanisms on a single material has been published yet. In addition, there are relatively few studies devoted to surface wettability and surface morphology are also closely related, and for the same conversion mechanism, the effect of different surface morphologies on the overall macroscopic wettability. Therefore, it is necessary to comprehensively study the transformation and suppression of surface wettability from the aspects of surface morphology and surface energy, which is of great importance for maintaining the stability of the wettability of hydrophilic surfaces, and find application where the functional surfaces with hydrophilic properties is desired.

Methods The employed laser was an amplified Ti: sapphire femtosecond laser system that generates light pulse at central wave-length of 800 nm, with pulse energy of 4 mJ and pulse duration of 50 fs at a 1 kHz repetition rate. Laser with different laser energy densities (adjusted via energy attenuator) were used to manufacture two kinds of sub-micron and columnar structure surfaces on the surfaces (304 stainless steel) via laser direct writing, denoted as S_L and S_H, respectively. The samples without chemical treatment were marked as S_L-Air and S_H-Air, respectively, and the sample after soaking in sodium hydroxide solution were marked as S_L-NaOH and S_H-NaOH, respectively, and all of samples were stored in a same dark environment. Optical contact angle measurement equipment was used to measure the contact angle and infiltration time every seven days in order to draw curves of contact angle with time. Meanwhile, scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) were used to analyze the change of surface morphology and chemical composition.

Results and Discussions Initially, 304 stainless steel surfaces with different structures had excellent hydrophilicity, but the changes of surface contact angle were more obvious for the surface via low-energy compare with high-laser energy (Fig. 3). Generally, the surface wettability is mainly affected by the surface morphology and surface energy. But, the surface morphology of S_L was relatively flat, the influence of the surface morphology on the wetting characteristics could be ignored. Therefore, the change in the surface wettability was mainly depends on surface energy which caused by the adsorption of foreign chemicals at the surface defect sites. According to XPS analysis, the defect sites on the S_L-Air could adsorb organic matter in the air (Fig. 6), resulting in a decrease in surface energy and reduced its wetting. The wettability of S_L-NaOH also decreased (Fig. 3), this was because the surface defect sites of S_L-NaOH were first occupied by hydroxyl groups, and then as the sample was stored in the air for time, the hydroxyl groups were replaced by oxygen (Fig. 8). But its surface energy decreased slower than S_L-Air, so the rate of the contact angle change in S_L-NaOH was smaller, which was the sodium hydroxide solution immersion method delays the transition of surface wettability. As for the surface process via high-laser energy, the contact angle of the sample surface was still 0° for a period of time in part due to the fact that the surface wettability was not only affected by surface energy, but also surface morphology and capillary effect. The increase in the infiltration time (Fig. 4) reflects the surface energy also decreased. However, the presence of roughness and capillary effect could slow down the influence of surface energy on wettability, resulting in a relatively stable surface in wettability for a period of time.

Conclusions In this article, the wettability changes of 304 stainless steel surface after femtosecond laser irradiation were systematically studied. The contact angle measurement and surface energy spectrum analysis showed that the change in wettability, which mainly related to the adsorption of surface chemical substances. The change of surface wettability without chemical treatment was mainly related to the adsorption of organic substances while for the surface after soaked in sodium hydroxide was not, which demonstrates that the post-treatment could effectively intervene in the process of wetting transition and change the transformation mechanism (from the original organic matters adsorption dominanced to oxygen adsorption dominances), therefore make an optimization for the hydrophilic properties of the surface. The way of post-processing intervention is expected to become a new method to maintain the hydrophilic properties of the surface which has a more significant impact on the surface with low roughness prepared by low laser energy. Moreover, when a superhydrophilic surface with a larger degree of roughness is fabricated, the superhydrophilicity can be maintained for a longer period of time due to the stronger capillary effect even though the wettability is reduced. Therefore, this article provides an effective method and strategy for maintaining the hydrophilicity of the surface in terms of micro-nano structure and surface chemical composition, which is of great significance for the application of hydrophilic surfaces.