KEYWORDS
Stainless steel mesh, superhydrophobic, superhydrophilic, vacuum ageing, air ageing
Introduction
The wettability of a material’s surface is an important property that plays a crucial role in the wide range of surface-related phenomena’s and defines different ranges of applications where materials can be used. In the case of stainless steel, the industrial applications are countless and can be enhanced by controlling its surface properties. Surface properties of steel such as color, chemical stability, roughness, and wettability have received special attention in biomedicine, optics and surface chemistry [1-4]. Stainless steel in form of meshes is a robust, durable and flexible construction material. Recently, controlling the wettability state of the metal meshes by femtosecond laser-induced structuring has found applications for efficient oil-water separation [5,6]. Wetting is the ability of a liquid medium to maintain contact with a solid medium due to intermolecular interaction when in contact with each other. Static contact angle θ is a quantitative way to measure wettability at a solid-liquid interface [7]. In our work the liquid is water and is termed as static water contact angle θ w. Different strategies have been proposed to tune the wetting behavior of metal surfaces. Most of them require the use of coatings using suitable materials or plasma/chemical etching. Recently, controlling the extreme wetting properties of materials by laser texturing of the substrate has gained much attention because it is maskless, chemical free, facile and robust technique [8].
Laser texturing of metal surfaces is proven to be a reliable way to change wettability of materials, with transitions towards the superhydrophilic or superhydrophobic properties depending on the material surface energy. For low surface energy material like polytetrafluoroethylene (Teflon) laser structuring can enhance the superhydrophobic characteristic [9]. Whereas, for other freshly laser textured materials it yields superhydrophilic response [10,11]. Laser processing of metals in ambient air by femtosecond laser pulses produces metal oxides on the textured surfaces. Metal oxides possess high surface energy and typically behave as hydrophilic material because oxides favour formation of hydroxylated layer through hydrogen bonding [12]. Laser texturing leads to formation of various micro- and nanostructures on the surface of the material. Shape and content of these structures can be controlled by laser parameters, such as pulse energy, polarization, scanning strategy and so on. Generally, laser surface structuring is accompanied with functionalization step to alter or retain the desirable surface wettability of materials. Typically, extreme wettability of laser textured surfaces was achieved through toxic and complex chemical reagent causing harm to the environment.
Moreover, the wettability of laser-textured metal surfaces can be changed from hydrophilic to hydrophobic if the textured surface is exposed to air for an extended period of time. Such transitions were observed for stainless steel [11,13], aluminum alloys [10, 14] and other metal alloys [15,16]. This transition is also observed for micro/nanostructures textured using the pulsed lasers operating in the nanosecond, picosecond and femtosecond time domains.
Generally, surfaces after the laser treatment are hydrophilic (θw < 30°). Within one week of exposure in air, the wettability changes to hydrophobic (θ w ≈ 90°), and within two weeks or more the surfaces become superhydrophobic with θ w ≈ 150°. The exposure to air does not change the morphology of the micro/nanostructures. However, the surface is influenced by chemical changes. Different processing environments including O2, air, N2, CO2, and Ar during laser texturing of AISI 304 stainless steel were studied in [17]. In one study, the textured samples were kept in sterile individual plastic bags filled with air for 7 days. It was shown that after 7 days the surface textured in argon environment achieves θ w = 125°, whereas surfaces processed in oxygen, air, carbon dioxide, and nitrogen had θ w= 31°, 46°, 50°, and 83°, respectively. XPS study demonstrated a significant amount of carbon on the surface, indicating the adsorption of organic or biomolecules during storage in sterile individual plastic bags. Those studies have shown that laser processing ageing environment influences the wettability transition rate.
Analysis of laser textured aluminum was performed previously [10] to identify the mechanisms contributing to long term air exposure-induced wettability transition. After laser texturing in ambient air, aluminum oxide surfaces are known to have many polar sites composed of unsaturated aluminum and oxygen atoms, which lead to an overall hydrophilic surface [18]. The textured samples that were kept in N2, O2, and CO2atmosphere remained superhydrophilic and these gases do not contribute to the wettability transition of aluminum. It was observed in [8] that surfaces kept in air and an atmosphere with volatile polar organics (4-Methyloctanoic acid) demonstrated a significant transition of wettability towards the superhydrophobicity. XPS analysis has shown that the amount of carbon on these surfaces and the carbon to aluminum ratio increased drastically over the same time periods. These measurements indicated that adsorbed organic compounds from ambient air are likely to form a hydrophobic coating on the initially hydrophilic metal-oxide surface.
In the present work, the role of the processing atmosphere, as well as the influence of the long term ageing in ambient air of the laser-processed stainless steel meshes on their wettability, are investigated. The role of low-pressure and vacuum ageing was also tested. Complex geometrical form and curved nature of meshes is crucial and not easy by itself and it was already demonstrated in [6]. In our work we focused on influence of processing atmosphere and subsequent long-term ageing on the wetting behaviour of laser treated meshes. For these purposes stainless steel mesh samples were processed in atmospheres of pure gases, ambient air and low pressure (vacuum) condition. After processing, long-term air ageing (30 days) was used to define the final wettability properties. We used 5 different gases - N2, O2, CO2, Ar, SF6, with the first three being part of the ambient air, which allows to define the possible role of these air components on the wetting properties after long-term air ageing. We also sufficiently decreased the role of water vapor during laser processing by conducting laser structuring inside closed chamber in pure gas atmospheres. Ar and vacuum conditions present inert conditions, whereas SF6gas, which is not a component of ambient air, can potentially affects the etching and enriching of samples surfaces with fluorine and its compounds.
Experimental setup
Meshes were processed by laser radiation in the presence of five gases (N2, O2, CO2, Ar, SF6), as well as in ambient atmospheric air and in vacuum conditions. In contrast to [17] where the used gases were purged on the surface of the samples, which does not completely excludes the influence of ambient air. In our work the stainless steel meshes were laser processed in pure atmosphere of each individual gases introduced inside the vacuum pumped chamber. Also we are using the femtosecond laser source at 1030 nm wavelength while authors of [17] utilized the nanosecond source at λ = 532 nm. After processing, the laser-textured meshes were aged in the ambient air conditions for 30 days under the same conditions, while maintaining the constant temperature and humidity. To analyze the effect of vacuum ageing on the wetting characteristic, two different chambers were used. One of them utilized oil-free vacuum pump system, while second chamber used the pump containing oil as lubricant for the moving parts.