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.