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A Simple Way to Achieve Self-Cleaning Surfaces with Unique Antifouling Property
Self-cleaning surfaces may have wide applications such as microfluidic devices, lab-on-a-chip, sensors, microreactors, air purification, and antimicrobial fields. In this article, by using a combination of femtosecond (fs) laser irradiation and fluorination technique, self-cleaning stainless steel surfaces with unique antifouling property were obtained. New insight is developed through a detailed analysis of the antifouling behavior of the self-cleaning surfaces. The surface free energy and its polar and disperse components were calculated by using the Owens–Wendt-–Rabel–Kaelble (OWRK) method. X-ray photoelectron spectroscopy was employed to analyse the surface elemental compositions and functional groups. The antifouling property of the surface was recorded by using a high speed camera. Water sliding angles (SAs) were reduced by fluorination treatment, resulting in low adhesive superhydrophobic surfaces with the self-cleaning property. The influences of micro/nanostructures, fluorination, and their combination on the surface free energy were investigated. The interaction process between water droplets and pollutants (inorganic and organic particles) on the treated surface was explored. The antifouling property of an optimized specimen (CA = 162° and SA = 1°) was tested and compared with the untreated sample.
Contaminants on surfaces may threaten the health of people and the quality of products. Cleaning methods, e.g., swiping, ultrasonic cleaning, illuminating, and water spraying have been used to maintain surface cleanliness, which consumes labor, energy, capital, and time. Recently, surface treatment with self-cleaning properties has generated worldwide interest [1, 2]. These surfaces are of great importance not only for the fundamental research but also for various practical applications. Antifouling paints/coatings are normally applied to metallic surfaces by using electrospinning [2, 3], spraying , vapor deposition , or electrochemical deposition . These paints/coatings have good self-cleaning property. However, their disadvantages of low adhesion, poor temperature resistance, and discoloration due to weathering or mechanical damaging severely limit their application in extreme conditions. Inspired by lotus leaf, self-cleaning property can be obtained by surface micro/nanostructuring. Water droplets pick up dirt particles and debris from a tilt surface and roll-off to realize self-cleaning [2, 7]. Metallic surfaces with self-cleaning property have huge potential applications in microfluidic devices, lab-on-a-chip, sensors, microreactors, air purification, and antimicrobial fields . For example, as the interior surfaces, the washing process for pipelines of milk or petroleum can be greatly simplified. The exterior surfaces would be cleaned by rain water without using detergent, saving the maintenance, and labor costs . Furthermore, ships’ body can be better protected due to the low adhesion of biological pollutants .
Femtosecond (fs) laser technique has proved to be a promising method to produce micro/nanostructures on surfaces for its unique abilities and advantages, including the following. (1) This is a maskless technique with 0–3 dimensional micro/nanoscale structures being able to form simultaneously . (2) It can be applied to a variety of materials such as polymers [11–15], metals [10, 16–19], and semiconductors  in various environments (e.g., air [7, 12–14, 16–18], gaseous atmosphere , vacuum, and liquid environments ). (3) Precise micro/nanostructure can be realized by controlling laser parameters and/or environments to fine-tune the surface wettability and adhesive property [9, 21]. (4) It has minimum thermal effect so that the property of substrate can be preserved [22, 23]. Despite its infancy, the outstanding micro/nanostructure fabrication of femtosecond laser has been raised worldwide attention, including Wolter from Laser Zentrum Hannover e.V. , Jeon from KIST Korea , Guo from University of Rochester [24, 25], Hatzikiriakos from UBC Canada , Li from USTC China , Chen from Xi’an Jiaotong University [7, 13, 15, 26], and Zhong from Tsinghua University . In his review article, Chen et al.  pointed out that laser microfabrication can realize special modulation ranging from superhydrophilic to superhydrophobic, allowing much more freedom to achieve complex multiple-wettability integration. Zhang et al.  adopted femtosecond laser irradiation technique to achieve the tunable adhesive superhydrophobic silicon surfaces and revealed that the adhesive forces of as-prepared surfaces can be tuned by varying the area ratio of superhydrophobic domain to hydrophobic domain. Jun et al.  generated microscale grooves on nanofibrous platforms using a femtosecond laser ablation process to develop engineered fibrous platforms with patterned hierarchical topographies. The fabricated platforms can regulate cellular adhesive morphology, proliferation, and distinct distribution of focal adhesion proteins. By tailoring the surface chemical composition and surface morphology, Moradi et al.  reported the effect of extreme surface wettability on platelet adhesion and activation in SS and Ti surfaces. Li et al.  reported the assembly of self-organized 3-dimensional porous metal micro/nanocages arrays on nickel surface by ethanol-assisted femtosecond laser irradiation. The underlying formation mechanism was also investigated. The 3D cage-like micro/nanostructures exhibit not only improved antireflection property but also enhanced hydrophobicity. However, the interaction behaviour and mechanism between water droplets and pollutants on self-cleaning surfaces were seldom reported.
In this paper, superhydrophobic surfaces were obtained by using one-step femtosecond laser irradiation. Its surface free energy was greatly reduced by a fluorination process to achieve excellent self-cleaning property. Surface morphologies, surface functional groups, surface free energy, static wettability, and dynamic wettability were characterized. The interaction behavior between water droplets and pollutants on self-cleaning stainless steel (SS) was investigated. The self-cleaning mechanism was also explored. This paper aims to bridge the gap between superhydrophobic surface and self-cleaning surface.
2. Experimental Process
2.1. Samples Preparation
Experiments were performed in a clean room at 23°C in ambient atmosphere. Mirror-like 304 SS coupons with a dimension of 20 mm × 20 mm × 2 mm were used as substrates. The schematic of experimental setting up can be referred to a previous study . In brief, a commercial Ti:sapphire chirped-pulse amplification laser system (Spectra Physics) was used to generate a 35 fs horizontally linear polarized laser with a central wavelength of 800 nm. The beam has a Gaussian profile. A neutral density (ND) attenuator was used to adjust the pulse energy. Femtosecond laser pulses were focused to e−2 spot diameter of 300 μm by using a 150 mm focal length lens. Substrates were mounted perpendicular to the laser beam on a precision computer-controlled 3D translation stage. The laser repetition rate is 1 kHz. The pulse number per spot was controlled by the combination of an electric shutter and the speed of translation stage. Laser scan irradiation mode was employed to get large and uniform surface areas for antifouling property tests.
In order to evaluate the surface morphologies of samples over a range of experimental parameters, the influence of fluence (F = 0.3, 0.5, 0.7, 1.0, 1.3, 1.5, 1.6, 1.9, 2.0 J/cm2, respectively) and pulse number per focal spot (PPS = 10, 20, 30, 100, 200, 300 pulses, respectively) were systematically investigated. After laser treatment, the fluorination process was performed to reduce surface free energy. Samples were cleaned by ultrasonic cleaning in water at 23°C for 10 min and then immersed into 1% fluoroalkylsilane (C14H19F13O3Si) solution for 30 min. The samples were immediately treated in a furnace at 135°C for 30 min. Mirror-like 304 SS coupons were fluorinated with the same procedure and tested for comparison purpose.
Images of surface morphologies and microstructures were obtained by using a Phenom scanning electron microscope (SEM) with an accelerating voltage of 10 kV. Samples were sputter-coated with a 5 nm gold layer before observation. Contact angles (CAs) and surface free energy were characterized by using interface tension meter (Kruss K100). Owens-Wendt-Rabel-Kaelble (OWRK) method was employed to calculate surface free energy and its polar and disperse components. The disparity between water and hexadecane in the appearance of CA was due to the difference of surface tension. Hexadecane has a lower surface tension of γHD = 26.7 mN/m in comparison with that of water γH2O = 72.0 mN/m. X-ray photoelectron spectroscopy (XPS) was used to test the surface elemental compositions and functional groups of specimens before and after fluorination. Al Kα was used as X-ray source. A step-scan mode was used at the binding energy ranging from 0 to 1360 eV with an energy step size 1 eV. Water bouncing and rolling behaviors on specimens were recorded by using a high speed camera (Pro.1200 hs). The interaction between water droplets and pollutants on self-cleaning SS was also monitored.
3. Results and Discussion
3.1. Surface Morphology and Microstructure
Systematic investigation of femtosecond laser-induced micro/nanostructures on 304SS was performed and a F-PPS model was built, which can be used to locate and fine-tune the laser settings to produce a specific surface microstructure in laser scan irradiation mode. As shown in Figure 1, two types of micro/nanostructures were observed: the laser-induced periodic surface structures (LIPSSs) on S1, S2, S3, and S4,and nodular structures on S5 and S6. LIPSSs on 304SS are located at F lower than 1 J/cm2. With increasing F, the nodular structures start to form. The two types of structures have different formation mechanism and different structure properties. This result is consistent with literatures [3, 11, 28], indicating that, at low fluences (<1 J/cm2), the ablation process is dominated by the optical penetration depth and, at high fluences (≥1 J/cm2), the electron heat diffusion length is the domination parameter. Interestingly, with increasing PPS from 5 pulses to 300 pulses at the same F, the structure types were kept almost the same. However, the dimensions of nodular structures increased dramatically, which may be due to the incubation effect [27, 29, 30]. Thus, in this F-PPS model, F determines the type of structures and PPS determines the dimensions of structures. This result is consistent with the numerical simulation results of Zhang et al. , who demonstrate that, according to the two-temperature model for multipulse laser irradiation, the lattice temperature will be much less affected if the time interval between two pulses is larger than the electron-lattice coupling time.
3.2. Surface Free Energy and Wettability Test
Surface free energy of a material is normally influenced by surface morphology and chemical composition. Their combined effects have been studied extensively. However, their respective influence on surface free energy and wettability of a material was seldom investigated, which is of great importance for the self-cleaning surface.
Table 1 and Figure 2 show the relations of surface free energies and CAs of specimens after femtosecond laser irradiation. The surface free energies vary significantly from 23.09 ± 0.44 mN/m to 60.12 ± 0.17 mN/m in comparison to the original 304SS (42.22 ± 1.98 mN/m). Tested surface free energy of S1 and S2 is lower than that of original 304SS, and it grows with increasing accumulated energy density. The water contact angle is slightly decreased with the increasing of surface free energy. S1 has a CA of about 152°, showing superhydrophobicity. S6 has a CA of about 9°, showing hydrophilicity. Similar results were reported by Pfleging et al.  on nanosecond laser irradiated titanium alloy. Low surface energy and decreased wettability may reduce the interaction between the metal surface and contaminants.
Table 2 and Figure 3 show the surface free energies are significantly reduced and CAs are greatly increased after fluorination. The surface free energy of fluorinated S1–S6 is much lower than that of 304SS (17.47 mN/m) with the same fluorination process. All specimens show near superhydrophobic property (∼150°) while fluorinated 304SS has CA = 103°. It can be concluded that proper micro/nanostructure fabrication and fluorination can be used to reduce surface free energy, and different micro/nanostructures have different fluorination effects.
According to the OWRK method, surface free energy is composed of disperse and polar components. Thus, a detailed analysis is performed on SS, S1, and S6 and their fluorinated samples FS, FS1, and FS6, to further understand the influence of fluorination on the free energy components. As shown in Table 3 and Figure 4, the values of polar component (P) and disperse component (D) of the three samples before fluorination follow the order of PS1 < PSS < PS6 and DS1<DSS < DS6. PS1 and DS1 are significantly lower than those of original 304SS and S6. After fluorination, the polar component and disperse component of these samples were greatly reduced. Both PFS1 and PFS6 are so small that can be ignored in comparison with PFS.
Polar component has significant effect on wettability. Polar molecules can enhance the wettability by interaction with dipole force and hydrogen bonds . Thus, before fluorination, the water contact angle of untreated 304SS is smaller than that of S1 and lager than that of S6. S1 shows superhydrophobicity due to the reduction of polar component. After fluorination, FS1 and FS6 show superhydrophobicity due to the elimination of polar component. FS6 has a lower surface free energy in comparison with FS1, which may be due to its large specific surface area. The deposition of more hydrophobic functional groups during the fluorination process can reduce the surface free energy of FS6. The relatively large polar component on FS can be used to explain why the fluorinated untreated 304SS surface does not have superhydrophobicity.
3.3. Surface Functional Groups and Adhesive Property
The dynamic wettability of original 304SS, superhydrophobic S1, and hydrophilic S6 samples was investigated by using high speed camera in Figure 5. A water droplet of 8 μl fell from the same height above the tested surface. S1 shows the absolute water repelling property, and S6 shows much higher wettability in comparison with the original 304SS due to the different types and dimensions of micro/nanostructures as shown in Figure 1.
S1 has a CA of 152°; however, it shows adhesive property (Figure 6(a)), which is unfavorable for the formation of self-cleaning surface. A self-cleaning surface should have a high water repelling property and a low adhesive property to contribute the water droplet rolling process . After treated by a fluorination process, the adhesive force is greatly reduced as shown in Figure 6(b). XPS analysis of S1 in Figure 7 and Table 4 indicates that this may be related to the formation of hydrophobic functional groups, e.g., -C-F, -C-F2, and -C-F3. The peak position of F1s locates at 688.9 eV. The atomic percent of -C-F2 increases to 26.94% after fluorination. During the fluorination process, metallic bonds and –OH will be replaced by hydrophobic groups. Hydrophobic groups have less adhesive force to water droplets, which contribute to the self-cleaning property.
3.4. Wettability Models
Figure 8 shows the wettability model of SS, S1, and S6 before and after fluorination. It has been reported that micro/nanobinary structures on solid surfaces can effectively enhance hydrophobicity of the surfaces [34, 35], giving the corroboration for the difference of hydrophobicity between the polished 304SS and laser treated surfaces. SS has a CA less than 90° due to the existence of hydrophilic metallic bonds or hydroxyl groups. As shown in Figure 8(b), the superhydrophobicity of S1 comes from its microstructures (Figure 1) and can be explained by Cassie model : when a water droplet is put on the surface of S1, the droplet will only contact with the protuberance of the LIPSSs, covered by hydrophilic metallic bonds or hydroxyl groups. Meanwhile, air bubbles are trapped in the bottom, greatly reducing the contact area between the water droplet and sample. The hydrophilic property of S6 may be attributed to the growing of microstructures. In this case, the sample has a large surface area covered by hydrophilic metallic bonds or hydroxyl groups. Water will take the place of air and infiltrate into the microstructures as shown in Figure 8(c).
The three samples are further fluorinated as shown in Figures 8(d)–8(f). During the fluorination process, hydrophilic metallic bonds or hydroxyl groups are replaced by hydrophobic functional groups, which, combined with the trapped air bubbles, could be the reason for the excellent superhydrophobicity of FS1 and FS6. This result is consistent with literature . The process of fluorination relies on the reaction of fluoroalkyl silane with -OH groups, leading to significant decrease in surface energy of sample. Moreover, heating in a furnace at 135°C may help to eliminate the surface hydroxyl groups, which could also be a reason for the hydrophilic-to-hydrophobic transition of samples.
3.5. Antifouling Modes
The interaction between water droplet and pollutant particle is crucial for the self-cleaning property. The most important adhesive forces between the pollutants and surface are the van der Waals’ and electrostatic forces. The cleaning process requires a repelling force larger than the adhesive force. Thus, the surface free energy of a specimen must be controlled as small as possible. In this research, the interaction behaviors between water droplets and different types of particulate pollutants on self-cleaning SS were monitored and recorded. Three types of antifouling modes were built.
Figure 9(a) is a series of photos taken by using a high speed camera, showing the “rolling” process of a water droplet on a contaminated surface. The surface is tilted with an angle of 5° to perform the rolling process. The water droplet has a volume of 8 μL and provides a shear force to the contaminant (a SiO2 particle indicated by the white arrow). When the shear force is large enough to overcome the adhesive force between the contaminant and surface, the water droplet will lift the contaminant up from the surface. Figure 9(b) is a schematic of the rolling cleaning process.
Figure 10(a) is a series of photos taken by a high speed camera, showing the “rebounding” process of a water droplet on a contaminated surface, which is tilted for 5°. SiO2 particles are indicated by white arrows. A water droplet with a volume of 8 μL came out from a needle, spreading on the surface, collected the contaminant from the surface, and rebounded back to the air. Interestingly, the “rebounding” mode has a much higher cleaning efficiency than the rolling mode due to the large spreading area of the water droplet, which depends on the initial energy of the droplet and the status of the surface. Figure 10(b) is a schematic of the “rebounding” process.
Figure 11(a) is the photos showing the contaminant absorbing process by a water droplet on a surface. Contaminants are sponge particles. A water droplet of 4 μL is suspended on the tip of a needle. When it is close to the contaminant particle, the particle is absorbed into the droplet due to the high surface tension. The water droplet does not need to contact with the surface to make it clean, which can be regarded a damage-free cleaning process. Figure 11(b) is a schematic of the “absorbing” cleaning mode.
This article presents techniques to make self-cleaning stainless steel (SS) surface, including tailoring surface morphology by femtosecond laser irradiation and reducing surface free energy by fluorination. Results revealed that different micro/nanostructures including laser-induced periodic structures and nodular structures were obtained on 304SS. Fluorine-free surfaces with controllable water contact angles (CAs) from hydrophilic to superhydrophobic were obtained by using one-step femtosecond laser scan, which are desirable for microfluidic devices, lab-on-a-chip, sensor, and microreactor applications. Surface free energies of laser irradiated surfaces vary significantly from 23.1 ± 0.4 mN/m to 60.1 ± 0.2 mN/m comparing to the untreated 304SS (42.2 ± 2.0 mN/m); and the water contact angle roughly decreased with the increasing surface free energy. After fluorination treatment, the surface free energies of all specimens were reduced and CAs were much increased. All specimens showed near superhydrophobic property (CA = ∼150°) except for the fluorinated 304SS (103°). The adhesive property of laser irradiated surface was reduced due to the formation of surface hydrophobic functional groups such as C–F, -CF2, and -CF3. A wettability model of specimens before and after fluorination was proposed, in which the influence of micro/nanostructures, fluorination, and their combined effects on CAs were analysed. The interaction behaviors between water droplets and different types of particulate pollutants on self-cleaning SS were monitored and recorded. Three particulate cleaning modes, rolling, rebounding, and absorbing modes, have been suggested.
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This work was supported by the National Natural Science Foundation of China (NSFC) (Grant nos.: 51701087 and 51535003) and Special Foundation for Young Scientists of LFRC.
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