《1. Introduction》

1. Introduction

Controlled modification of a material’s surface wettability has been the subject of significant scientific research due to its importance for various applications, which include easy cleaning, adhesion mitigation, and microfluidic and biomedical applications. In terms of wettability, the water contact angle (WCA) and contact angle hysteresis are affected by both material surface chemistry and topology [1]. Based on the Wenzel [2] and Cassie-Baxter [3] models, the roughness of the sample surface is critical in increasing its hydrophobicity or hydrophilicity. Surface morphology is one of the key factors determining the wettability of a solid surface. Various techniques have been used for surface texturing in order to create the desired surface morphologies. These techniques can be classified into two major categories: top-down processes such as the lithographic process, the template-based method [4], plasma treatment [5,6], and the laser surface treatment process [7,8]; and bottom-up processes that mainly consist of self-assembly and self-organization processes [9,10]. As a top-down approach, laser micro-and nano-processing has been extensively investigated and is developing rapidly. Picosecond (ps) and femtosecond (fs) ultrafast lasers are the most employed tools for surface texturing. The ps or fs laser pulse-induced ablation is able to produce well-controlled two-scale micro- and nano-scale surface textures [11–13] in order to fabricate certain types of surfaces with improved surface functional properties.

Thus far, a significant amount of research has been carried out on the surface treatment and modification of various types of materials, such as metallic [11,12,14–16], semiconductor [17], and polymeric [18–20] materials, to modify their wetting properties using various lasers. The mechanisms involved in the wettability conversion of structured metallic surfaces from super-hydrophilicity to super-hydrophobicity have also been well studied [12,21–22]. The mechanism for the wettability change has been found to be dependent on the specific material to be laser beam treated. Regarding laser surface-treated stainless steel, Kietzig et al. [12] have proposed that the conversion in wettability from the original hydrophilicity to super-hydrophobicity after laser treatment is due to the accumulation of a certain amount of carbon onto the laser-treated surface through the dissociation of carbon dioxide (CO2) into carbon, with the active magnetite acting as a catalyst. For copper (Cu) [21], it is the adsorption of organic materials from the surrounding environment that induces the change in wettability from hydrophilic to highly hydrophobic. For lasertextured nickel (Ni) [22], the change in wettability from hydrophilicity to super-hydrophobicity has been attributed to the accumulation of a thin layer of carbon on the laser-textured Ni surface. Stavax steel is well known to be the metal that is most commonly used to fabricate mold inserts for the large-volume production of plastic or polymer products, including microfluidic devices and consumer products. The laser-produced micro-/nanoscale surface patterns on a Stavax mold surface can be duplicated onto the molded polymer product surfaces so as to improve the product’s surface functional properties such as superhydrophobicity; this is beneficial for the production of consumer plastic products with the property of easy cleaning, among other useful properties.

In this study, ps laser surface texturing on Stavax steel was systematically investigated. Three types of surface textures were demonstrated: periodic ripples, a two-scale hierarchical two dimensional (2D) array of micro-bumps, and a micro-pits array with nano-ripples. After the laser-texturing treatment, the Stavax steel initially showed hydrophilicity with a WCA of less than 10°; it then gradually acquired high hydrophobicity, and finally acquired super-hydrophobicity with a WCA of more than 150°. This wettability conversion is considered to be due to a laser irradiation-induced change in the surface topography and chemistry through the fabrication of a two-scale hierarchical 2D surface structure and the rendering of a carbon formation, resulting in a decrease of the surface energy. Laser-induced surface textures on metal mold substrates were then replicated onto polypropylene substrates via the polymer injection molding process. The surface wettability of the molded polypropylene was found to be changed from the original hydrophilicity to super-hydrophobicity, with a WCA of more than 150°.

《2. Experimental》

2. Experimental

A 5 mm thick Stavax steel plate with mirror finishing on one side was used for the laser surface texturing. The Stavax steel plate first underwent sonication cleaning with acetone and methanol. A Time-Bandwidth’s Duetto ps laser system was employed in the experiments to perform the laser surface treatment. The laser had a Gaussian-profiled spatial mode (TEM00) with a beam quality of M2 < 1.3 (M2 represents the degree of variation of a beam from an ideal Gaussian beam) and a beam divergence of less than 0.3 mrad. The laser wavelength was near-infrared at 1.06 μm, the output power was up to 10 W, and the laser pulse width was 10.3 ps. The pulse repetition rate could be set to a minimum value of 50 kHz and continuously tuned up to 8200 kHz. The incident laser beam was focused and manipulated by a galvo-scanner, which was equipped with a telecentric f-θ lens. The focal length of the f-θ lens was 100 mm. The linear polarized raw laser beam had a diameter of 7 mm (at 1/e2, which is the distance between points where the intensity falls to 1/e2 = 0.135 times the maximum value for a Gaussian beam), which was focused and directed onto the sample surface with a beam diameter of 25 μm. The laser machining control software that was used was able to produce various machining patterns, including lines, circular or rectangular shapes, hatched patterns, and so forth. The maximum scanning speed of the galvo-scanner was 2000 mm·s-1. For the surface treatment process, the focused laser beam either raster scanned a designed pattern or performed spot-by-spot ablation with a certain pulse number on each spot. The effects of the following factors were investigated: the rate of repetition, velocity of scanning, laser power density, pass number, and pulse number on each spot. In addition, we investigated the effect of the pitch of the hatched pattern on properties of the produced surface textures such as morphology, feature size, and uniformity of the produced pattern.

After laser treatment, the sample surfaces were characterized using scanning electron microscopy (SEM). The nano-features were characterized using atomic force microscopy (AFM). The surface wettability was characterized by measuring the WCA. Using the sessile drop method, the WCA was obtained with a VCA Optima contact angle measurement system (VCA-2500XE, AST Products, Inc.). During the WCA measurement, a water droplet with a specific volume was created with a syringe connected to a capillary tip. The water droplet was then gently detached onto the substrate. The image of the droplet was recorded with a camera and analyzed with the software to determine the tangent line for the contact angle. The chemical status and elemental composition of the original and laser-treated sample surfaces were analyzed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific).

《3. Results and discussion》

3. Results and discussion

Fig. 1 schematically shows the two main types of designed patterns that were employed to produce the desired texturing. The line-hatched pattern was produced by scanning the focused laser beam along the regular hatched raster pattern, as shown in Fig. 1(a), so as to yield desired surface textures such as a microbumps array pattern. The dot array pattern, as shown in Fig. 1(b), was produced via dot-by-dot ablation with the laser beam so as to yield a 2D array of the micro-pits texturing pattern. In addition to the ultrafast laser-induced self-organizing effect, we investigated the dependence of the induced surface patterns on the different laser parameters, including the laser fluence, pulse repetition rate, beam-scanning velocity, pass number or pulse number on each spot, and hatching density. It was found that three types of surface texture patterns could be produced: periodic ripples, a two-scale hierarchical 2D array of micro-bumps, and a micro-pits array with nano-ripples.

《Fig.1》

Fig. 1. Schematic of the designed laser-textured patterns. (a) Line-hatched pattern; (b) dot array pattern.

First, the Stavax steel substrate was treated with the following laser parameters: The laser energy fluence was set at 1.18 J·cm-2, the repetition rate was set at 1 MHz, the scanning velocity was set at 500 mm·s-1, the repetition pass number was set at 30, and the hatched distance was set to be 25 μm for both directions in the hatched pattern, as shown in Fig. 1(a). A uniform surface pattern comprising a micro-bumps array was produced on the Stavax substrate with an area of 20 mm × 20 mm, as shown in Fig. 2. The estimated processing time was 5 min. It was observed that a uniform and consistent surface pattern was produced. Fig. 2(c) illustrates the measured feature profile of the micro-bumps array surface texture. The diameter of a micro-bump was 12.5 μm, the period of the pattern was 25 lm, and the height of the microbumps pattern was about 21.9 μm. Fig. 2(b) shows a magnified SEM image of a micro-bump; periodic surface ripples were observed to be superimposed onto the surface of the microbump. The period of this ripple was 800 nm and the modulation depth was 220 nm. Thus, a two-scale hierarchical 2D surface texture based on a micro-bumps array was produced.

《Fig.2》

Fig. 2. Laser-produced two-scale micro-/nano-bumps array surface texturing. (a) SEM image under ×500 magnification; (b) SEM image under ×4000 magnification; (c) profile measurement.

Next, the Stavax substrate was treated with the following laser parameters: The laser fluence was set at 0.8 J·cm-2, the repetition rate was set at 1 MHz; the scanning velocity was set at 1000 mm·s-1, with a pass number of 1; and the hatched distance was set to be 5 lm for both directions in the hatched pattern, as shown in Fig. 1(a). A large-area uniformly distributed periodic ripple pattern was produced with an area of 20 mm × 20 mm, as shown in Fig. 3. The estimated processing time was 3 min. Using AFM measurement, the period of the ripple was found to be 800 nm with a modulation depth of 220 nm.

《Fig.3》

Fig. 3. Laser-produced periodic ripple surface textures on a Stavax surface. (a) SEM image under magnification of ×1000; (b) SEM image under magnification of ×4000; (c) AFM image.

Laser surface treatment was then conducted in two steps. The first step involved the dot array pattern (Fig. 1(b)) with laser parameters set as follows: a laser energy fluence of 1.43 J·cm-2, a pulse repetition rate of 1 MHz, and a dot pitch of 35 lm for 5000 pulses on each location. The second step involved the linehatching pattern (Fig. 1(a)) with laser parameters set as follows: a laser energy fluence of 0.8 J·cm-2, a pulse repetition rate of 1 MHz, a scanning velocity of 1000 mm·s-1 for only one pass, and a hatching pitch of 5 μm for both directions in the line-hatching pattern, as shown in Fig. 1(a). A 2D pattern of a micro-pits array covered with nano-scale periodic ripples in between the pits was fabricated, as shown in Fig. 4. The micro-pit diameter was 13 μm, the pit depth was 15 μm with a pitch of 35 lm, the period of the ripple was around 800 nm, and the ripple depth was 220 nm, as measured with AFM (Fig. 4(c)). With this combined two-step process, the scanned area was 20 mm × 20 mm and the estimated total processing time was 8 min.

《Fig.4》

Fig. 4. Laser-produced 2D pattern of a micro-pits array covered with nano-scale periodic ripples. (a) SEM image under × 500 magnification; (b) SEM image under ×2500 magnification; (c) AFM image.

It was observed that the laser fluence used for the ripple formation was 0.8 J·cm-2; this was a bit higher, albeit on the same order of magnitude, as the ablation threshold of 0.5 J·cm-2 that was used on stainless steel with the ps laser [23]. Previous studies [24,25] have clearly shown that when a solid surface is irradiated with a certain number of laser pulses with a laser energy fluence close to the single pulse ablation threshold, a periodic surface-modulation pattern is produced with a period that is close to the laser wavelength. It has been suggested that the incident laser light may interfere with the scattered waves from the surface disturbance so as to create a standing wavethat leads to theformationof ripples [26]. In this case, the laser energy density is higher than the threshold fluence for the periodicsurfacematerialmeltingfollowingthe interference pattern. The thin layer of molten material then re-solidifies immediately to form a ridged structure. A certain degree of laser power intensity is required in order for the interference pattern to be engraved or etched on the target sample surface. The sample surface is initially roughened by the first laser pulse irradiation; after that, subsequent pulses create periodic ripples via the interference effect [27]. The laser beam parameters, which include the laser wavelength, beam polarization, and incident angle of the radiation, determine the period of the ripple pattern that is produced.

Fig. 2 shows a laser-produced pattern that is a regular microbumps array. From the magnified SEM image, it was found that the surface of the micro-bumps was covered with nano-ripples, creating a micro-nano hierarchical 2D surface pattern. It was expected that this type of two-scale surface pattern would be able to modify the surface wettability into hydrophobicity and even super-hydrophobicity by increasing the WCA. However, it was found that the sample surface displayed super-hydrophilicity immediately after laser surface treatment, with a WCA of less than 10°. The WCA of the sample surface then gradually increased to 120° within 3 d, indicating a shift to a highly hydrophobic surface. Eventually, after 30 d, the WCA of the laser-treated Stavax sample surface increased to more than 150, indicating that a superhydrophobic surface had been achieved. Fig. 5 illustrates the measured WCA of the Stavax sample substrate before and after laser treatment. The WCA of the untreated Stavax steel was 73°; after laser surface texturing, the surface WCA increased to 154°. This result indicates that laser surface treatment was able to convert the original hydrophilic Stavax steel surface into a superhydrophobic surface under specific laser parameters.

《Fig.5》

Fig. 5. The WCA of the (a) untreated and (b) laser-treated Stavax substrate surface.

The surface wetting property of a solid substrate is determined by its surface morphological structure and surface chemistry properties. Therefore, an XPS analysis was conducted to characterize the elemental composition and chemical compositions of the Stavax substrate surface before and after laser treatment to study its surface chemistry effects. Fig. 6 illustrates the XPS survey spectra of the untreated and laser-treated Stavax steel surface. Before collecting the spectra, the samples were treated with argon ion (Ar+) sputtering for 20 s to ensure that the surface was clean with no contaminants from the ambient atmosphere.

《Fig.6》

Fig. 6. XPS spectra of the (a) untreated and (b) laser-treated Stavax substrate surface.

As shown in Fig. 6, it was observed that for untreated Stavax steel, the carbon (C) 1s peak intensity was very weak. In contrast, for laser-treated Stavax steel, the C 1s peak intensity was much higher. The increment of the relative C 1s peak intensity indicated that the elemental concentration of C on the laser-textured Stavax steel surface had increased. Based on the relative intensities of elements in the XPS spectra, the elemental concentrations (at%) of C, oxygen (O), iron (Fe), and chromium (Cr) on the untreated and laser-treated steel substrate surfaces were derived (Table 1). It was found that the C concentration on the laser-treated Stavax steel surface had increased from 9.6 at% on the untreated Stavax steel surface to 29.7 at%. Therefore, we believe that a thin layer of nonpolar carbon had accumulated on the laser-treated Stavax steel surface. The dissociation of CO2 into carbon is considered to be the main reason for the carbon layer formation. The dissociative reaction of CO2 was initiated by active magnetite Fe3O4-δ (0 < δ < 1), an oxygen-deficient iron oxide that may have been induced by the ultrafast laser surface treatment [12,15]. We believe that this nonstoichiometric magnetite Fe3O4-δ (0 < δ < 1) acted as a catalyst to initiate the dissociative adsorption of CO[28–30], producing carbon monoxide and carbon as the outcome of the CO2 dissociation, and moving oxygen anions into lattice vacancies of the steel alloy to form stoichiometric Fe3O4. Therefore, a nonpolar amorphous carbon accumulation layer formed on the laser-treated Stavax steel surface due to the CO2 decomposition reaction. This amorphous carbon layer has a low surface energy. Thus, the combination of the CO2 decomposition reaction and the laser-induced two-scale hierarchical 2D surface texture (Fig. 2) resulted in the formation of a super-hydrophobic Stavax steel surface with a WCA greater than 150°.

《Table 1》

Table 1 Concentrations of the elements C, O, Fe, and Cr (at%).

Next, the laser-induced micro-pits array surface pattern on the metal mold substrate was replicated onto a polypropylene substrate via the polymer injection molding process. Fig. 7(a) shows the laserproduced surface textures on the Stavax steel substrate. From the three-dimensional profile measurement shown in Fig. 7(c), it can be seen that the micro-pit diameter is 15 μm, the period of the pits is 30 μm, and the depth of the pits is 17 μm. With the polymer injection molding process, the micro-pits array pattern on the Stavax steel mold was converted into a micro-bumps array pattern on the polypropylene substrate, as shown in Fig. 7(b). The molded microbump diameter was about 15 μm, the period was 30 μm, and the bump height was about 20 μm, as shown in Fig. 7(d). Fig. 8 shows the measured WCA for the original and for the molded and textured polypropylene substrate. It was found that the WCA increased from 96° for the original polypropylene to 156° for the injection-molded textured polypropylene. This increase of 60° indicates that a super-hydrophobic polypropylene surface was achieved.

《Fig.7》

Fig. 7. SEM images on (a) a laser-textured Stavax steel surface and (b) a molded polypropylene surface; measured surface profiles on (c) laser-textured Stavax steel and (d) molded polypropylene.

It was observed that due to the formation of the 2D array of micro-bumps surface structures, as shown in Fig. 7(b), when the water droplet comes into contact with the laser-textured surface, the water droplet may not be able to fully wet the whole surface area; rather, it may stay on top of the micro-bumps, causing some air to be entrapped between the water droplet and the textured bottom surface, and resulting in a composite interface. Following the Cassie-Baxter [3] model, this may be the cause of the WCA increasing to more than 150°, as shown in Fig. 8(b).

《Fig.8》

Fig. 8. WCA of (a) original and (b) molded textured polypropylene surface.

《4. Conclusions》

4. Conclusions

In conclusion, a systematic investigation of the ps laser surface texturing of a Stavax steel substrate was conducted. Three types of surface textures were achieved: periodic ripples, a two-scale hierarchical 2D array of micro-bumps, and a micro-pits array with nano-ripples. After laser surface treatment, the Stavax surface showed super-hydrophobicity with a WCA of more than 150°. The XPS analysis indicated that a nonpolar amorphous low-surfaceenergy carbon layer accumulated on the laser-treated Stavax steel surface via the CO2 decomposition reaction. The combination of the CO2 decomposition reaction and the laser-induced two-scale roughness structure produced a super-hydrophobic Stavax steel surface. Furthermore, the laser-induced surface textures on the metal mold substrate were replicated onto a polypropylene substrate via the polymer injection molding process. The results showed that the laser-induced surface textures were successfully duplicated onto the polypropylene surface. The WCA of the molded textured polypropylene surface changed from the original 96° to 156°, indicating that a super-hydrophobic polypropylene surface was achieved. This developed process holds potential for application in the fabrication of microfluidic devices and consumer plastic products with an easy cleaning function.

《Acknowledgements》

Acknowledgements

The authors would like to thank Ms. Hong Xie for her help with XPS measurement. They would also like to thank the Agency for Science, Technology and Research (A*STAR) of Singapore for financial support.

《Compliance with ethics guidelines》

Compliance with ethics guidelines

Xincai Wang, Hongyu Zheng, Yinchi Wan, Wenhe Feng, and Yee Cheong Lam declare that they have no conflict of interest or financial conflicts to disclose.