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Abstract
Ordered hierarchical structures were fabricated on a stainless steel surface using a single picosecond laser for highly controllable dimensions. Picosecond laser induced periodic structures were firstly used to create large-scale nano-structures with a period of ~450 nm. Subsequently, laser direct writing, by simply changing process parameters was employed to create micro squared structures with 19 μm width, 19 μm interval and 3-7.5 μm depth on the previously created nano-structures. As a result, micro squared structures covered by uniform nano-structures, similar to examples present in nature, were successfully fabricated. Additionally, the wettability of the created hierarchical structures was analyzed. The results demonstrated that the combination of both micro- and nano-structures allowed to tune the wetting behavior, presenting a great potential for wettability applications.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In the course of evolution, creatures in nature have developed variety of functional surfaces to ensure their survival. These functional surfaces have attracted considerable interest owing to potential applications in solving industrial problems like friction and wear [1, 2], wettability [3, 4], adhesion [5, 6], reflectivity [7, 8] and so on. It has been proved by researchers that these special functions are a direct consequence of their hierarchical surface structures on the micro- and nano- scale, as well as the surface chemistry [9]. In the last few decades, various bio-inspired hierarchical structures presented in plants’ or animals’ surfaces have been designed and fabricated [10, 11]. The dimensions of the structures have a high dependence on tailoring of the special functions in the aforementioned applications.
At the present, numerous different mechanical, physical and chemical approaches are available to fabricate hierarchical structures [10,12]. Among these technologies, ultra-shot laser micro/nano machining has been proved to be an effective way to create hierarchical structures with micro- and nano- features. For example, Rukosuyey et al. [13] created micro-patterned structures on the surfaces of metal samples using femtosecond laser in a single step, combined with submicron scale features on the surfaces due to rapid cooling and redeposition. Long et al. [14] used similar method to fabricate structures in the micro-scale covered by some nano-folds, redeposited nano-particles and laser induced nano-structures, by simply changing the scanning speed of a femtosecond laser. Both of their studies mainly focused on the generation of micro-scale structures using femtosecond laser with a high fluence, companied by the generation of random nano-features. In addition, Bizi-Bandoki et al. [15] varied the pulse numbers of a femtosecond laser at a low fluence to create hierarchical structures on the surfaces of AISI 316L stainless steel and Ti-6Al-4V. The created structures consisted mainly of nano-scale ripples with a micro periodic interval, which corresponded to the imposed laser scanning overlap. Femtosecond laser induced ripples with a micro interval were also used by Martínez-Calderon et al. to create multi-scale structures on stainless steel [16]. Their studies mainly focused on the formation of nano-scale patterns at a micro periodic interval. The above mentioned works fabricated hierarchical structures in one step, implying that the micro- and nano-structures were hard to tune separately.
However, there is a few literature on the fabrication of precisely controlled hierarchical structures in both micro- and nano-scales (i.e., ordered controllable micro-structures covered by precise nano-patterns). Huerta-Murillo et al. [17] employed nanosecond direct laser writing to create well-defined micro-cells with squared shape, following by picosecond direct laser interference pattering to create a dual-periodic structures. As a result, hierarchical structures were generated on Ti-6Al-4V alloy. Two different lasers increased the complexity of the process to a certain extent. Martínez-Calderon et al. [18] further used a femtosecond laser at a high fluence to create micro-structures with precise dimensions, and then to create nano-patterns at a low fluence in a controlled way, covering the previously micro-patterns. Their results showed that the created micro-structures had relatively small impact on the creation of nano-structures due to the minimal thermal effect of the femtosecond laser.
In ultra-shot laser micro/nano machining, laser direct writing is an effective method to remove a controlled amount material in micro-scale, while laser induced periodic structures (LIPSS) is commonly accepted to fabricate precise nano-features. According to the previous works mentioned above, most hierarchical structures are generated by femtosecond lasers. A simpler and faster process of fabricating precisely controlled hierarchical structures with a dual scale roughness is still a challenge. In this work, the combination of micro squared structures and low spatial frequency LIPSS (LSFL) with nano-scale is used to fabricate hierarchical structures with controllable dimensions. Picosecond laser induced periodic structures at a low laser energy is firstly used to create large-scale nano-structures, following by picosecond laser direct writing at a high laser energy to create micro squared structures. Accordingly, apparent contact angle (CA) and contact angle hysteresis (CAH) measurements are carried out to analyze the wettability of the fabricated hierarchical structures.
2. Experimental
A linearly polarized Nd: YVO4 picosecond laser (PX50, Edgewave, Germany) with 10 ps pulse duration operating at 532 nm wavelength was used to fabricate ordered hierarchical structures under atmospheric conditions. The laser beam with a Gaussian profile was delivered to the sample surface via a beam delivery system consisting of a half-wave plate, a liner polarizer, a diaphragm, several turning mirrors and a plano-convex focal lens. Through the focusing lens, the obtained spot diameter at the focal plane was approximately 16 μm. The laser machine was equipped with a 3-axis motorized x-y-z stage (PS-30, Borui, China) capable of moving the workpiece with a translation precision of 500 nm. The laser setup is depicted in Fig. 1.
AISI 304 stainless steel samples with a dimension of 25 mm × 25 mm × 0.5 mm were used in this study. The sample has been processed with an average surface roughness of 10 nm by the suppliers. No further treatment was used before laser surface texturing. Two laser beam scanning strategies with different pulse energies were employed to fabricate hierarchical structures, as shown in Fig. 2. The first raster trajectory aimed to create a homogeneous LIPSS nano-structures. The operating frequency and the scanning speed were fixed at 50 kHz and 40 mm/s, respectively, yielding a 95% pulse overlap in the scanning direction. The scanning space between two adjacent lines was fixed at 6 μm. The pulse energy was varied from 0.24 to 0.62 μJ, as measured by an external power meter (Maestro, Gentec–EO, Canada). The single overscan number (i.e., the number of laser path repetitions) was used. In order to create micro-structures, the laser scanning was performed using raster trajectory in the vertical direction (y direction) first and then in the horizontal direction (x direction), as shown in Fig. 2(b). The operating frequency and the scanning speed were as same as that of creating nano-structures, while the pulse energy was held constant at 5.1 μJ. The laser scanning was repeated from 3 to 15 times to obtain the ablated micro-channels with various depths. The intervals of adjacent laser scanning lines in two directions were kept constant at 38 μm resulting in the creation of micro squared protrusions. The combination of micro squared structures and LIPSS nano-structures was employed to fabricate hierarchical structures with a dual scale roughness.
After laser irradiation, ultrasonic cleaning was performed to remove the debris from the textured zone. The morphologies of the surfaces were characterized by a confocal laser scanning microscope (OLS-400, Olympus, Japan) and a scanning electron microscope (Quanta FEG 450, FEI, USA). After surface characterization, the samples were silanized using C14H19F13O3Si ethanol solution with a concentration of 1% for 1 h, followed by washing with ethanol and drying in an oven at 90 °C for 1 h. Subsequently, the wetting behavior of the samples was evaluated by measuring the apparent CA and CAH using an optical contact angle measuring instrument (OCA-20, Dataphysics, Germany). The deionized water with a droplet volume of 3 μL was used for CAs, while CAHs (i.e., the calculated difference between advancing and receding CAs) were performed starting with a 3 μL droplet which was slowly increased to 15 μL and then decreased to 3 μL. The CA and CAH measurements for each sample were repeated three times to obtain an average value.
3. Results and discussion
3.1 Nano structures
Figure 3 shows SEM images of picosecond laser irradiated lines with different laser energies (i.e., 0.24 μJ, 0.32 μJ, 0.48 μJ, 0.62 μJ). For a laser energy of 0.24 μJ, a nonuniform distribution of nano-structures was observed in the laser irradiated region, as shown in Figs. 3(a) and 3(b). Most ripples had the orientation parallel to the laser polarization direction and the spatial periods of ~130 nm significantly below the laser wavelength, which were belonging to high-spatial frequency LIPSS (HSFL). Low fluence related to low pulse energy results in the formation of HSFLs, which may be due to self-organization [19,20]. A small amount of LSFLs with the orientation perpendicular to the laser polarization and the spatial of ~450 nm were generated on the central region. This is attributed to the Gaussian distribution of laser energy, resulting in the increasing laser fluence at the central. Interference effects in the high fluence region is responsible for the generation of LSFLs [21]. With the increasing laser energy to 0.32 μJ, homogeneous LSFLs with periods of ~450 nm appeared on the central region, while HSFLs were simultaneously observed in the periphery, as shown in Figs. 3(c) and 3(d). At a laser energy of 0.48 μJ, similar nano-structures consisting of LSFLs and HSFLs were formed in the laser irradiated region, as demonstrated by Fig. 3(e). However, the laser fluence at the central exceeded the melting threshold of stainless steel, leading to a blurring of the LSFLs, as illustrated in Fig. 3(f). With the further increasing energy to 0.62 μJ, LSFLs were awfully blurred, due to surface melting, as shown in Figs. 3(g) and 3(h).
Fig. 3 SEM images showing the created nano-structures for various pulse energies: (a, b) 0.24 μJ, (c, d) 0.32 μJ, (e, f) 0.48 μJ, (g, h) 0.62 μJ. The red arrow shows the polarization direction.
Obviously, the laser energy significantly affected the formation of nano-structures. In this paper, the regular LSFL was selected for creating the hierarchical structures being the convenient one to obtain homogenous structures and to control the geometrical parameters as well. Hence, a laser energy of 0.32 μJ was applied to create large-area uniform nano-structures by overlapping the adjacent irradiated lines at an interval of 6 μm. The perpendicular LSFLs with periods of around 450 nm were uniformly distributed on the stainless steel surface, as shown in Fig. 4.
3.2 Micro structures
Bidirectional raster scanning was used to create perpendicular micro-channels, which contributed to further formation of micro squared protrusions. Micro square-shaped structures with four different depths were created on the stainless steel surfaces by varying overscan number from 6 to 15. Figure 5(a) shows an example of planar image of created micro-structures with overscan number of 12, while corresponding 3D image is given in Fig. 5(b). It is clearly seen that well-defined periodic micro square-shaped protrusions were formed. Since the laser fluence is far beyond the ablation threshold of the stainless steel, thermal effects are hard to completely eliminated, leading to minor recast formation of the molten material. Consequently, micro-walls were created around the unprocessed surfaces of the protrusions resulting in the formation of a closed packed or μ-cell, as illustrated in Fig. 5(b). The surface profiles corresponding to the selected lines are shown in Fig. 5(c). The depth and width of the micro protrusions were approximately 6 μm and 19 μm, respectively.
Fig. 5 Micro-structures at laser energy of 5.1 μJ and oversacen number of 12: (a) planar image (b) 3D image (c) surface profiles.
Figure 6 shows SEM images of created micro-structures at overscan number of 12. Some nano-structures similar to LSFL were generated inside the ablated micro-channels. At the intersection of two perpendicular channels several holes were observed due to the fact that more laser slots were irradiated on these areas. Meanwhile, the surface was almost in its original state in the region where no laser scanning has been performed, with the exception of the appearance of particles in the peripheral region.
The dimensions of micro squared structures strongly depend on the ovescan number used for the ablation. Figure 7 shows the relation between the depth and width of the micro squared structures for four different overscan numbers. As the overscan number increases, the depth of the micro channels increases almost linearly from 3 μm to 7.5 μm, while the width has no obvious changes. It can be concluded that, by simply varying the overscan number, the profiles of the micro-structures can be tuned precisely.
3.3 Hierarchical structures
The combination of micro squared structures and LSFL nano-structures was used to fabricate hierarchical structures with a dual scale roughness. Initially, the fabrication sequence of micro- and nano- structures was tested. Figure 8 shows SEM images of the hierarchical structures created by micro-structures firstly following by nano-structures (micro-nano) at overscan number of 12. It is clearly seen that no LSFLs were generated on the surrounding region of the micro-squares. This is because the subsequent nano-structures were created on the surrounding residues, which were eliminated by ultrasonic cleaning, resulting in the absence of LSFLs in the periphery. SEM images of the created hierarchical structures by nano-structures firstly following by micro-structures (nano-micro) with different overscan numbers are shown in Fig. 9. As can be observed, LSFLs covered all over the micro-squares and similar nano-structures were also appeared inside the micro-channels, showing a representative micro/nano scale binary structure. Hence, the sequence of nano-structures firstly following by micro-structures was selected to fabricate hierarchical structures. With the increasing of overscan number, more holes were generated at the intersection. Simultaneously, more particles were observed on the surrounding of the micro-squares due to the resolidification of the splashed materials.
Fig. 8 SEM images of created hierarchical structures using micro-nano sequence at overscan number of 12.
Fig. 9 SEM images of created hierarchical structures using nano-micro sequence at overscan number of (a, b) 9, (c, d) 15.
The corresponding dimensional measurements obtained using confocal microscopy are shown in Fig. 10. Compared to micro-structures shown in Fig. 2(a), darkening of the surface on the micro-squares was observed, as revealed by Figs. 10(a) and 10(d), which is due to the creation of nano-structures. The profiles of hierarchical structures in the horizontal as well as in the vertical direction are also illustrated. The achieved depths at overscan number of 9 and 15 were 4.5 μm and 7.5 μm, respectively, which were roughly the same as that of just micro-structures. It can be concluded that the nano-structures had no significant effect on the machined depth of the hierarchical structures.
Fig. 10 Hierarchical structures using nano-micro sequence with different overscan numbers: (a, b, c) 9, (d, e, f) 15.
3.4 Wetting behavior
In order to characterize the wetting properties of the stainless steel surfaces, CAs and CAHs of water droplet were measured, including a non-irradiated surface as a reference. For non-irradiated surface, an average CA value of 117° was obtained after chemical treatment. Figure 11 shows the captured images of droplets in the CA measurements. The micro-patterned surface with a depth of 3 μm, reached a CA value of 121.5°, slightly improving wetting property of the stainless steel. With the increase of machined depth, the CA significantly enhanced, reaching a value of 138° at a depth of 7.5 μm. The surface roughness induced by laser ablation leads to the change of the CA.
Fig. 11 Water droplets on stainless steel surfaces with micro-structures and hierarchical structures.
In order to further explain the wetting behavior, the measured CAs and CAHs of water droplets on the micro-structures are presented in Fig. 12. It is observed that the CAH of water droplets on the micro-structures decreases dramatically from 63.8°to 12.5°with increasing machined depth from 3 μm to 7.5μm.
The created micro squared structures could be simplified into a geometric model, as shown in Fig. 13. According to Wenzel’s theory, the contact angle is expressed as [22]:
where θw and θ0 are contact angles on rough and smooth surfaces, respectively, and f is the roughness factor, which can be given by:According to the previously measured results, the width of micro-squares, a, and the interval, b, were 19 μm and 19 μm, respectively, while the depth, h, varied from 3 μm to 7.5 μm. Figure 12(a) also depicts the predicted CAs using Wenzel model, as a function of the machined depth. As the figure indicates, the CA predicted by Wenzel’s model is in good agreement with measured CA at the depth of 3 μm. The low CA and high CAH of this surface can be explained according to the Wenzel model. However, with the increasing depth from 4 μm to 7.5 μm, the experimental CAs are larger than that of the predicted values. Meanwhile, with an increase in the machined depth, the CAH decreases dramatically, as shown in Fig. 12(b). This is due to the fact that the liquid does not completely wet the rough surface at a higher depth, leading to the trap of air pockets in the micro squared structures [23]. Cassie-Baxter’s model is more suitable for describing the wetting behavior at this situation [24]:
where f1 can be described by:However, the experimental CA at the depth of 6 μm or 7.5 μm is still not accord with the predicted value of 149.7° using Cassie-Baxter’s model. In this case, the droplet partially wets the side face of micro-channels and partially sits on air pocket. With the increasing machined depth the liquid-solid contact area decreases, resulting in increased CA and reduced adhesion to water. This state between Wenzel and Cassie-Baxter states is named mixed or combined state in some papers [25, 26].
Compared to only micro-structures, the hierarchical structures (i.e., micro-structures coved LSFL nano-structures) presented higher values of CA and lower values of CAH. This increased CA and decreased CAH can be attributed to the presence of nanostructures. The regular LSFLs, which are similar to those reported by Long et al. [27], allow the water penetrate into nanostructures, corresponding to Wenzel state. However, plenty of random nanoparticles were formed on the regular LSFLs due to the fabrication of micro squared structures, especially after numerous laser scanning. These complicated nanostructures make the water partially wet the micro squares’ surfaces, instead of completely wetting, thus leading to an increased CA and a reduced CAH. The measured CA of hierarchical structures at depth of 7.5 μm, 143°, is incredibly close the Cassie-Baxter model’s value.
4. Conclusions
Hierarchical structures were created on stainless steel surfaces using a single picosecond laser fabrication process for highly controllable dimensions. Picosecond laser induced periodic structures were firstly used to create large-scale nano LSFLs with a period of ~450 nm. After that, laser direct writing, by simply changing process parameters was employed to create micro squared structures (19 μm width, 19 μm interval and 3-7.5 μm depth) on the previously created nano LSFLs. Consequently, micro squared structures covered by uniform nano LSFLs, similar to examples present in nature, were successfully fabricated. In addition, CA and CAH measurements were carried out to analyze the wetting behavior of the created hierarchical structures. For separate microstructures, with the increasing machined depth the liquid-solid contact area decreases, resulting in increased CAs and reduced adhesion to water. The combination of both micro- and nanostructures allowed the water to partially wet the micro squares’ surfaces, instead of completely wetting, thus leading to an increased CA and a reduced CAH. These results demonstrated that created hierarchical structures were easy to turn micro- and nano-structures separately, presenting a great potential for wettability applications.
Funding
Natural Science Foundation of Jiangsu Province (BK20150685); the Fundamental Research Funds for the Central Universities (KYZ201659); National Natural Science Foundation of China (51705258); Foundation for Distinguished Young Talents, College of Engineering, Nanjing Agricultural University (YQ201604); Young Teachers Fund of Nanjing Agricultural University (rcqd16-05).
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