Fast Fabrication of Superhydrophobic Copper Alloy Fabricated by Commercial Fiber Laser-1062 nm and Stability of Superhydrophobic Surface

We present a fast, straightforward fabrication method to produce stable superhydrophobic copper alloy surfaces. The technique consists of irradiating copper with a fiber laser and then coating the surface with ethanol. The laser ablation generates surface morphology on the copper surfaces, such as line and grid patterns. We can modify the wetting properties and surface morphology by changing the step size. We assessed the change of the water contact angle on the raw surface, laser-based textures after natural aging, and laser-based textures post-low annealing. After that, we measured the stability of the water contact angle till 120 days. After low annealing heat treatment, water contact angles on grid pattern surfaces have superhydrophobic surfaces and are more stable till 120 days.


Introduction
Copper (Cu) is used in many industrial sectors, such as automobiles, home appliances, air conditioning, power electronic devices, solar, biomaterials, and heat sinks.However, Cu is easily affected by environmental conditions.Cu's hydrophilicity enables it to condense water from humid air, even frost.The ice and water on the Cu surface offer the perfect conditions for bacteria to grow and contaminate the incoming air [1][2][3].The superhydrophobic surface plays an essential key role in inhibiting ice nucleation and postponing the freezing of the droplet, exhibiting good anti-icing/frost performances [2].Superhydrophobic surfaces offer special functions such as self-cleaning, corrosion protection, drag reduction, and antibacterial [4].
The crucial factors affecting the increasing or decreasing water contact angle (CA) are the surface energy, chemical elements, and surface textures.A metal's surface textures can be altered to provide the necessary wettability by applying a particular combination of surface chemistry and surface texturing [5].The fiber lasers laser-based textures are a promising war for modifying the surface textures of metals for generating superhydrophobic surfaces since these can be produced quickly, with minimal waste, without requiring a mask, in a single procedure, and at an affordable price [6,7].Fiber lasers offer a flexible solution, effective pulse repetition rates, and fast processing times with sufficient results for texturing various materials with varying surface patterning.The laser energy density of fiber lasers can be controlled by modulating the laser power and scanning speed to obtain an ideal surface patterning [8,9].Materials and laser interaction can produce micro or nano-structures, including ripples, nanoparticles, and other structures [10].The different textures of metals are formed either by melting and re-solidifying or removing the materials substrate through laser ablation with a single or a train of laser pulse.Micro or nano-structure structure formation depends on the ability of material and laser pulse parameters such as wavelength, pulse energy, pulse duration, and pulse rate [3].According to previous literature, surface wettability can be altered to have hydrophobic or superhydrophobic properties by laser treatment, depending on surface patterning.Yang [11] reported that a nanosecond laser with different patterning, such as spot pattern, line pattern, and grid pattern, was employed on Inconel 718 to fabricate a superhydrophobic surface with a CA of 156.8 ± 1˚.Line pattern and grid pattern have superhydrophobic surfaces of 153 ± 1,2˚ (line pattern) and 156,8 ± 1,1 (grid pattern) compared to spot pattern (140,8 ± 2,8˚).Dinh [12] reported the effects of different surface patterning and step size on superhydrophobicity.The grid pattern showed the greatest contact angle > 150˚ and sliding angle < 10˚ compared to the line pattern.The CA of the line pattern decreased with increased step size because, at small step sizes (50 and 100 µm), the recast layer still supported the water droplet; nevertheless, at a large step size (200 to 300 µm), the water droplet might penetrate between recast layer, and the water droplet could touch on the flat surface.
Nowadays, two-step processes needed to form metals have superhydrophobic surfaces because transforming from a laser-ablated hydrophilic surface to a superhydrophobic surface takes several days to month [6, [13][14][15].The first step is laser beam machining to fabricate micro or nano-structures, and the second step is applying coatings on the laser-based textures with chemical agents such as perfluorinated octyl trichlorosilane (FOTS), fluoro silane, fluoroalkyl silane (FAS), and ethanol to have a lower energy at the surface [5,7,[16][17][18][19].
We prepared superhydrophobic surfaces on commercial Cu plates using a fiber laser in this research.Laser scanning in a perpendicular direction was utilized to achieve superhydrophobicity on the Cu surfaces, form rough hierarchical structures, and then low-annealing heat treatment to accelerate superhydrophobicity to superhydrophobic.This work aims to make superhydrophobic on Cu surfaces within minimum processing time.

Materials
The experiments detailed in this paper were performed on commercially available copper (purity 98,1%)-square samples with sizes of 20 mm and thicknesses of 0.5 mm used as the substrate material.The Cu surface has not undergone any treatment before laser ablation.The Raw Cu surface is shown in figure 1. Ethanol of analytical grade was used for the low annealing heat treatment.

Methods
The copper sheets are irradiated by fiber laser from Epilog Laser.Laser wavelength of 1062 nm, and the fibre laser source produces laser light by forcing intense diode light into fiber optic cables doped with ytterbium (Yb3+).Raster mode, laser power of 24 W, frequency of 12%, scanning speed of 8%, and 600 dots per inch (DPI) used to fabricate laser-based textures-figure 2 shows the fabrication processes.During the laser ablation process, the laser beam is controlled to scan the Cu sheet fixed on the working platform line-by-line in one direction (X-direction) by a computer program-rotating Cu sheet needed to create a grid pattern.The frequency and scanning speed were also controlled automatically by the computer program.The laser ablation operation is carried out in an air-atmospheric environment.Before surface patterning by a fiber laser, the surface patterns were drawn using Adobe Illustrator.Line (LP) and grid (GP) surface patterns were used in this research with different step sizes: 20 µm, 100 µm, and 150 µm.The line width of both patterns is 100 µm.Then, some samples of the laser-ablated textures are post-processed by ethanol-assisted low annealing heat treatment at a temperature of 110˚C for 2.5 h in the air to accelerate the transformation of CuO to Cu2O.Another sample is natural aging at room conditions.

Surface analysis
The samples were left in the air, under ambient conditions, before studying their wettability by measuring the contact angle.A ̴ 5 µL droplet of deionized water was injected into the sample surface using a syringe, and then a camera (SANYO VCC-5775P) captured a picture.At least three times, the water droplet contact angle (WDCA) was measured in different places for each sample.The contact angle was then determined by analyzing the droplet image using ImageJ software.The CA was measured on different natural aging days of 5, 14, 30, and 120 days for HT samples measured on different days of 5, 21, 30, and 120 days.To analyze the surface textures of the laser textures samples, a digital microscope (Endoscope), optical microscopy (Seek), and atomic force microscopy (Park NX10) were used.The chemical composition of the surface was investigated by Raman spectroscopy (Horiba) on three random areas and X-ray diffractometer (Bruker, D2 Phaser) measurements.

Analysis of surface morphology
The digital and optical microscopes of the surface topography of Cu surface after laser-based textures (LBT) are shown in figure 3. The line textures are different by different step sizes, as shown in figures 3a-c-step size of 100 µm shown as more precise and solid than other step sizes.The different line textures after laser ablation due to plasma temperature do not optimum to irradiate the Cu surface.Two factors influence laser plasma during laser ablation: imperfection on the Cu surface and optical breakdown [20].Figures 3d and 3h show that laser scanning did not move symmetrically on the Cu surface.The scan velocity, laser power, and strategy during the laser processing greatly control the texture line and surface roughness.The energy density of a fiber laser depends on scan velocity and laser power [9].Figures 5 and 6 show that the error bar was slightly different.The optical configuration and the laser control, such as laser power, scanning speed, frequency, and step size, allow the accurate determination of the surface texture modification.Figure 5 shows that the average surface roughness increased after laser ablation because of the formation of a dual-scale structure.From the error bar, the small step size of 20 µm has a slightly higher average surface roughness and Sz than 100 µm due to the interaction of splashing and solidification.However, average surface roughness decreased with increasing step size because the un-ablated area increased, and interaction between adjacent holes was weakened [17,18].Increasing step size provides more space between holes for materials to melt and increases the height of recast layers [4].Figure 6 shows that the average H/D (high/diameter) ratio increased from 0.27 ± 0,065 to 0.37 ± 0.006 with an increase in the step size from 20 µm to 100 µm.The different H/D ratios between 20 µm and 100 µm were slightly different from the error bar.However, H/D decreased in the sample of 150 µm step size (0,07 ± 0,02).

Surface wettability
The CA on the raw surface and laser-based textures are shown in figure 7. The raw surface has a hydrophobic surface (119˚).However, the CA changed to hydrophilic on all surfaces after laser ablation.The CA decreased from hydrophobic to hydrophilic after laser ablation due to the CuO structure (protuberant part or recast layer) presence on the Cu Surface [19].Figure 7 shows the CA of line and grid patterns increased with increased step size from 20 µm to 150 µm, except for the step size of 150 µm of the line pattern surface.The CA increased because of the raised un-ablated surface (hydrophobic surface).However, the CA decreased to 43 on a line pattern of 150 µm step size; the decrease in CA may be due to line patterns having irregular ablation spot distribution.Figure 8a shows that the CA increased with time.After five days, the LBT surface changed from hydrophilic to hydrophobic surfaces on LP 100, GP 20, and GP 100 samples.However, when the step size of LP (line pattern) and GP (grid pattern) increased to 150 µm, the CA decreased to 117˚ and 111˚.The decreasing CA with increasing step size is due to the water droplet's easy contact with a flat area [4,23].H/D ratios have an impact on the CA transformation.Figure 6 shows a line pattern with a step size of 100 µm has the highest CA because this pattern has the highest H/D ratio.The H/D ratio influences the effectiveness of the air pocket formation under the water surface [24].The increase in depth (Sz) leads to an increased CA because the water droplet cannot touch the bottom of the valley.When the valley's depth is not deep, the droplet quickly touches the bottom of the holes and then promotes the Wenzel state on the surface [4].
In contrast, after low annealing heat treatment (HT) at a temperature of 110˚C for 2.5 h on the same days as the laser ablation caused, the CA increased from lower CA (<70˚) to high CA (>150˚), except LP 20 significantly.However, CA on the raw surface has not changed to superhydrophobic.Those indicated HT affects the laser-based textures to increase CA by accelerating air pockets on the ablation surfaces.He [7] reported a superhydrophobic surface on laser-based textures after low annealing, which causes the fast transition from the hydrophilic layer (CuO) on the recast layer after surface patterning to the hydrophobic layer (Cu2O) after low annealing, and adsorption of hydrophobic organic such as C-C, and C-H group during low annealing.Figures 8a and 8b show that the wettability of LP 20 changed from hydrophilic to hydrophobic surface because the air pocket may only occur at the main grove (the ablation area).When the step size became minor, deformed Cu covered the protuberant part and welded the island.Then, the surface exhibits a form of semi-regular pattern.A secondary air pocket between two recast surfaces can be encouraged, and the Cu melt flow to cover the protuberant part can be impeded by increasing the step size to the optimal step size [20,22].Compared to line patterns, the CA on grid patterns is more stable, as shown in figures 8c, because grid patterns have a small portion of a flat surface area surrounded by much-trapped air (pillars); those pillars prevent liquid from wetting the surface.After HT, the CA on LBT surfaces has greater stability than NA, as shown in figure 8d.The laser-ablated textures have lower adhesion force and sliding angle than the raw surface.The decreasing adhesion force and sliding angle because of micro/nano-villi-like (figure 8f) structure on the laser-based textures.The increasing CA to superhydrophobic decrease the adhesion force due to air trapped between two pillar or inside hole and the presence of micro/nano-villi-like oxide, which structures can trap air.Micro/nano-villi-like oxide on laser-based textures can trap air to form an air film on which the droplets seem to float [2,23].Decreasing adhesion force promoted a low sliding angle, as shown in figure 8e.

Surface composition analysis
Cu peak intensity of 111 on the laser-based textures is higher than the raw surface, and Cu peak intensity of 200 slightly decreased, as shown in figure 9.However, Cu peak intensity of 220 showed no noticeable change.Laser processing is one of the most crucial requirements for patterning crystals with a favoured growth orientation.The high energy and impact of the laser irradiation caused the increase in crystallinity, which induced the preferred direction.After laser processing, the grain size increases with increasing diffraction peak intensity [26].Different crystallographic planes of a pure, single crystal are known to have different densities of matter and surface energy; because of that, the CA of a liquid will be different from other planes [27].Wettability is changed by surface energy to vary between wetting and non-wetting levels [28].Jing Li [26] reported the correlation between XRD pattern and wettability by laser processing before and after surface patterning  The surface wettability change in figure 8 after NA or HT is due to the change in surface chemical; for example, metallic oxide on the surface.Metallic oxides have high surface energy.Figure 9 shows the Raman spectroscopy results to investigate the CuO layer, Cu2O layer, and airborne organic adsorption.CuO layer presence was dominant on the raw surface (Raman shift of 606 cm -1 ).The Cu2O (Raman shift of 625 cm -1 ) layer showed on area 1 only, and the CuO layer (292 and 591 cm -1 ) showed on areas 2 and 3 on the laser-based textures-NA.The CA of LP 100 changed to superhydrophobic after natural aging.The hydrophilic surface of the CuO layer changes slowly and becomes hydrophobic with time in the air at the ambient temperature.Some of the CuO layers on the laser-based textures can adsorb the airborne organic constituents or decomposition of CO2 or be treated chemically with low surface energy functional groups such as alkyl or fluoro silane groups, and CuO becomes a Cu2O-like layer (2CuO→Cu2O+1/2 O2) [29].The stability of CA on an ablated surface depends on the amount of CuO layer transformed to Cu2O [7].Cu2O layer distribution was not uniform; it means that the CuO NPs distributed heterogeneously during laser ablation.It might caused by preferential metal vapor condensation and aggregation of ablated material during processing due to topography shadowing and heterogeneous surface temperature [30].The non-homogeneous distribution of the Cu2O layer on laserbased textures NA influences the stability of CA, as shown in figure 8c.Cu2O was distributed uniformly on the laser-based textures HT samples showed a Raman shift of 625 cm -1 in area 1 and area 2, and a Raman shift of 520 cm -1 and 623 cm -1 in area 3, as shown in figure 10.Low annealing heat treatment in the air can accelerate the CuO layer to the Cu2O layer [7].The homogeneous distribution of Cu2O makes CA more stable, as shown in Fig 8d.
Homogeneous adsorption of C-H groups on all areas of laser-based textures NA (Raman shift of 2860 cm -1 and 2858 cm -1 ) or HT (Raman shift 2823, 2859, and 2989 cm -1 ) shown in figure 10.C-H groups are only on the raw surface at area 3 (Raman shift 2988 cm -1 ).The C-C groups were distributed heterogeneously on all samples.The C-C/C-H group is nonpolar, which is beneficial to surface hydrophobicity.Hydroxylation may happen as a result of a surface crystalline defect (or ultrafine grain structure) after laser-ablated surface [31][32][33].More stable CA on laser-based textures after HT because of the homogeneous distribution of Cu2O and C-H groups on all surface areas.The CA of laser-based textures (LP 100) NA was not stable due to the heterogeneous distribution of Cu2O.

Figure 3 .
Figure 3. Laser-based textures for (a) line pattern of 20 µm step size, (b) line pattern of 100 µm step size, (c) line pattern of 150 µm step size, (d) microstructure of line pattern of 100 µm step size, (e) grid pattern of 20 µm step size, (f) grid pattern of 100 µm step size, (g) grid pattern of 150 µm step size, and (h) microstructure of grid pattern of 100 µm step size.

Figure 5 .
Figure 5. Surface roughness and height of valley (Sz) of the raw surface and LBT.

Figure 7 .
Figure 7. CA before and after LBT on the Cu surfaces.

Figure 8 .
Figure 8. CA evolution on laser-based textures (LBT): (a) natural aging for five days, (b) after HT, (c) stability of CA of LBT natural aging samples, (d) stability of CA on LBT-HT samples, (e) adhesion force and sliding angle of water droplets on 120 days of Cu samples and (f) illustration of micro/nanovilli-like structure.
. The superhydrophobic surface (low adhesion) has the lowest peak intensity of Al (111), the highest peak intensity of Al (200), Al (220), and Al (311).However, high adhesion has a high peak intensity of Al (111), the low peak intensity of Al (200), Al (220), and Al (311).The peak intensity of Al (222) on superhydrophobic surface disappeared.In this study, LP 100 and GP 100 have a high CA compared to raw surfaces because the peak intensity of Cu (111) increased after laser ablation, and the peak intensity of Cu (200) decreased after laser ablation.The peak intensity of Cu (200) is dominant on the raw surface, and the raw surface has hydrophobic properties.NA and HT have not influenced the increase in the crystallographic planes' peak intensity.

Figure 9 .
Figure 9. XRD results on Cu surface with different surface patterns and treatments.The surface wettability change in figure8after NA or HT is due to the change in surface chemical; for example, metallic oxide on the surface.Metallic oxides have high surface energy.Figure9shows the Raman spectroscopy results to investigate the CuO layer, Cu2O layer, and airborne organic adsorption.CuO layer presence was dominant on the raw surface (Raman shift of 606 cm -1 ).The Cu2O (Raman shift of 625 cm -1 ) layer showed on area 1 only, and the CuO layer (292 and 591 cm -1 ) showed on areas 2 and 3 on the laser-based textures-NA.The CA of LP 100 changed to superhydrophobic after natural aging.The hydrophilic surface of the CuO layer changes slowly and becomes hydrophobic with time in the air at the ambient temperature.Some of the CuO layers on the laser-based textures can adsorb the airborne organic constituents or decomposition of CO2 or be treated chemically with low surface energy functional groups such as alkyl or fluoro silane groups, and CuO becomes a Cu2O-like layer (2CuO→Cu2O+1/2 O2)[29].The stability of CA on an ablated surface depends on the amount of CuO layer transformed to Cu2O[7].Cu2O layer distribution was not uniform; it means that the CuO NPs distributed heterogeneously during laser ablation.It might caused by preferential metal vapor condensation and aggregation of ablated material during processing due to topography shadowing and heterogeneous surface temperature[30].The non-homogeneous distribution of the Cu2O layer on laserbased textures NA influences the stability of CA, as shown in figure8c.Cu2O was distributed uniformly on the laser-based textures HT samples showed a Raman shift of 625 cm -1 in area 1 and area 2, and a Raman shift of 520 cm -1 and 623 cm -1 in area 3, as shown in figure10.Low annealing heat treatment in the air can accelerate the CuO layer to the Cu2O layer[7].The homogeneous distribution of Cu2O makes CA more stable, as shown in Fig8d.Homogeneous adsorption of C-H groups on all areas of laser-based textures NA (Raman shift of 2860 cm -1 and 2858 cm -1 ) or HT (Raman shift 2823, 2859, and 2989 cm -1 ) shown in figure10.C-H groups are only on the raw surface at area 3 (Raman shift 2988 cm -1 ).The C-C groups were distributed heterogeneously on all samples.The C-C/C-H group is nonpolar, which is beneficial to surface hydrophobicity.Hydroxylation may happen as a result of a surface crystalline defect (or ultrafine grain structure) after laser-ablated surface[31][32][33].More stable CA on laser-based textures after HT because of the homogeneous distribution of Cu2O and C-H groups on all surface areas.The CA of laser-based textures (LP 100) NA was not stable due to the heterogeneous distribution of Cu2O.

Figure 10 .
Figure 10.Raman spectra of raw Cu surface and laser-based texture (LBT) after NA and HT.
Our study has shown that the fast fabrication of superhydrophobic Cu alloy fabricated by commercial fiber laser-1062nm and stability of superhydrophobic can achieved by two-step fabrication.After surface patterning, the irregular Cu surface was successfully transformed into a line and grid pattern.Laser ablation has increased surface roughness; the crystallographic planes increased (111) and decreased (200) planes after laser ablation.The rapid evolution of the LBT surface from hydrophilic to superhydrophobic surface causes the rapid change from hydrophilic CuO to hydrophobic Cu2O and airborne organic (C-H groups) adsorption.The transition of hydrophilic to hydrophobic or superhydrophobic surfaces on natural aging samples depends on step size, H/D, Sz, and crystallographic plane.Laser-based textures after low annealing heat treatment are higher and more stable than natural aging because of the more uniform distribution of Cu2O and the adsorption of C-H groups.The grid pattern of 100 µm has a superhydrophobic surface [163 ˚ (NA) and 167˚ (HT) on 120 days], low adhesion force (59 µN and 50 µN and low sliding angle of 4.3˚).Superhydrophobic with low adhesion force could be used in self-cleaning applications.