Simultaneous Manipulation of the Optical and Wettability Properties of Metal Surfaces Using 150 kHz Femtosecond Fiber Laser

: We demonstrate the formation of permanent and iridescent colors on aluminum, copper, steel, and brass surfaces using femtosecond laser-induced periodic and non-periodic nanostructuring. We show that both the permanent and iridescent colors of the metal surfaces can be erased and re-colored using a second stage of laser processing. A correlation was found between the spectral reflective properties of the laser-processed surfaces and their wettability properties. Transition from superhydrophilic to superhydrophobic response is observed while tailoring the optical reflectance of the metal surfaces. We employ a high power femtosecond fiber laser at 150 kHz repetition rate, which notably reduces the processing time, making this technique attractive for practical applications.


Introduction
Laser-induced structuring has become a versatile technique to modify the surface properties of various materials. Whether used for art features and decorative purposes or for scientific applications to produce multifunctional surfaces, laser structuring has become highly captivating in technological and scientific communities [1][2][3][4]. The unique ability of high-power short laser pulses to modify almost all kinds of materials on the nanoscale poses great potential for various novel applications in optics, optoelectronics, microfluidics, color marking, and mechanics [5,6]. Femtosecond laser processing allows precise control over local nanosized structures on material surfaces to make them highly effective and multifunctional, particularly, in the coloring and modification of surfaces wettability [7][8][9][10]. Laser-induced coloring is environment-friendly as it eliminates the usage of any pigments or chemicals, while being quite flexible in achieving the desired results by optimizing laser irradiation parameters [11].
During the interactions of intense laser irradiation with material surfaces, different features may be induced. Highly-localized laser energy deposition can be achieved by a variety of means, the most efficient of which are using a tightly focused beam with its peak power just above the melting threshold [12], or by utilizing the optical near field effects [13], as well as the interference of several Gaussian laser beams [14]. The former method can be applied for generating jet-like structures and their arrays, including the fabrication of spiral surface structures [15], whereas the latter was recently utilized in the generation of periodic patterns on metals with UV laser pulses [16]. Table 1. Laser induced structures and potential areas of applications.

Structures Features Potential Application Areas Reference
Hierarchical micro/nanostructures Hydrophobicity, water collection [35] Scale-like structures Friction reduction [36] Pillar-like, nanowires nanostructures Anti-reflection [37] Periodic ripples, micrograting Structural coloring, iridescence [38,39] Micro/nano spikes Anti-corrosion, Anti-mechanical fatigue resistance [40] Laser ablation can be considered one of the most efficient techniques to directly produce superhydrophobic surfaces on a wide range of materials [41]. By optimizing the laser processing parameters, it is possible to produce robust 3D structures that are necessary to prolong the durability of the processed surfaces [42]. Femtosecond lasers have been used dominantly to produce nanostructures on different metal surfaces to alter their wettability behavior. Following laser ablation, processed surfaces may initially behave as superhydrophilic, but when kept in ambient air for many days, they transform into super-hydrophobic ones with very small contact angle hysteresis that makes them suitable for a wide range of technological and bio-medical applications [43]. Some divergences of aging-dependent contact angle hysteresis were observed for surfaces processed by femtosecond pulses compared to those processed by nanosecond pulses [44,45]. Superhydrophobicity of laser-ablated surfaces is attributed to the surface chemistry and topography of the processed surface [29,46]. The ultimate degree of the achieved superhydrophobicity depends on the produced surface roughness. Furthermore, the changes in the wettability behavior of the air-exposed laser-processed surfaces can be also attributed to the increase of carbon content and decrease of surface polarity due to chemisorbed organic substances from the air moisture [47].
The important issue of all these laser processing procedures is their duration. In most of the above-considered cases, the used lasers operate at pulse repetition rates in the range of tens Hz to a few kHz, which results in a relatively slow speed of processing the surfaces. The application of advanced high pulse repetition rate lasers (i.e. those operating at >100 kHz regime) along with high scanning speed (i.e. >100 mm/s) can significantly accelerate the modification of coloring and wettability properties of materials. To operate with such class lasers for LIPSS formation, one has to deal with issues related to the higher loading of laser power on the metal surfaces preventing the optimization of coloring and wettability properties of the processed surfaces. Once a robust methodology of laser-matter interaction at these high pulse repetition rate conditions is developed, the processing time of coloring and wettability modifications can be decreased by orders of magnitude, which will make it attractive for industrial applications.
In this work we used 150 kHz laser for LIPSS formation and describe methods here for optimizing such a high pulse repetition rate regime of the laser-matter interaction that leads to modifying the coloring and wettability properties of the developed surfaces. We show how nanostructures produced by ultra-short laser pulses can change the optical diffractive effects leading to the generation of angle-dependent colors on steel, copper, aluminum, and brass surfaces. We present a simple process of re-coloring of aluminum to prove the sustainability and reusability of the processed samples. We analyzed the wettability properties of such surfaces and demonstrate the relation between these properties and the lasting coloring of surfaces. We also demonstrate how a superhydrophobic aluminum surface can be developed by femtosecond laser processing when stored in vacuum for several hours after the laser treatment.

Experimental Arrangements
An Ultrafast high-performance Yb-doped fiber laser (UFFL_300_2000_1030_300; Active Fiber System), with a central wavelength of 1030 nm, a repetition rate of 150 kHz, and a pulse duration of 40 fs, was used to irradiate the surfaces (Figure 1a). The laser polarization was linear with beam quality close to the diffraction limit (M 2 < 1.3), and average power stability <1% of the root mean square (RMS) value. The laser beam was directed to a computer-driven scan head (FARO Scan System). The scan head had the flexibility to control the scanning speed along the metal surface, line spacing between the markings, and some other features such as the shape of the irradiated area, angle of incidence, direction of the laser beam, and others. The laser system controller maintained the parameters of laser pulses like repetition rate and pulse duration. A half wave plate (HWP) and a thin film polarizer (TFP) were used to control the power of laser pulses on the target surfaces ( Figure 1a). A 3-D motorized stage (3DMS) was used to adjust the position of the sample under the scan head. The scan head was equipped with an F-Theta lens, which provided almost constant spot size across the entire image plane (90 × 90 mm 2 ). The spacing between two lines was maintained at 50 µm, which nearly corresponded to the spot size of the focused beam. The sample position was kept constant and the scan head software controlled the beam motion. The optimal laser irradiation of surfaces for different metallic samples was determined by varying the scan speed of the laser beam.
Stainless steel, brass, aluminum, and copper plates were irradiated at different speeds of the scan head. Except for the 5 mm thick brass sample, the thickness of the processed samples was 2 mm. The surfaces of samples were cleaned with isopropanol before irradiation. A scanning electron microscope (SEM) was used to characterize the surfaces of different treated samples. The surface optical characteristics were analyzed using the experimental arrangement shown in Figure 1b. The horizontally mounted sample was illuminated by an un-polarized light source with continuum spectrum in the visible spectral range, which was transformed to a parallel beam by two lenses and an aperture. In our study, the incident position angle (α) was fixed at 20 • . To inspect the colorizing effect of the treated area, the spectra of the scattered light from different angles (β) were measured by a fiber optics spectrometer (Flame, Ocean Optics).
, 10, x FOR PEER REVIEW 1. (a) Experimental setup for metal surfaces treatment using a femtosecond laser at e duration 40 fs, central wavelength 1030 nm, and repetition rate 150 kHz coupled w H) containing an F-Theta objective lens. The sample was placed on a 3D motion stag ave plate (HWP) and a thin film polarizer (TFP) were used to control the average lses. (b) Experimental set up for the measurements of the angle-dependent color r e processed surfaces. Light (tungsten bulb) was shone on the processed samples at a ith regard to the axis perpendicular to the sample; and the diffracted light was meas pectrometer at variable angle (β).

nd Discussion
observations showed that the processed stainless steel sample demonstrated dent colors when irradiated by sun light. The copper brightness was next t en followed aluminum, while the brass sample showed the least brightness. lors were attributed to the formation of LIPSS on the irradiated surfaces, as w analysis below. allets of different metals are shown in Figure 2. Pallets (a-d) correspond to s

Results and Discussion
Visual observations showed that the processed stainless steel sample demonstrated the brightest angle-dependent colors when irradiated by sun light. The copper brightness was next to the stainless steel and then followed aluminum, while the brass sample showed the least brightness. The observed different colors were attributed to the formation of LIPSS on the irradiated surfaces, as will be shown in the SEM analysis below.
Color pallets of different metals are shown in Figure 2. Pallets (a-d) correspond to stainless steel, copper, aluminum, and brass samples, respectively. All samples were prepared at different scan speeds to optimize LIPSS for the observation of different colors. The optimal scanning speeds for stainless steel, brass, aluminum, and copper were 350, 400, 500, and 450 mm/s, respectively. Pictures of the samples were taken in sunlight at fixed incident position (α = 20 • ). The angle of sample observation was changed from 0 • to 40 • when different colors were observed. The processing times for disks of 20 mm diameter of stainless steel, brass, aluminum, and copper were 15 s, 14 s, 11 s, and 12 s respectively.
Scanning electron microscope (SEM, TESCAN Vega 3) images show that all angle-dependent colors were features of the periodic structures imprinted on the surface of each metal ( Figure 3). The directions of all nanoripples were orthogonal to the polarization axis of the laser beam. The distance between nanoripples was measured by SEM and was found to be approximately 1 µm, which is very comparable to the wavelength of the used femtosecond laser). Scanning electron microscope (SEM, TESCAN Vega 3) images show that all angle-dependent colors were features of the periodic structures imprinted on the surface of each metal ( Figure 3). The directions of all nanoripples were orthogonal to the polarization axis of the laser beam. The distance between nanoripples was measured by SEM and was found to be approximately 1 µm, which is very comparable to the wavelength of the used femtosecond laser). The measurements of the angle-dependent color reflectance from the surfaces were conducted using the setup shown in Figure 1b. The broad-bandwidth emission from the tungsten lamp was directed on the laser-treated area of the sample and the diffracted light was measured using the fiber spectrometer at different angles with respect to the normal of the processed surface. The diffraction equation mλ = d (sinα + sinβ) was used to determine the direction from which the different colors resulting from LIPSS were viewable. Here m is the diffraction order, λ is the wavelength, d is the LIPSS spacing, α is the angle of incident light, and β is the angle of diffracted light. The spectral analyses of the treated aluminum, copper, and stainless steel surfaces are shown in Figure 4a-c. These graphs show a very consistent pattern for all metallic samples and hence serve as evidence of reflectance similarities due to the same LIPSS on different metallic surfaces. Meanwhile, in the case of non-treated surfaces (Figure 4d-f), the reflectance spectra remain the same at different angles of observation, contrary to the case of treated surfaces (Figure 4a-c). Scanning electron microscope (SEM, TESCAN Vega 3) images show that all angle-dependent colors were features of the periodic structures imprinted on the surface of each metal ( Figure 3). The directions of all nanoripples were orthogonal to the polarization axis of the laser beam. The distance between nanoripples was measured by SEM and was found to be approximately 1 µm, which is very comparable to the wavelength of the used femtosecond laser). The measurements of the angle-dependent color reflectance from the surfaces were conducted using the setup shown in Figure 1b. The broad-bandwidth emission from the tungsten lamp was directed on the laser-treated area of the sample and the diffracted light was measured using the fiber spectrometer at different angles with respect to the normal of the processed surface. The diffraction equation mλ = d (sinα + sinβ) was used to determine the direction from which the different colors resulting from LIPSS were viewable. Here m is the diffraction order, λ is the wavelength, d is the LIPSS spacing, α is the angle of incident light, and β is the angle of diffracted light. The spectral analyses of the treated aluminum, copper, and stainless steel surfaces are shown in Figure 4a-c. These graphs show a very consistent pattern for all metallic samples and hence serve as evidence of reflectance similarities due to the same LIPSS on different metallic surfaces. Meanwhile, in the case of non-treated surfaces (Figure 4d-f), the reflectance spectra remain the same at different angles of observation, contrary to the case of treated surfaces (Figure 4a-c). The measurements of the angle-dependent color reflectance from the surfaces were conducted using the setup shown in Figure 1b. The broad-bandwidth emission from the tungsten lamp was directed on the laser-treated area of the sample and the diffracted light was measured using the fiber spectrometer at different angles with respect to the normal of the processed surface. The diffraction equation mλ = d (sinα + sinβ) was used to determine the direction from which the different colors resulting from LIPSS were viewable. Here m is the diffraction order, λ is the wavelength, d is the LIPSS spacing, α is the angle of incident light, and β is the angle of diffracted light. The spectral analyses of the treated aluminum, copper, and stainless steel surfaces are shown in Figure 4a-c. These graphs show a very consistent pattern for all metallic samples and hence serve as evidence of reflectance similarities due to the same LIPSS on different metallic surfaces. Meanwhile, in the case of non-treated surfaces (Figure 4d-f), the reflectance spectra remain the same at different angles of observation, contrary to the case of treated surfaces (Figure 4a-c).
aluminum, stainless steel, brass, and copper satisfy the conditions of SPP excitation [51], which results in generation of LIPSS similar to those detected in both single-pulse [52] and multi-pulse experiments [53]. Theoretical interpretation of the SPP influence on the mechanism of metal surfaces restructuring with laser pulses was recently reported in [23]. The interference of the incident laser field with the excited SPP field may lead to laser intensity redistribution across the irradiated surface, which results in modulation of the sample surface heating and hence the growth of periodic patterns [21]. Moreover, the laser intensity redistribution across the surface can induce the growth of the periodic nanostructures even though the averaged incident fluence is below the melting threshold. In addition, it was theoretically identified that due to the correlation established between the laser intensity redistribution and the pre-structured surface, the LIPSS could be produced by pre-structured beams [12,16] or due to excitation of SPP waves [48], i.e. surface plasmons coupled to an incident laser pulse. In the latter case, any pre-existing structures or initial surface roughness can serve as centers for SPP excitation [49]. At these centers, normally comprising a local electronic density jump, the fast oscillating electromagnetic field can result in a local collective motion of electrons. Such SPP waves can travel across the surface with distances up to tens of microns, depending on the optical properties of the medium [50]. The optical properties of aluminum, stainless steel, brass, and copper satisfy the conditions of SPP excitation [51], which results in generation of LIPSS similar to those detected in both single-pulse [52] and multi-pulse experiments [53]. Theoretical interpretation of the SPP influence on the mechanism of metal surfaces restructuring with laser pulses was recently reported in [23].
The interference of the incident laser field with the excited SPP field may lead to laser intensity redistribution across the irradiated surface, which results in modulation of the sample surface heating and hence the growth of periodic patterns [21]. Moreover, the laser intensity redistribution across the surface can induce the growth of the periodic nanostructures even though the averaged incident fluence is below the melting threshold. In addition, it was theoretically identified that due to the correlation established between the laser intensity redistribution and the pre-structured surface, the formation of LIPSS becomes more preferable in the multi-pulse regime [54]. The latter was confirmed in recent experimental studies on Si [55]. Our experiments were carried out in the multi-pulse regime, when the range of overlaps between nearby beams was adjusted by the scan speed, while maintaining suitable average power and fluence of the heating pulses. Thus, application of high pulse repetition laser-matter interactions can be considered an advanced approach for simultaneous modifications of coloring and wettability properties of the processed surfaces.
Besides showing the ability to produce new colors on the pristine samples, below we demonstrate that the color on the surface of processed metals can be erased and re-colored by using the same laser radiation. As shown in Figure 5, a sample of aluminum was made dark golden (A) and then erased to restore it to almost its original color. The same sample was then colored to a whitish color (B), part of which was erased again, and colored to the black semi-circle (C).
Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 14 formation of LIPSS becomes more preferable in the multi-pulse regime [54]. The latter was confirmed in recent experimental studies on Si [55]. Our experiments were carried out in the multi-pulse regime, when the range of overlaps between nearby beams was adjusted by the scan speed, while maintaining suitable average power and fluence of the heating pulses. Thus, application of high pulse repetition laser-matter interactions can be considered an advanced approach for simultaneous modifications of coloring and wettability properties of the processed surfaces.
Besides showing the ability to produce new colors on the pristine samples, below we demonstrate that the color on the surface of processed metals can be erased and re-colored by using the same laser radiation. As shown in Figure 5, a sample of aluminum was made dark golden (A) and then erased to restore it to almost its original color. The same sample was then colored to a whitish color (B), part of which was erased again, and colored to the black semi-circle (C). In the center of the circle a strip was colored to light golden (D), and finally, part of the whitish surface of the same sample was colored to dark golden (E). Such manipulation of colors demonstrates the reproducibility and sustainability of these samples. The erasing and alteration of colors were achieved by changing the fluence of the laser beam by varying the position of the sample from the focus position (0.5 J/cm 2 to 1.0 J/cm 2 ). The surface was restored to each previous color by irradiating at a smaller fluence with regard to the initial coloring procedure by changing the processed sample position with respect to the focal plane of F-Theta lens and ablating the existing patterns. In that case, the number of pulses on the surface area and the resulting incident fluence were governed by the scanning speed for each color at 150 kHz repetition rate. The slower speed (i.e; below 70 mm/s) In the center of the circle a strip was colored to light golden (D), and finally, part of the whitish surface of the same sample was colored to dark golden (E). Such manipulation of colors demonstrates the reproducibility and sustainability of these samples. The erasing and alteration of colors were achieved by changing the fluence of the laser beam by varying the position of the sample from the focus position (0.5 J/cm 2 to 1.0 J/cm 2 ). The surface was restored to each previous color by irradiating at a smaller fluence with regard to the initial coloring procedure by changing the processed sample position with respect to the focal plane of F-Theta lens and ablating the existing patterns. In that case, the number of pulses on the surface area and the resulting incident fluence were governed by the scanning speed for each color at 150 kHz repetition rate. The slower speed (i.e; below 70 mm/s) resulted in an excessive absorbed energy and a rather roughly treated surface due to its ablation, whereas the Appl. Sci. 2020, 10, 6207 8 of 14 higher speed (i.e; above 200 mm/s) resulted in an under-structured LIPSS pattern. The former case corresponded to the black color due to random non-uniform structure formation similar to that demonstrated in [56], whereas the latter case resulted in less pronounced periodic structures with faint images of diffracted golden or brown colors. Finally, we found an optimal scanning speed for every color formed, with speeds ranging from 10 mm/s for the black color up to 1500 mm/s for the whitish color. The formation of LIPSS was found to be most efficient at an optimum scanning speed of 500 mm/s, at which the number of pulses and total incident fluence per unit area resulted in the generation of structures with more pronounced periodicity, sharp images of ripples, and multicolor properties as a function of the viewing angle.
Alongside the manipulation of the colors of the treated samples, we also characterized the wettability of the surfaces possessing different colors. Characterization of wettability was done by measuring the static contact angle, using a sessile drop optical measurement method. We used drop shape analyzer (100 Expert, KRUSS) to measure the contact angle of smooth surfaces where the drop lies on the solid surface. We measured the surface morphology of different colors using SEM. Variable scanning speeds were used to achieve different colors on the aluminum surface. Black, brown, golden, and iridescent colors were obtained at the speeds of 10, 70, 200, and 500 mm/s, respectively.
SEM images ( Figure 6) show the morphologies of the surfaces processed at the above-mentioned scanning speeds. It is clearly seen from the images that the low speeds of processing (10 and 70 mm/s) resulted in the formation of non-periodic micro-and nanostructures on the surfaces ( Figure 6A,B). This is well understood as a higher number of overlapped laser shots hit the surface resulting in greater absorbed energy and stronger ablation of the material. One can see the appearance of nanoto micro-sized particles between the "mountains" and "valleys" of those images. The golden color was produced at 200 mm/s and at this speed the periodic structures started to appear ( Figure 6C). However, the LIPSS in that case was of poor quality and showed some nano/micro sized bead-like structures on the ripples. For the angle-dependent (iridescent) samples prepared at 500 mm/s, the SEM images show the appearance of sharp ripples ( Figure 6D). For comparison we show here the SEM image of untreated surface as well ( Figure 6E).
The last column of Figure 6 shows the results of the wettability measurements of the aluminum surfaces demonstrating different colors. While laser processing of the samples initially resulted in their wettability shift towards hydrophilic behavior, dramatic transformation into superhydrophobic response was observed after those samples were kept in vacuum for 24 h. In order to reveal the effect of vacuum treatment explicitly, two sets of samples of each color were prepared at the same conditions. The wettability response of the first set of samples was measured right after the laser processing. All colored samples exhibited contact angle values less than 15 • and were correspondingly classified as hydrophilic. The second set of processed samples, was kept in vacuum at 2.5 × 10 −4 mbar for 24 h prior to the contact angle measurements. It was found that, after vacuum storage, different colors provide different variations of wettability. We observed that the golden color surface had the highest contact angle close to 156 • , thus exhibiting superhydrophobic behavior. The contact angles of black, brown, and multicolor samples were 149, 147, and 145 respectively. When we placed 5 µL water droplets, the droplet was repelled back by the hierarchical rough micro-nanostructures and the low surface free energy of the structured material. The presence of nano/micro-sized bead-shaped particles on top of the ripples of golden color samples resulted in a decrease of the surface free energy and caused a high contact angle. This set of experiments showed that the laser-structured surfaces that were stored in vacuum for 24 h were extremely water repellant and hence superhydrophobic. Moreover, to test the sustainability of these superhydrophobic surfaces, we repeated the contact angle measurements after keeping the samples in air for many weeks; and yet they showed the same superhydrophobic behavior. Earlier, vacuum storage of nanosecond and picosecond laser-structured aluminum and titanium allowed the superhydrophobicity response of their processed surfaces to be increased due to enhanced adsorption of carbonaceous species on the laser textured surfaces [57,58]. These organic compounds were assumed to emanate from the walls of the chambers and other remaining hydrocarbons in the vacuum chamber maintained at the base pressure~10 −4 mbar [58]. While hydrogen-bonded H 2 O molecules on the hydroxylated layer were believed to hinder further adsorption of the hydrocarbons in ambient atmosphere, the extremely low water vapor pressure in the vacuum chamber allows faster chemisorption of hydrocarbons. This gave rise to speedy realization of the superhydrophobic characteristics on the structured surface, as observed in our studies.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 14 This set of experiments showed that the laser-structured surfaces that were stored in vacuum for 24 h were extremely water repellant and hence superhydrophobic. Moreover, to test the sustainability of these superhydrophobic surfaces, we repeated the contact angle measurements after  Figure 7 shows the Atomic Force Microscope and Confocal Microscope topographic profiles for the surfaces of different colors of the aluminum sample. As shown in the figure, the black and brown colored surfaces had larger as well as irregular structures on the surface. The features are similar, but the black surface shows higher average surface roughness, and hence demonstrated slightly higher contact angle as compared to the brown color. On the other hand, the golden and multicolor surfaces have more regular ripple-like structures. The average roughness of golden color is higher than the multicolor. Those regular surface features are much smaller compared to the black and brown colors. The golden colored surface has ripple-like and hierarchical structures on the ripples. When a drop is placed on such hierarchical structures, the contact area between the drop and the metal surface is reduced and the air pockets underneath drop are higher compared to the other colors. Such state of contact is well explained by Cassie-Baxter's theory [59]. The presence of nano-scale hierarchical structures on the ripples of the golden color leads to the highest contact angle among the four colors. The black surface was the roughest but since the metal-drop contact was much bigger compared to the golden color, it resulted in a few degrees less contact angle.

Conclusions
In conclusion, we demonstrated the appearance of permanent and angle-dependent colors using femtosecond laser surface structuring of aluminum, copper, steel, and brass through the formation of laser-induced periodic surface structures and non-periodic nano/microstructing. We showed that both these groups of colored species can be either erased or re-colored using an additional step of laser processing. The process of material surface restructuring was attributed to the SPP excitation

Conclusions
In conclusion, we demonstrated the appearance of permanent and angle-dependent colors using femtosecond laser surface structuring of aluminum, copper, steel, and brass through the formation of laser-induced periodic surface structures and non-periodic nano/microstructing. We showed that both these groups of colored species can be either erased or re-colored using an additional step of laser processing. The process of material surface restructuring was attributed to the SPP excitation and its interference with the incident laser irradiation. This interference resulted in the laser intensity redistribution across the surface and its periodic patterning. The scanning speed was found to be a key factor in determining the resulted optical properties of the generated structures. There is an optimal scanning speed at which the pre-modified surface structure can serve as a feedback for each subsequent pulse, which enhances the accuracy of the produced LIPSS in a multipulse regime.
The relation between the spectral reflective properties of different colored metals and the wettability of the processed metal surfaces was also studied. We demonstrated the possibility to simultaneously tailor the coloring as well as the wettability properties of metals by optimizing the set of laser parameters employed in the processing. Furthermore, vacuum storage of the laser-treated samples dramatically changed their wettability response from hydrophilic to superhydrophobic lasting permanently even after leaving the vacuum-stored sample in an ambient atmosphere. Vacuum ageing allows the surface energy of the sample to be changed via desorption of the oxide layer followed by adsorption of carbon species from the surrounding environment.
All these manipulations of metal surfaces were carried out using the fiber femtosecond laser at 150 kHz repetition rate, which notably reduces the processing time to make this technique attractive for industrial applications. An 80 mm square stainless steel sample was processed in only 227 s to achieve LIPSS features on the surface.