1. Introduction Porous copper surfaces with micro-pores have been widely utilized in many fields, e.g., chemical reaction, sound absorption and heat transfer[1-3]. Their large surface area, good performance of sound absorption, and excellent thermal properties make them as good candidates for catalyst support for microreactors [4], sound absorption medium [2], wicks of heat pipes [5] and heat transfer surface in two-phase heat sinks[6]. To date, several methods, such as sintering, foaming, electrodepositing and painting means, have been developed to fabricate porous copper surfaces. Weibel et al. [5] prepared copper powder surfaces by sintering to supply as the wick of heat pipes and vapor chambers. The excellent heat transfer capacities of these sintered powder wicks have been experimentally demonstrated. Ru et al. [2] fabricated porous copper surfaces using a resin curing and foaming method. Good sound absorption performance of these porous copper were obtained. El-Genk and Ali [6] fabricated

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micro-porous surface layers on Cu substrates using a two-stage electrodepositing process. Dendritic micro-structure masked by the clusters of Cu micro-particles and surface macro-pores filled up with depositing Cu atoms and micro-particles were obtained. Li et al. [7] fabricated enhanced nanostructured porous surfaces using an electrodepositing method and obtained a 3D macro-porous dendritic Cu surface layer with nanostructured porosity. These electrodeposited porous copper surfaces were found to enhance the boiling heat transfer performance considerably. You and Chang [8] also developed porous copper layers by painting a mixture of copper powder binder and carrier. Moreover, some advanced techniques were employed to fabricate copper porous surfaces. Tang et al.[9] proposed a hot-dip galvanizing and dealloying process to fabricate nanoporous copper surfaces by a two-step procedure. First, the copper specimen was degreased and dipped into a flux solution(180g/L ZnCl2, 120g/L NH4Cl), and was further dipped in a 540℃ zinc bath for 2 min. Then the zinc was removed by immersing the as-alloyed test section in 5 wt% NaOH solution for 48h. Nanoporous copper surfaces with the pore sizes of approximately 50–200 nm and a porosity of 33-44% was obtained. Among the above fabrication methods for the copper porous surfaces, like sintering, electrodepositing and other advanced techniques, the high cost, complicated process steps, and long production period may hinder their wide applications. Furthermore, most of the porous copper surfaces fabricated by the above methods are constructed by separated particles, which form a porous layer. The strength of the porous layers is limited, and they may tend to separate from the substrates after long time runs[6]. To address the above issues, laser micromilling process may be another good choice. Laser micromilling, as one of laser manufacturing processes, is able to fabricate textured surfaces on the bulk substrates in a one-step process [10]. It fabricates a part via laser ablation by controlling the materials removal. Due to its physical characteristics, such as high power density, non-contact and easy process, it can fabricate complex geometries with high machining accuracy, and allow to process a wide variety of materials, such as metals [11], polymer [12-13], silicon[14], and glass [15]. Therefore, the laser micromilling method shows many promising merits to fabricate micro-structures for a wide range of applications. Snakenborg et al. [16] fabricated V-shaped microchannels using a CO2 laser direct-writing method on the PMMA substrate. Wang et al. [17] fabricated microfluidic channels with controlled surface wettability by using femtosecond laser direct ablation of polymethyl methacrylate at various laser fluences. Results shows that varied flow velocities and separation ratios of water in microfluidic channels have been successfully obtained by modifying the wetting characteristics of the microchannel surfaces. Pham et al. [18] machined micro-cavities on the alumina and silicon nitride ceramics substrates by the laser micromilling technology, and explored the process parameter effects on the part qualities. Aoyagi et al. [12] fabricated high aspect ratio thin holes on biodegradable polymer using laser micromilling method for their applications in a micro-needle. Zhou et al.[19] processed micro-pillars arrays on copper by the laser micromilling for their applications in biomedical electrodes. The wettability of these micro pillars arrays was also explored. As enumerated above, the current efforts concerning the laser micromilling have been devoted to the fabrication of microchannels, micro-holes, micro-cavities or micro-pillars. Nevertheless, the knowledge of fabrication of porous copper surfaces by laser micromilling process is still limited. To this aim, we in this study introduce the laser micromilling technique to fabricate porous copper surfaces without any pre-and post-processing procedures. A series of porous copper surfaces were obtained using a pulsed fiber laser on pure copper plates in a one-step process. The effects of processing parameters, such as laser fluence, scanning speed, number of scanning cycles and scanning interval, on the formation and surface properties of porous surfaces were comprehensively investigated. Moreover, the wetting characteristics of these

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porous copper surfaces were also assessed using a sessile drop method. The results in this study are believed to provide critical information for the fabrication and application of the developed porous copper surfaces, and are of considerable practical importance.

2. Experiment

2.1 Laser micromilling process Commercial pure red copper plates (99.9% Cu) with a thickness of 2mm, length of 10mm and width of 10mm were utilized to prepare porous copper surfaces. Before the laser micromilling process, all the copper plates were polished and cleaned with deionized water and anhydrous ethanol in an ultrasonic bath each for 15 minutes. Then they were dried by the compressive air. The laser micromilling process was carried out in a pulsed fiber laser (IPG, No: YLP-1-100-30-30- HC-RG, Russia), which equipped with a fiber laser, scanning galvanometer, focusing lens, computer controlled systems, power equipment, CCD camera, LED light and working table. The maximum output power of the laser was 30W. The laser was set to produce 100 ns pulses with a 1064nm wavelength (λ) at a repetition rate (f ) of 20kHz, which is the same as the previous study of Deng et al. [20]. The specifications of characteristic parameters of the fiber laser system are given in Table1. Figure 1(a) shows a schematic of the laser micromilling process on copper surfaces. The cross machining route and loop multiple-pass reciprocating scanning strategy utilized in many studies [14,21] are adopted to process the copper surfaces. The material was removed by the translation of laser beam in x and y directions, and the porous surface was formed after the laser micromilling process. Figure 1(b) shows the details of laser spot in the scanning pattern. The pulse overlap denotes the distance between the pulses, which is given by the ratio of laser scanning speed(v) and repetition rates [22]. The step overlap is induced by the intersection of two subsequent beams, which is dependent on the scanning interval. Laser fluence, F, is an important factor in the laser micromilling process. It is can be calculated as follows:

F

4P (1)  f  2 spot

Where P is the laser fluence, f is the repetition rate, and  spot is the focused diameter of the laser spot. In the laser micromilling process, the processing parameters, such as laser fluence (F), scanning speed (V), the number of scanning cycles (N) and scanning interval (S), may play notable roles on the formation of porous surfaces. In order to assess processing parameters effects on the geometrical morphology and wettability of porous copper surfaces, a series of laser fluence,

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scanning speed, number of scanning cycles and scanning interval were utilized in the laser micromilling process, as shown in Table 2.

2.2 Geometric dimensions measurement After the laser micromilling process, the porous copper surfaces were created on pure copper plates. The obtained porous copper surfaces were ultrasonically cleaned in ethanol for about half an hour to eliminate possible residues or contaminants. Then the samples were dried by compressive air for about 20 minutes. The morphologies of porous surfaces were measured by a scanning electron microscope (SEM, HITACHI SU-70) with energy-dispersive X-ray measurements (EDX). EDX measurements were made to evaluate the change of the chemical elements on the laser micro-manipulated surfaces. The 3D profiles of the porous copper surfaces and geometry were measured by a 3D laser scanning confocal microscope (Keyence VK-X200), which provides non-contact and high-accuracy surface measurement with 0.5 nm Z-resolution. Figure 2 illustrates the geometric dimensions of porous copper surfaces, where Di（i=1,2,...,n）is the measured depth of any individual micro-holes. The average depth value (Dave ) of the scanned regions can be calculated in the image software of laser scanning confocal microscope. Ten randomly selected scanned regions of 718.8×500μm were measured in each porous surface sample, and the means of all ten measurements of Dave was then utilized for the results of depth of micro-holes in each sample. The averaged values of the arithmetic mean surface roughness (Ra) of porous copper surfaces were also obtained by averaging the results of the above ten randomly selected scanned regions.

2.3 Contact angle measurement Surface wetting properties are critical for the frictional, corrosion and heat transfer performances of porous surfaces [23-24]. In order to evaluate the wetting properties of the obtained porous copper surfaces, the static contact angles were measured at about 2 hours later after the laser fabrication process. The contact angle measurement was conducted by a sessile drop technique using an automatic contact angle meter (Model: DCAT21, Dataphy Instrument Co., Ltd, Germany). A ~3μl droplet of deionized water was dispensed onto the sample surface using a syringe pump under atmospheric conditions. After the droplet remained on the porous surface for about 30s, images of the droplet were taken by the CCD camera. The contact angle was obtained by analyzing the images through the supplied software. All mean contact angle values were calculated from at least five individual measurements on randomly selected areas. The standard deviation and the average error for contact angle measurements were estimated to be 1◦. All the measurements were performed at room temperatures (~25◦C ).

3. Results and discussion

3.1 Laser micromilling process of porous copper surfaces Laser micromilling, as a thermal machining process, removes materials by the reaction between the laser beam and the workpiece. The incident laser beam is directed at the material to be removed, and the surface material is heating by the laser beam. Then the surface material is removed following the procedures of rapidly heating, melting, evaporation, re-solidification and ablation [25-26]. It consists of the following procedures. When the laser beam irradiates on the material surface, surface thermal energy accumulated quickly, resulting in a significant increase in the temperature of material surface. When the incident laser fluence is close to the laser damage threshold, the material in the workpiece begins to melt within the laser irradiated zone. Due to the Gaussian intensity distribution in the laser spot, the materials absorbs more energy at the center of the circular spot than that at the edge, leading to the formation of a large temperature gradient in the irradiated area. As a result of the radial surface tension and

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temperature gradient, the liquid material would be expelled to the periphery of the molten pool and rapidly solidify to form protruded surfaces. When the laser intensity at the center of the laser irradiated area is sufficiently high to produce ablation, the material particles would be ejected outside the edge of the molten pool, resulting in the formation of micro-holes and micro-cavities in the pool position. The ejected materials cool rapidly and re-solidify to form the recast layer around the micro-holes and micro-cavities, and induced ripples-like structures. With the movement of the laser beam, more micro-holes and micro-cavities, as well as ripples-like structures were formed on the copper, and the porous copper surfaces were finally formed. Fig.3 illustrates the morphologies of porous copper surfaces by SEM images and their composition elements by EDX measurement. It can be noted that numerous homogeneous micro pores and cavities with the size of about 1~15μm are formed after the laser micromilling process. Besides of the main elements of copper, oxygen can be also notable, which is induced by the oxidation during the vaporization and evaporation process as well as the re-solidification of material. There are very little carbon elements, which come from the decomposition of carbon dioxide into carbon with the aid of active magnetite in the laser ablation process [27].

3.2 Effects of processing parameters on the porous copper surfaces

3.2.1 Effect of laser fluence Figures 4 and 5 show the SEM and 3-D profile images of porous surfaces fabricated with different laser fluences. Five laser fluences of 77, 96, 115, 135 and 154 J/cm2 were imposed in the laser micromilling process at the same scanning speed of 250mm/s, number of scanning cycles of 10 and scanning interval of 5μm. It can be noted that the micro-holes and cavities were hardly to be formed when the laser fluence was smaller than 115 J/cm2, and the smooth copper surfaces did not changed remarkably after the laser irradiation. The average depth and roughness of copper surface were also small, as shown in Fig.6(a) and (b), respectively. This can be linked to the fact that the laser fluence is too low and the material does not absorb enough laser energy, which hinders the sufficient melting and evaporation process of surface material. Few material particles were ejected outside the laser irradiated area, and the micro-holes and micro-cavities were hardly to form. Nevertheless, when the laser fluence increased to 115 J/cm2 or higher, the micro-holes and cavities were easily formed on the copper substrates. The average depth of micro-holes and roughness of porous surfaces increased greatly with increasing the laser fluence when the laser fluence was higher than 96 J/cm2. For example, the Dave increases for about three times (from 32μm to 93μm) when the F increased from 96 J/cm2 to 154 J/cm2, indicating that the laser fluence plays a significant role on the formation and geometry of porous surfaces on copper. With the increase in laser fluence, more laser energy was absorbed by the material in a certain area, and the energy density, i.e., energy per area, increased considerably. It resulted in more ablation and ejection of materials and the formation of deeper molten pool, thus the depths of micro-holes and cavities increased. More ripples-like structures were formed, as shown in Fig.4 and Fig.5, which increased the surface roughness of laser irradiated surfaces at larger laser fluence.

3.2.2 Effect of scanning speed Figures 7 and 8 show the SEM and 3-D profile images of porous copper surfaces fabricated with different scanning speeds. Five scanning speeds of 100mm/s, 200mm/s, 250mm/s, 300mm/s and 400mm/s were utilized at the same laser fluence of 115 J/cm2, number of scanning cycles of 10 and scanning interval of 5μm. Porous surfaces with a large number of micro-holes and cavities were all formed in these cases. Nevertheless, the micro holes tend to be smaller and close at the largest scanning speed of 400mm/s, indicating that large scanning speed plays a negative role on the formation of micro holes on copper surface. The average

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depth of micro pores and roughness of porous copper surfaces are shown in Fig.9. The copper porous surfaces presented large depth and roughness at low scanning speeds of 100mm/s and 200mm/s. In these cases, the surface material was irradiated by the laser beam for a long time. The energy density was large due to the heat transfer and diffusion effect. This resulted in more material removal to form deeper holes and more solidification of surface recast layer. Therefore, micro-porous surfaces with large depth and roughness can be obtained. When the scanning speed increased from 200mm/s to 400mm/s, the average depth and roughness of porous copper surfaces presented a decreasing trend. With the increase of laser scanning speed, the distance between adjacent pulses increased. This induced the decrease in the overlapping region between two adjacent laser spots (Fig.1(b)). The interaction time (n) of a certain area irradiated by the laser spot decreased, which can be calculated by [28]  spot (2) n V where  spot is the focused diameter of the laser spot, V is the scanning speed. At a constant laser fluence, the decrease of interaction time contributed to the decrease of total energy absorbed by a certain area. Less material removal by the melting, evaporation and ejection process was thus induced, and smaller depth and roughness of porous copper surfaces was obtained. The above trend accorded with the findings of Saklakoglu and Kasman [28], in which smaller milling depth and surface roughness was also obtained at larger scanning speed when tool steel AISI H13 were processed by the laser micromilling method. Besides, it can be noted when the scanning speed increased from 200mm/s to 400mm/s, the Dave just decreased from 77 to 60μm, and the Ra decreased from 8.8 to 6μm. Such magnitude of variation was much smaller than those at different laser laser fluences, indicating that the scanning speed played less influence on the formation of porous copper surfaces.

3.2.3 Effect of number of scanning cycles Figures 10 and 11 shows the SEM and 3-D profile images of porous surfaces fabricated with different number of scanning cycles. Five number of scanning cycles of 5, 8, 10, 12 and 15 were selected at the laser fluence of 115 J/cm2, scanning speed of 250mm/s and scanning interval of 5μm. It can be found that relatively regular and round micro-holes were formed at the smallest number of scanning cycles (N=5). Nevertheless, these micro-holes tended to become irregular when the number of scanning cycles increased. Fig.12 shows the average depth of micro-holes and roughness of porous surfaces fabricated with different number of scanning cycles. Both Dave and Ra presented a monotonic increase tendency with the increase in number of scanning cycles, e.g., the Dave increased from 50 to 79μm, and the Ra increased from 4.4 to 8.7μm when the number of scanning cycles increased from 5 to 15. As mentioned in Section 2.1, the multiple-pass reciprocating scanning strategy was utilized in the laser micromilling process. The porous copper surfaces were fabricated by repeating scanning sequence, and the surface material was removed by the laser ablation. As the scanning path was consistent, the surface structures formed in the former milling process may be re-melted in the subsequent scanning process. When the number of scanning cycles increased, the energy density absorbed by the surface material increased, inducing more re-melting of the surface structures. More melted debris was ejected towards the beam and fell around the machined micro-holes. The debris was finally cooled to form thick recast layer on the copper surface. This promoted the development of molten pool in the vertical direction greatly. Therefore, a deeper molten pool was created. Moreover, with more and more repeated ablation, the surface material as well as the recast layer around the micro-holes tended to be re-melted for more times, which induced rougher surfaces

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with numerous ridges. Therefore, the roughness of porous copper surfaces also increased with the number of scanning cycles.

3.2.4 Effect of scanning interval Figures 13 and 14 shows the SEM and 3-D profile images of porous surfaces fabricated with five different scanning interval of 3, 5, 8, 12 and 20μm. The laser fluence, scanning speed and number of scanning cycles were set to be 115 J/cm2, 250mm/s and 10, respectively. It is obvious that the micro-holes changed to be much harder to form when the scanning intervals increased to 12μm and 20μm. The two cases with the scanning interval of 12μm and 20μm produced much fewer and smaller tiny holes, and they featured more or less smooth surfaces. The average depth of micro pores and the roughness of porous surfaces were shown in Fig.15. With the increase of the scanning interval, the average depth of micro pores and the roughness of porous surfaces were greatly decreased, i.e., the Dave decreased from 83 to 42μm, and the Ra decreased from 8.8 to 3.2μm when the scanning interval increased from 3μm to 20μm. This can be linked to the fact that a larger scanning interval was associated with a smaller step overlap fraction, as shown in Fig.1(b). This decreased the repetition time of laser irradiation on a certain area in the overlapping regions [29], and resulted in smaller energy density absorbed by the surface materials. The re-melting and ablation by the subsequent laser scanning in the overlapping regions were thus reduced, and less recast layer was re-deposited around the former molten pool. Therefore, the depths of the molten pool and laser ablated micro-holes were considerably reduced. The roughness of the processed copper surface also decreased notably.

3.3 Effects of processing parameters on wettability The measurement results of static contact angle in porous copper surfaces fabricated with different processing parameters are shown in Fig. 16. All the porous copper samples featured hydrophobic behaviors in water after the laser micromilling process, which can be helpful for their applications, such as the anti-icing, defrosting, reduction of friction resistance and enhanced heat transfer [30]. It can be found that the laser fluence played the most significant role on the variation of the wetting properties of porous copper surfaces, i.e., at small laser fluence (77 and 96 J/cm2), the contact angles were 95-100°. When large laser fluence(154 J/cm2) was applied, the copper porous surfaces approached to be superhydrophobic (145°). From Fig. 16(b) and (c), it can be also found that there were no remarkable change of the contact angles on the porous copper surfaces fabricated with different scanning speed and number of scanning cycles, indicating that the laser scanning speed and number of scanning cycles showed little effects on the wettability of porous copper surfaces. The above variations in the wettability can be correlated to the changes in the surface morphology of the porous copper samples. Generally, there are two states for liquid droplets on the solid surface, Wenzel state [31] and Cassie-Baxter state [32]. When the Wenzel state occurs, the increase in surface roughness contributes to the enhancement of wetting behavior, i.e., a hydrophobic surface tends to be more hydrophobic, and a hydrophilic surface becomes more hydrophilic with an increase in roughness as well. As copper surfaces are initially hydrophilic, the porous copper surface fabricated by laser micromilling process would make them more hydrophilic with smaller contact angles. This is apparently opposite to the observed hydrophobic properties for the present porous copper surface. Thus, the water droplets on the porous copper surface can not be explained by the Wenzel state. It is more likely that the wetting behaviors of porous copper surfaces are governed by the Cassie-Baxter state, cosC  f cos  f  1 (3)

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where θC is the apparent contact angle on the rough surfaces, θ is the contact angle of a liquid droplet on a flat solid surface, f is the fraction of the project area of the solid surface in contact with liquid. In Cassie-Baxter’s state, the liquid droplets could not completely wet the rough surfaces, and some air trapped beneath the liquid drop will enlarge the contact area with the solid surface. Therefore, the rough surfaces introduce a decrease in the f and a increase in the contact angle. As aforementioned, when the laser fluence of 77 and 96 J/cm2 were imposed, the micro-holes and cavities were hardly to form. Nevertheless, when the laser fluence increased to 115 J/cm2 or higher, numerous micro-holes and cavities were formed on the copper substrates after the laser micromillig process. The depth of micro-holes and the roughness of porous surfaces increased greatly with laser fluence. More air could be encapsulated between the interfaces of the liquid and solid, which enlarged the solid-liquid and liquid-gas composite interface, thereby resulting in an increasing contact angle with the laser fluence. Moreover, for the samples processed in different scanning speeds and numbers of scanning cycles, the porous surface morphology and geometries changed much smaller than those in different laser fluences. Therefore, the air trapped inside the micro-holes and cavities of porous copper surfaces may did not vary significantly, and smaller variations of contact angles can be observed. It should be noted that for the porous copper samples in different scanning intervals, the contact angles presented an increasing tendency when the scanning interval increased from 3μm to 12μm, and then decreased notably with a further increase to 20μm. Such behaviors can not be explained only by the above surface morphology effects. This may be associated to the fact that the surface chemical composition also plays a key role on the wettablity. As mentioned before, the EDX measurement results showed there is a certain portion of oxygen element (O) after the laser micromilling process on copper plates. These O compositions come from the oxidation of copper material during the evaporation, ejection and re-solidification of copper materials in the laser micromilling process, which usually exist in the form of CuO [33]. As the surface of CuO exhibits hydrophilic nature [33-34], the more portion of O in the porous copper surfaces tended to intensify the hydrophilicity. Fig.17 illustrates the variation of the atom ratio of O in porous copper surfaces with the increase in the scanning interval. When the scanning interval increased from 3μm to 12μm, the decrease of the atom ratio of oxygen induced the continuous increase in the contact angles. The porous copper surfaces changed to be more hydrophobic. Nevertheless, the atom ratio of O increased sharply when the scanning interval increased further to 20μm, and a decline of the contact angles can be noted.

4. Conclusions In this study, a laser micromilling technique was proposed to fabricate porous surfaces with the pore size of about 1~15um on copper plates. A series of porous copper surfaces were fabricated to explore the processing parameters effects on the formation, geometric dimensions and wetting properties of porous surfaces. It was found that the laser fluence played the most significant role on the surface morphology of porous copper surfaces. The porous surfaces with numerous micro-holes can be steadily formed when the laser fluence was 115 J/cm2 or higher. A distinct increase of the average depth of porous surface and roughness can be observed with laser fluence. A monotonic increase in the average depth and surface roughness was incurred when the number of scanning cycles increased. On the contrary, the increase in the scanning interval introduced a reverse trend. The scanning speed, however, played little influence on the formation of porous copper surfaces. The above processing parameters effects can be closely related to the variation of energy density and re-melting process during the laser micromilling process. On the other side, all the porous copper surfaces featured hydrophobic behaviors in water after the laser micromilling process. The wettability of porous copper surfaces was found to be most significantly dependent on the laser fluence, but weakly affected by the scanning

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speed and number of scanning cycles. The wetting properties of the porous copper surfaces are believed to be governed by the Cassie-Baxter state. The present studies indicate that the laser micromilling process can be a promising candidate for the fabrication of porous copper surfaces in one-step process. Acknowledgement This research was financially supported under the Grants of the National Nature Science Foundation of China (No. 51405407), the Natural Science Foundation of Fujian Province (No. 2015J05112), the Fundamental Research Funds for the Central Universities, Xiamen University (No. 20720150094). Furthermore, the financial support of Collaborative Innovation Center of High-End Equipment Manufacturing in FuJian is also acknowledged. References [1] Banhart J. Manufacture,;1; characterisation and application of cellular metals and metal foams. Progress in Materials Science 2001;46: 559–632. [2] Ru JM, Kong B, Liu YG, Wang XL, Fan TX, Zhang D.;1; Microstructure and sound absorption of porous copper prepared by resin curing and foaming method. Materials Letters 2015; 139: 318-321. [3] Patil CM, Kandlikar SG.;1; Review of the Manufacturing Techniques for Porous Surfaces Used in Enhanced Pool Boiling. Heat Transfer Engineering 2014; 35: 887–902. [4] Tang Y, Zhou W, Pan MQ, Chen HQ, Liu WY, Yu H.;1; Porous copper fiber sintered felts: An innovative catalyst support of methanol steam reformer for hydrogen production. International Journal of Hydrogen Energy 2008; 33: 2950-2956. [5] Weibel JA, Garimella SV, North MT.;1; Characterization of evaporation and boiling from sintered powder wicks fed by capillary action. International Journal of Heat and Mass Transfer 2010; 53: 4204–4215. [6] El-Genk MS, Ali AF.;1; Enhanced nucleate boiling on copper micro-porous surfaces. International Journal of Multiphase Flow 2010; 36: 780-792. [7] Li SH, Furberg R, Toprak MS, Palm B, Muhammed M.;1; Nature-inspired boiling enhancement by novel nanostructured macroporous surfaces. Advanced Functional Materials 2008;18: 2215-2220. [8] Chang JY, You SM.;1; Enhanced boiling heat transfer from microporous surfaces effects of a coating composition and method. International Journal Heat and Mass Transfer 1997; 40: 4449-4460. [9] Tang Y, Tang B, Qing J., Li Q, Lu LS.;1; Nanoporous metallic surface: Facile fabrication and enhancement of boiling heat transfer. Applied Surface Science 2012; 258: 8747-8751. [10] Campanelli SL, Ludovico AD, Bonserio C, Cavalluzzi P, Cinquepalmi M.;1; Experimental analysis of the laser milling process parameters. Journal of Materials Processing Technology 2007;191: 220–3. [11] Teixidor D, Ferrer I, Ciurana J, Oezel T.;1; Optimization of process parameters for pulsed laser milling of micro-channels on AISI H13 tool steel. Robotics and Computer-Integrated Manufacturing 2013; 29: 209-18. [12]Aoyagi S, Izumi H, Isono Y, Fukuda M, Ogawa H.;1; Laser fabrication of high aspect ratio thin holes on biodegradable polymer and its application to a microneedle. Sensors and Actuators A-Physical, 2013;139: 293-302. [13] Yong JL, Fang Y, Chen F, Huo JL, Yang Q, Bian H, Du GQ, Hou X.;1; Femtosecond laser ablated durable superhydrophobic PTFE films with micro-through-holes for oil/water separation: Separating oil from water and corrosive solutions. Applied Surface Science 2016; 389: 1148-1155.

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Fig. 1. Schematic illustrations of laser milling process of copper surface: (a) three-dimensional laser milling process (b) details of laser spot in the scanning pattern

Fig. 2. Geometric dimensions measurement of porous copper surfaces

Fig. 3. SEM images of porous copper surface together with their composition elements by EDX. Processing conditions: F = 115 J/cm2, V=250mm/s, N=10, S=5μm.

Fig. 4. SEM images of porous surfaces fabricated with different laser fluences: (a)F=77 J/cm2; (b) F =96 J/cm2; (c) F =115 J/cm2; (d) F =135J/cm2; (e) F =154 J/cm2. Processing conditions: V=250mm/s, N=10, S=5μm.

Fig. 5. 3D profile images of porous surfaces fabricated with different laser fluences: (a) F=77 J/cm2; (b) F =96 J/cm2; (c) F =115 J/cm2; (d) F =135J/cm2; (e) F =154 J/cm2. Processing conditions: V=250mm/s, N=10, S=5μm.

Fig. 6. Average depth and surface roughness of porous copper surfaces fabricated with different laser fluences. Processing conditions: V=250mm/s, N=10, S=5μm.

Fig. 7. SEM images of porous copper surfaces fabricated with different scanning speeds: (a) V =100mm/s; (b) V =200mm/s; (c) V =250mm/s; (d) V =300mm/s; (e) V =400mm/s. Processing conditions: F = 115 J/cm2, N=10, S=5μm.

Fig. 8. 3D profile images of porous surfaces fabricated with different scanning speeds: (a) V =100mm/s; (b) V =200mm/s; (c) V =250mm/s; (d) V =300mm/s; (e) V =400mm/s. Processing conditions: F = 115 J/cm2, N=10, S=5μm.

Fig. 9. Average depth and surface roughness of porous copper surfaces fabricated with different scanning speeds. Processing conditions: F = 115 J/cm2, N=10, S=5μm.

Fig. 10. SEM images of porous surfaces fabricated with different number of scanning cycles: (a) N=5; (b) N=8; (c) N=10; (d) N=12; (e) N=15. Processing conditions: F = 115 J/cm2, V=250mm/s, S=5μm.

Fig. 11. 3D profile images of porous surfaces fabricated with different number of scanning cycles:(a) N=5; (b) N=8; (c) N=10; (d) N=12; (e) N=15. Processing conditions: F = 115 J/cm2, V=250mm/s, S=5μm.

Fig. 12. Average depth and surface roughness of porous copper surfaces fabricated with different number of scanning cycles. Processing conditions: F = 115 J/cm2, V=250mm/s, S=5μm.

Fig. 13. SEM images of porous surfaces fabricated with different scanning intervals: (a) S=3μm; (b) S=5μm; (c) S=8μm; (d) S=12μm; (e) S=20μm. Processing conditions: F = 115 J/cm2, V=250mm/s, N=10.

Fig. 14. 3D profile images of porous copper surfaces fabricated with different scanning intervals: (a) S=3μm; (b) S=5μm; (c) S=8μm; (d) S=12μm; (e) S=20μm. Processing conditions: F = 115 J/cm2, V=250mm/s, N=10.

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Fig. 15. Average depth and surface roughness of porous copper surfaces fabricated with different scanning intervals. Processing conditions: F = 115 J/cm2, V=250mm/s, N=10.

Fig. 16. Contact angles of porous copper surfaces fabricated with different processing parameters. (a) different laser fluence; (b) different scanning speed; (c) different number of scanning cycles; (d) different scanning interval;

Fig. 17. Variation of atom ratio of oxygen in porous copper surfaces fabricated with different laser scanning interval. Tables Table 1 Specifications of characteristic parameters of the used fiber laser system Characteristic Parameter Process Unit range conditions Wavelength 1000-1200 1064 nm Nominal average output power 29-31 30 W Pulse duration 1-1000 100 ns Repetition rate 20-200 20 kHz Beam quality(M2) <1.1 1 Incident beam diameter 6-9 7 mm Focused diameter 24.3-37.3 31.5 μm

Table 2 Laser micromilling experiments design Laser Scannin Sample fluence, g speed, V F[J/cm2] [mm/s] 77 250 1 96 250 2 115 250 3* 135 250 4 154 250 5 115 100 6 115 200 7 115 250 8* 115 300 9 115 400 10 115 250 11 115 250 12 115 250 13* 115 250 14 115 250 15 115 250 16 115 250 17* 115 250 18 115 250 19 115 250 20 *The same sample TDENDOFDOCTD

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Number of scanning cycles, N 10 10 10 10 10 10 10 10 10 10 5 8 10 12 15 10 10 10 10 10

Scannin g interval, S[μm] 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 3 5 8 12 20

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