Evolution of a Superhydrophobic H59 Brass Surface by Using Laser Texturing Via Post Thermal Annealing

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Article Evolution of a Superhydrophobic H59 Brass Surface by Using Laser Texturing via Post Thermal Annealing

Xizhao Lu 1,2,*, Lei Kang 1,3, Binggong Yan 1 , Tingping Lei 1, Gaofeng Zheng 4 , Haihe Xie 5, Jingjing Sun 6 and Kaiyong Jiang 1,3,*

1 College of Mechanical Engineering and Automation, Huaqiao University, Xiamen 361021, China; [email protected] (L.K.); [email protected] (B.Y.); [email protected] (T.L.) 2 Department of Electronic Science, Xiamen University, Xiamen 361005, China 3 Fujian Key Laboratory of Special Energy Manufacturing, Huaqiao University, Xiamen 361021, China 4 Department of Mechanical Engineering, Xiamen University, Xiamen 361005, China; [email protected] 5 Department of Mechanical Engineering, Putian University, Putian 361110, China; [email protected] 6 School of Materials Science and Engineering, Xiamen University of Technology, Xiamen 361024, China; [email protected] * Correspondence: [email protected] (X.L.); [email protected] (K.J.)

Received: 26 September 2020; Accepted: 26 November 2020; Published: 29 November 2020

Abstract: To fabricate an industrial and highly efficient super-hydrophobic brass surface, annealed H59 brass samples have here been textured by using a 1064 nm wavelength nanosecond fiber laser. The effects of different laser parameters (such as laser fluence, scanning speed, and repetition frequency), on the translation to super-hydrophobic surfaces, have been of special interest to study. As a result of these studies, hydrophobic properties, with larger water contact angles (WCA), were observed to appear faster than for samples that had not been heat-treated (after an evolution time of 4 days). This wettability transition, as well as the evolution of surface texture and nanograins, were caused by thermal annealing treatments, in combination with laser texturing. At first, the H59 brass samples were annealed in a Muffle furnace at temperatures of 350 ◦C, 600 ◦C, and 800 ◦C. As a result of these treatments, there were rapid formations of coarse surface morphologies, containing particles of both micro/nano-level dimensions, as well as enlarged distances between the laser-induced grooves. A large number of nanograins were formed on the brass metal surfaces, onto which an increased number of exceedingly small nanoparticles were attached. This combination of fine nanoparticles, with a scattered distribution of nanograins, created a hierarchic Lotus leaf-like morphology containing both micro-and nanostructured material (i.e., micro/nanostructured material). Furthermore, the distances between the nano-clusters and the size of nano-grains were observed, analyzed, and strongly coupled to the wettability transition time. Hence, the formation and evolution of functional groups on the brass surfaces were influenced by the micro/nanostructure formations on the surfaces. As a direct consequence, the surface energies became reduced, which affected the speed of the wettability transition—which became enhanced. The micro/nanostructures on the H59 brass surfaces were analyzed by using Field Emission Scanning Electron Microscopy (FESEM). The chemical compositions of these surfaces were characterized by using an Energy Dispersive Analysis System (EDS). In addition to the wettability, the surface energy was thereby analyzed with respect to the different surface micro/nanostructures as well as to the roughness characteristics. This study has provided a facile method (with an experimental proof thereof) by which it is possible to construct textured H59 brass surfaces with tunable wetting behaviors. It is also expected that these results will effectively extend the industrial applications of brass material.

Keywords: superhydrophobic; thermal annealing; laser texture; muﬄe furnace; nanoparticles

Micromachines 2020, 11, 1057; doi:10.3390/mi11121057 www.mdpi.com/journal/micromachines Micromachines 2020, 11, 1057 2 of 14

1. Introduction Today, an anti-corrosion related technology based on brass has increased many people’s interest in mimicking the pattern of the lotus leaf surface. The reason is that a nanosecond fiber laser can be used to create this type of pattern on an H59 brass surface, thereby forming super-hydrophobic surfaces after an incubation period of about 2 weeks. This means that an increase in water contact angle (WCA) of up to 150◦ can be obtained after about 14 days of evolution [1–3]. However, the possibility of producing a superhydrophobic H59 brass surface in less than 14 days remains a big challenge. While annealing the brass material, the temperature will continuously increase, and the laser-induced texture will become both broader and wider. This process will simultaneously reduce the surface energy, and speed up the super-hydrophobic properties of the brass surface. As an important engineering metal material, H59 brass has become widely used in various types of applications (in industrial hydraulic systems, for transportation, in ship propellers, etc.). For this brass, a quick transformation from hydrophilicity to hydrophobicity should effectively improve the anti-corrosion properties. Examples of potential applications for this type of modified H59 brass material are self-cleaning surfaces [4,5], oil/water separations [6], anti-icing/frosting [7,8], drag reduction [9], and microfluidic devices [10]. As a result of recent research efforts, appropriate brass surfaces with hierarchic micro/nanostructures (from here on called micro/nanostructures), containing nanosized grains and Cu2O nanoclusters, have been manufactured with the purpose of producing hydrophobic surfaces with very short evolution times. These hydrophobic surfaces have been formed by using various types of deposition techniques; the sol–gel method, chemical vapor deposition [11,12], chemical etching [13,14], nanoimprint lithography [15], and self-assembly [16]. However, these methods are limited in terms of high cost, complicated processes, and poor mechanical properties of the textured surfaces. Recently, some environmentally friendly chemical solutions (like isopropyl alcohol, sodium bicarbonate solution, etc.) have been used to shorten the wettability transition time (down to about 5 days) [17]. In recent years, different types of annealing treatments, at lower temperatures, have been used to speed up the super-hydrophobic wettability transition [18]. The surface energy, roughness, and morphology, as well as the dual-scaled particles in the surface micro/nanostructure, play important roles in this evolution procedure (with an exception for the Cu2O nanoclusters) [19,20]. As a matter of fact, the physical profile of the hierarchic micro/nanostructures, in addition to the large oxygen content, will speed up the variation of Cu2O material on the surface [21,22]. It is well known that the super-hydrophobic property of a surface is determined by a combination of physical roughness and chemical composition. In fact, super-hydrophobic surfaces do not only depend on dual micro/nanostructures, but also on the existence of low surface energy Hence, key to the evolution of Cu2O during the laser processing of the brass surfaces is the reversible wettability induced by different laser parameters [13]. Different annealing treatments will also induce phase transitions between the alpha and beta phases of brass, with different metallographic sizes. Furthermore, the different WCAs are determined by the different nanograin sizes, which in turn depend on the different hierarchies in the micro/nanostructure (as manufactured by the combination of laser processing and Cu2O evolution).

2. Setup and Experiments

2.1. Experimental Setup A fiber laser (PicoYL-40-600-20, Yangtze Soton Laser Co., Ltd., Wuhan, China), with a wavelength of 1064 nm, 0.6–4 ns Pulse duration was used for the texturing of different micro/nanostructures on heated brass surfaces. After annealing in a Muffle furnace, the sample surfaces were polished with silicon carbide abrasive papers (500#, 800#, and 1200#, YING QIU, China). The micro/nanostructures on the brass surfaces were characterized by using a field emission scanning electron microscope (FESEM) (Zeiss SUPRA 55, Munich, Germany). Finally, the chemical compositions were determined by using energy-dispersive X-ray spectroscopy (EDS). 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The heated surface brass was pronounced withquitehydrophobicsubstrateshydrophobic a higher small, showed which annealingityity. . wasa small observed temperature reflection to enhance index (see with the Table speeddeeper1). for When grooves revers the,ing and brass the the super wasdistance hydrophilicity annealeds between to the 350to super nano◦C,- substrates showed a small reflection index with deeper grooves, and the distances between the nano- the main phase ofhydrophobicclustersquite theAsAs brasssmall, awere resulta resultity waswhichenlarge . of of thermally the wasthermallyd. alpha The observed contactannealing, phase.annealing, to areaenhance After different betweendifferent annealingthe speedtypes a types water offor toof metallographic reversdrop metallographic 600 andingC, the this super metallictypewere were hydrophilicityof observed observedcoarse alpha surface on and on t heto t betahe superH59was H59 - clusters were enlarged. The contact area between a water drop◦ and this type of coarse surface was brassquitebrasshydrophobicAs small,substrate. substrate.a result whichity ofThose . Thosewasthermally surfaceobserved surface annealing, structures structuresto enhance different are arethe shown shownspeed types in for in Tableof reversTable metallographic 2 ing[222 [22– the24].–24]. super L aser Lwereaser hydrophilicity grating observedgrating was was on thereafter to tthereafterhe super H59- phases did coexist.quite When small, heating which was the observed sample to above enhance 800 the◦ speedC, an for amorphous reversing the state super was hydrophilicity formed [23 to– super25]. - brassusedhydrophobicused tosubstrate. Asto form forma resultity dual dual . Those of hierarchy thermallyhierarchy surface micro/nanostructures structuresannealing,micro/nanostructures aredifferent shown typeson inon the Table the of brass metallographicbrass 2 [22 surfaces surfaces–24]. L (i.e.,aser (i.e., were gratingnano nano observed-grains- grainswas thereafterevoluton evolut theion H59ion In addition, the collectionhydrophobic ofity nanograins. became large. The heated brass substrates showed a small usedononbrass the Astheto micro substrate.form amicro result- groovesdual-grooves of Those thermallyhierarchy). ). surface micro/nanostructuresannealing, structures different are shown types on inthe of Table metallographicbrass 2 surfaces [22–24]. (i.e., Lwereaser nano observedgrating-grains was on evolut tthereafterhe H59ion reflection index with deeperAs a result grooves, of thermally and theannealing, distances different between types theof metallographic nano-clusters were were observed enlarged. on the The H59 onbrassused the substrate. microto form-grooves dual Those hierarchy). surface structuresmicro/nanostructures are shown inon Table the brass 2 [22 surfaces–24]. Laser (i.e., grating nano- grainswas thereafter evolution brass substrate. Those surface structures are shown in Table 2 [22–24]. Laser grating was thereafter contact area betweenusedon Table theto aTable form watermicro 1. 1. Laserdual -Lasergrooves drop hierarchy processing processing and). this micro/nanostructuresof of typep olishedpolished of coarse samples samples’ surface metal’ metalon theafter wasafter brass thermal thermal quite surfaces annealing small, annealing (i.e., which, nanow, armedwarmed-grains was in in observedm evolutufflemuffle ion used to form dual hierarchy micro/nanostructures on the brass surfaces (i.e., nano-grains evolution to enhance the speedon theTablefurnace furnacemicro for 1.reversing for -Lasergrooves for 2 hours2 hours processing) .and the and thereafter super thereafter of polished hydrophilicity cooled cooled samples in inambient ambient’ metal toair air . super-hydrophobicity. after. thermal annealing, warmed in muffle on theTable micro 1. -Lasergrooves processing). of polished samples’ metal after thermal annealing, warmed in muffle furnacePolishedPolished for 2 hours and thereafter cooled in ambient air. MicroscopMicroscopy ofy of metallographic metallographic substrates substrates furnace for 2 hours andLaser Laserthereafter processing processing cooled results inresults ambient air. Tablesamplessamples 1. Laser processing of polished samples’ metal after thermalsurface surface annealing structures structures, warmed [24] [24] in muffle Table 1. Laser processingPolishedTable 1.of Laser polished processing samples’ of polished metal after samples thermal’ metal annealing, Microscopafter thermaly warmed of annealingmetallographic in mu, warmedffle substrates furnace in muffle furnacePolished for 2 hours and Laserthereafter processing cooled inresults ambient air. Microscopy of metallographic substrates for 2 h and thereafterfurnacesamples cooled for 2 inhours ambient and thereafter air. cooled in ambient air. surface structures [24] Laser processing results [24] Polishedsamples Microscopy surfaceof metallographic structures substrates BrassBrass sample sample Laser processing results Polished MicroscopyMicroscop ofy of Metallographic metallographic[24] Substrates substrates Polished Samplessamples LaserLaser Processing processing Results results surface structures afteraftersamples 350 350 °C ° C surface structures [24] Brass sample Surface Structures [24 ] annealingBrassannealing sample after 350 °C smallsmall-sized-sized grains grains [24] [24] after 350 °C Brassannealing sample small-sized grains [24] Brassannealing sample Brass sample after 350after◦C 350 annealing °C small-sized grains [24]

annealingafter 350 °C small-sizedsmall-sized grains grains [24] [24 ] annealing small-sized grains [24] BrassBrass sample sample

Brassafterafter sample600 600 °C ° C

annealingBrassannealing sample Brass sample after 600after◦C 600 annealing °C mediummedium-sized-sized grains grains [24] [24] after 600 °C Brassannealing sample medium-sized grains [24] Brassannealing sample medium-sized grains [24 ] after 600 °C medium-sized grains [24] after 600 °C annealing Brass sample medium-sized grains [24] Brassannealing sample medium-sized grains [24]

Brass sample after 800afterafter◦C 800 annealing 800 °C ° C Brass sample annealingBrassannealing sample large-sized grains [24] after 800 °C large-sizedlarge-sized grains grains [24] [24 ] after 800 °C Brassannealing sample large-sized grains [24] annealing afterBrass 800 sample °C large-sized grains [24] after 800 °C annealing large-sized grains [24] As a result of thermallyannealing annealing, different types of metallographic were observed on the H59 large-sized grains [24] brass substrate. Those surface structures are shown in Table 2[ 22–24]. Laser grating was thereafter used to form dual hierarchy micro/nanostructures on the brass surfaces (i.e., nano-grains evolution on the micro-grooves). In the present study, the areas of micro/nanostructured hierarchic morphologies were chosen from different parts of the sample surfaces, which were due to the appearance of different phases. After the optimization of the laser parameters (see Figure1), characterizations by FESEM were made to obtain information about the surface morphologies of the fabricated surfaces of the brass samples. In addition, WCA measurements were performed to obtain information about the wettability transitions. The evolution mainly implied the growth of chemical composition, mainly depended on the morphologies of laser processing. The Silicon carbide abrasive papers (500#, 800#, and 1200#) had been used in polishing the different brass samples (after the annealing processes), the reflection indexes of the surfaces were found to be solely dependent on the different nano tissues. Micromachines 2020, 11, 1057 4 of 14

Table 2. Laser texturing parameters (used in deriving the results presented in Figures1–3).

Tuned Items of Column 1 Column 2 Column 3 Column 4 Column 5 Other Parameters Laser Period: 100 µm Laser fluence 7.16 8.60 10.02 11.46 12.88 Scan speed: 1 mm/s (J/cm2) Repetition rate: 200 kHz Laser fluence: 8.60 J/cm2 Period (µm) 80 90 100 110 120 Scan speed: 1 mm/s Repetition rate: 200 kHz Period: 100 µm Scan speed (mm/s) 0.1 0.5 5 10 100 Laser fluence: 8.60 J/cm2 Repetition rate: 200 kHz Period: 100 µm Repetition rate 100 500 1000 2000 5000 Laser fluence: 11.46 J/cm2 (kHz) Scan speed: 5 mm/s Micromachines 2020, 11, x 6 of 14

Figure 1. (a) shows the dependence of the WCA evolution on the different annealing treatments and Figure 1. (a) shows the dependence of the WCA evolution on the different annealing treatments laser fluence. The coverage of different laser fluence included 7.16 J/cm2, 8.6 J/cm2, 10.02 J/cm2, 11.46 2 2 2 and laserJ/cm fluence.2, 12.88 J/cm The2 in coverage Figure 1a of, which different is included laser fluencein Table 2 included; the average 7.16 WCA J/cm can, 8.6almost J/cm stand, 10.02 the J/cm , 2 2 11.46 J/cmhydrophobic, 12.88 J /wettability,cm in (a), the which abnormal is included WCA is influenced in Table2 in; theterm averages of the waterdrop WCA can contact almost during stand the hydrophobicthe last wettability, measurement. the (b abnormal) shows the WCAdifferent is influencedwettability transition in terms speed of thes after waterdrop annealing contact and by during the lastusing measurement. various laser ( bfluence) showss. The the diagrams different of wettabilitycurves show transitionthe WCA transition speedssafter for laser annealing-textured and by using variousbrass samples laser fluences. that have The been diagrams annealed ofat ( curvesc) 350 ° showC, (d) the600 WCA°C, and transitions (e) 800 °C (being for laser-textured affected by brass different laser scan speeds). (f) a 350 °C annealed brass sample with larger nanograins (and finer samples that have been annealed at (c) 350 ◦C, (d) 600 ◦C, and (e) 800 ◦C (being affected by different metallic texture), as compared with the samples in (g) and (h). (g) the beta phase of brass with a coarse laser scan speeds). (f) a 350 C annealed brass sample with larger nanograins (and finer metallic metallic texture, as compared◦ with the result shown in (d). texture), as compared with the samples in (g,h). (g) the beta phase of brass with a coarse metallic texture,B asecause compared of the with existence the result of surface shown oxidation in (d). , the significant hydrophilic effect was shown already in the first measurement. When the laser fluence increased, the large distances between the Tablenano2 showsgrains made the laser the WCA texturing increase parametersd very fast. The that hierarch were usedic nanostructure in texturings, produced the first at row different of samples in Tableannealing1. As can temperatures, be seen in Table can be2 ,seen the in laser Figure fluence 1f–h. After was tunedan incubation to 7.16 time J /cm of two2, 8.60 weeks J/cm, the2, 10.02low- J/cm2, 11.46 J/cmtemperature2, and 12.88 annealed J/cm 2brass. Moreover, sample had the developed waist of the a dense laser nano spotgrain was formation about 20 (alsoµm, including while the laser nanograin boundaries). scan speed, period, and repetition rate were set to 1 mm/s, 100 µm, and 200 kHz, respectively. These The coarseness of the nanograins made the surface energy to increase quite slowly. On the laser fluencescontrary, have the high the- capacitytemperature to annealed change bothbrass thesample brass showed roughness small nano andgrains brass (see oxidation Figures 1 [f–21h)].. When scanningThe the W gratingCA increased with very a higher quickly laser for this fluence, type of deepersurface. T grooveshe large distances will bedug. between At the nano samegrains time, laser could easily become filled with, for example, air. According to the Cassie–Baxter formula, water drops in contact with a large volume of air will have a large capacity to reduce the surface energy (which implicates a fast increase in the WCA). The developing speed of the WCAs for the 800 °C annealed sample was found to be higher than the developing speed for the 350 °C annealed sample.

3.2. Dependence on WCA Evolution by Scan Periods and Dual Hierarchy Micro/Nano-Structures

Micromachines 2020, 11, 1057 5 of 14 energy beyond the brass threshold will enhance the oxidation on the surface. As is clear from Table1, a higher energy density grating will enhance surface roughness, and increase surface oxidation. A deeper groove means that there is a larger surface area for oxidation to take place. For this type of surface, wettability will be able to go from hydrophilicity to hydrophobicity in a very short time. For the second row of samples in Table1, the following parameter values have been used: 80 µm, 90 µm,100 µm,110 µm,120 µm (laser grating period), 1 mm/s (laser scan speed), 8.60 J/cm2 (laser fluence), and 200 kHz (repetition rate). According to the Cassie–Baxter formulation, a microstructure grate will be formed by the laser. The dimension of the microstructure is about 100 µm, which will enhance the surface roughness in accordance with the increased laser fluence. The chosen size has been experimentally discovered to be reasonable. For the third row on the samples in the Table1, the laser scan speeds were tuned to 0.1 mm/s, 0.5 mm/s, 5 mm/s, 10 mm/s, and 100 mm/s, respectively. The period, laser fluence, and repetition rate were kept fixed (100 µm, 8.60 J/cm2, and 200 kHz, respectively). Finally, for the samples in the last row in Table1, the laser repetition rates were 100 kHz, 500 kHz, 1000 kHz, 2000 kHz, and 5000 kHz, with the constant period, laser fluence, and scan speed (100 µm, 11.46 J/cm2, and 5 mm/s, respectively). The WCAsMicromachines were 2020 measured, 11, x after laser processing. In addition, the laser parameters7 of 14 were also optimized. FromAccording the resulting to the graphs Cassie– inBaxter Figures and1 – Wenzel3, including formula, information the dimension about of relevant the surface nanoparticles, nanograins,microstructure surface roughness, is related to and the surface wettability roughness. transition, A laser scanned the following the annealed important surface (with information 100 can be understood.μm periods After) will the enhance annealing the annealed processes, brass surface nanoparticles roughness. became attached to, and even in some According to the result of the experimental measurements, and related references [27–30], the cases includedmicrogroove in, the di andfferent dual microstructure nanograins. act Ined addition,as an airbag therewhile the was droplet a lesser fell back amount to the rough of nanograinsened on the sample surface.surface. Furthermore, A fiber laser spot the’s texturingdiameter was on arranged the brass in continuously surface became linear grooves. more Moreover pronounced., the However, the smaller dimensionsstudies of microstructure of the micro related/nanostructures to super-hydrophobic meanity were that concentrated there is aon much some specific larger size areas. where the The experimental scan speed was set to 1mm/s. While the dual micro/nanostructure enlarges the formation ofcontact compounds area with oxygen, like CuO the nanostructure and Cu2O will could speed take up the place. cuprous oxide’s evolution in the groove.

Figure 2. (a)Figure shows 2. (a the) shows dependence the dependence of theof the WCA WCA evolution on onthe different the diff annealingerent annealing treatments and treatments and scan periods. The coverage of WCA refers to the different grating periods, which are 80 μm, 90 μm, scan periods. The coverage of WCA refers to the different grating periods, which are 80 µm, 90 µm, 100 μm, 110 μm, 120 μm in Table 2; the average WCA almost reveals the hydrophobic wettability, 100 µm, 110theµm, abnormal 120 µ mWCA in is Table influenced2; the in average terms of the WCA waterdrop almost contact reveals during the the hydrophobiclast measurement wettability,. (b) the abnormal WCAshows is the influenced different wettability in terms transition of the waterdropspeeds after annealing contact duringand by using the lastvarious measurement. periods. The (b) shows the differentdiagram wettabilitys of curves transition show thespeeds water contact after angle annealing transition and for ( byc) 350 using °C, (d various) 600 °C, and periods. (e) 800 ° TheC diagrams annealed laser-textured brass surfaces (being affected by different laser scan periods). Surface of curves showmorphologies the water of laser contact-textured angle and annealed transition samples for at ( c(f)) 350350 °C◦C,, (g)( 600d) 600°C, and◦C, (h and) 800 ° (Ce.) 800 ◦C annealed laser-textured brass surfaces (being affected by different laser scan periods). Surface morphologies of As can be seen from this experiment, there was a stable influence by the micro-groove on the laser-textured and annealed samples at (f) 350 ◦C, (g) 600 ◦C, and (h) 800 ◦C. wettability (Figure 2); on the contrary, the laser-textured roughness was apparently affected by thermal annealing processing. The high temperature is most probably the major cause of the superhydrophobic surfaces in Figure 2f–h. From these experiments, optimal periods were chosen, for which microstructure morphologies were further analyzed. The formed surface roughness, including the temperature factors, was found to have a certain impact on the surface properties.

3.3. Dependence on WCA Evolution by Scan Speed and Annealing Temperature

Micromachines 2020, 11, x 8 of 14

When the laser scanned the annealed surface at a certain high speed, the laser spots were not arranged in continuously linear grooves. The studied hot spots were instead concentrated in some specific locations. The scan speed was also observed to have a major influence on both the Micromachinesaccumulated2020, 11 temperature, 1057 and the metal oxidation, while the WCAs on the brass surfaces did not6 of 14 differ from each other.

FigureFigure 3. (a 3.) dependence(a) dependence of of the the WCA WCA evolution evolution on on the the different different annealing annealing treatments treatments and scan and scanspeed. speed. The coverage of WCA refers to the different scan speeds were tuned between 0.1 mm/s, 0.5 mm/s, 5 The coverage of WCA refers to the different scan speeds were tuned between 0.1 mm/s, 0.5 mm/s, mm/s, 10 mm/s, 100 mm/s in Table 2; the average WCA can almost stand the hydrophobic wettability, 5 mm/s, 10 mm/s, 100 mm/s in Table2; the average WCA can almost stand the hydrophobic wettability, the abnormal WCA is influenced in terms of the waterdrop contact during the last measurement. (b) the abnormal WCA is influenced in terms of the waterdrop contact during the last measurement. different wettability transition speeds after annealing and by using various speeds. The diagrams of (b) dicurvesfferent show wettability the water transition contact angle speeds transition after annealingfor (c) 350 °C and, (d) by 600 using °C, and various (e) 800speeds. °C annealed The laser diagrams- of curvestextured show brass the surfaces water (being contact affected angle by transition different forlaser (c scan) 350 speed◦C, (s)d.) Surface 600 ◦C, morpholog and (e) 800ies ◦ofC laser annealed- laser-texturedtextured and brass anneal surfacesed sample (beings at ( afff) 350ected °C, by (g) di 600fferent °C, and laser (h) scan800 °C speeds).. Surface morphologies of laser-textured and annealed samples at (f) 350 ◦C, (g) 600 ◦C, and (h) 800 ◦C. When the laser scanning speed is lower, the laser beam stays longer on a certain area on the 3. Evolutionsurface of of the WCAs brass. onMoreover, the Brass the Samplelarger the Surfaces nanoparticles on the surface, the easier it will be for the Thetemperature evolution to increase of WCAs. However, was observed this is not for the 2 weeks, situation and for statistical the WCA, analysis which will was not conducted. be easy to The increase. According to the Cassie–Baxter formula, the more hydrophilic the nanograins are, the more laser parameters (like laser fluence, scan speed, repetition rate, and period) were also optimized by hydrophobic they will become. Moreover, the smaller the micro-groove spacings, the greater the usingsurface the di ffroughnesserent micro. In /additionnanostructures, the large as oxidation quality indicators.degree of the To surface be more can specific,be seen by these the color parameters of relatethe to brass the di surfacefferent. The micro-configurations, black and dark colors nanosized show a high grains, oxidation nanoparticles, concentration etc. [31,32] The. I nanograinsn the initial and nanoparticlestests, large that surface were energy produced was byconnected the nanosecond to a more fiber hydrophilic laser were surface relatively [33–35] coarse.. In addition However,, the this was notincrease the casein annealing for other temperature types of nanostructures.was found to have The a significant different effect laser on parameters both the increase therefore in the had to be optimized.number of nanograins and of the hydrophobicity [36–38]. When a water-drop falls to the textured surface, the most prominent part of the hierarchic surface will be3.4. the Dependence first to beon inWCA contact Evolution with by the Repetition water. Rate According and Annealing to the Temperature Wenzel formula, when the surface roughness increases, the contact area between the surface and the water droplet decreases. As a result, the intensity of the contact pressure becomes enlarged, and the water drop must overcome a much larger repellant. On the other hand, the surface energy will decrease and the water contact angle will quickly become quite large. In addition, the surface attractive forces toward the liquid drop will become smaller for the polished surface. When the sizes of the nanograins become smaller, the distances between the nanoclusters will be longer. This is the result of a transformation of the metal phase. Hence, Cu2O nanoclusters can be formed quite quickly. The air will thereafter become attached to the surface as a cushion-like structure; nanograins on the surfaces resemble and are looked like American Acacia when the temperature is sufficiently high. According to the Cassie–Baxter formula, the distances between Micromachines 2020, 11, 1057 7 of 14

the clusters of nanograins at 600 ◦C and 800 ◦C are much longer than those at 350 ◦C. This will result in a decrease in nanograin density, while the nanoparticles will become larger, and the surface roughness is more pronounced. The gap between nanoclusters will be decreased, the oxygen breath of brass will be blocked and Cu2O will be halted.

3.1. Dependence on WCA Evolution by Laser Fluence and Annealing Temperature Figure1 shows the evolution in WCAs when going from hydrophilicity (with a contact angle smaller than 90◦) to hydrophobicity (with a contact angle larger than 90◦). Brass surfaces have been textured after being annealed at different temperatures. As can be seen in Figure1, the laser impact on the water contact angle is large. For instance, the laser fluence will determine the depth of the grooves. There is also a directly proportional relationship between the laser power and the laser fluence, in that low laser power is directly correlated with low laser fluence. As a result, a low laser fluence relates to the processing of plain grooves. Moreover, the WCA will increase slowly for surface texturing with a low laser fluence, in comparison to the corresponding treatment with high laser fluence. When the polished brass mesh (as textured by the laser process) was very shallow, the WCA could remain through the micro/nanostructure evolution. As the processed depth increased, there was a larger volume of processed brass material removed from the grooves (which was due to an increased laser fluence). The laser fluence affected the sizes of both the grooves and the grains in the hierarchic micro/nanostructure on the brass surface. As a first measurement, water drops were pumped to a textured brass surface. This low-energy surface was expected to form a smaller amount of Cu2O and to also enlarge its WCA. While the WCA was measured in the first measurement, a textured brass surface, with deeper grooves, was processed by using a higher laser fluence. Since the higher laser fluence was an indication of a higher laser single pulse energy, which excavated a broader and deeper micro groove size, further oxidation on the surface did also take place [26]. As a result, a higher proportion of CuO will be manufactured on the surface. Besides, there was an increase in attractive forces to this roughened surface area. The gap is big; therefore, the proportion of oxygen will run away. As a result, the surface showed much higher surface energy and developed initial hydrophilicity. Most of the surface will be endowed with large hydrophilicity attraction; after all, C-O will release its oxygen because of the large gap between hierarchy structure, and the transformation to hydrophobicity will take place more quickly. Because of the existence of surface oxidation, the significant hydrophilic effect was shown already in the first measurement. When the laser fluence increased, the large distances between the nanograins made the WCA increased very fast. The hierarchic nanostructures, produced at different annealing temperatures, can be seen in Figure1f–h. After an incubation time of two weeks, the low-temperature annealed brass sample had developed a dense nanograin formation (also including nanograin boundaries). The coarseness of the nanograins made the surface energy to increase quite slowly. On the contrary, the high-temperature annealed brass sample showed small nanograins (see Figure1f–h). The WCA increased very quickly for this type of surface. The large distances between the nanograins could easily become filled with, for example, air. According to the Cassie–Baxter formula, water drops in contact with a large volume of air will have a large capacity to reduce the surface energy (which implicates a fast increase in the WCA). The developing speed of the WCAs for the 800 ◦C annealed sample was found to be higher than the developing speed for the 350 ◦C annealed sample.

3.2. Dependence on WCA Evolution by Scan Periods and Dual Hierarchy Micro/Nano-Structures According to the Cassie–Baxter and Wenzel formula, the dimension of the surface microstructure is related to the surface roughness. A laser scanned the annealed surface (with 100 µm periods) will enhance the annealed brass surface roughness. According to the result of the experimental measurements, and related references [27–30], the microgroove and dual microstructure acted as an airbag while the droplet fell back to the roughened Micromachines 2020, 11, 1057 8 of 14 surface. A fiber laser spot’s diameter was arranged in continuously linear grooves. Moreover, the studies of microstructure related to super-hydrophobicity were concentrated on some specific sizes. The experimental scan speed was set to 1mm/s. While the dual micro/nanostructure enlarges the contact area with oxygen, the nanostructure will speed up the cuprous oxide’s evolution in the groove. As can be seen from this experiment, there was a stable influence by the micro-groove on the wettability (Figure2); on the contrary, the laser-textured roughness was apparently a ffected by thermal annealing processing. The high temperature is most probably the major cause of the superhydrophobic surfaces in Figure2f–h. From these experiments, optimal periods were chosen, for which microstructure morphologies were further analyzed. The formed surface roughness, including the temperature factors, was found to have a certain impact on the surface properties.

3.3. Dependence on WCA Evolution by Scan Speed and Annealing Temperature When the laser scanned the annealed surface at a certain high speed, the laser spots were not arranged in continuously linear grooves. The studied hot spots were instead concentrated in some specific locations. The scan speed was also observed to have a major influence on both the accumulated temperature and the metal oxidation, while the WCAs on the brass surfaces did not differ from each other. When the laser scanning speed is lower, the laser beam stays longer on a certain area on the surface of the brass. Moreover, the larger the nanoparticles on the surface, the easier it will be for the temperature to increase. However, this is not the situation for the WCA, which will not be easy to increase. According to the Cassie–Baxter formula, the more hydrophilic the nanograins are, the more hydrophobic they will become. Moreover, the smaller the micro-groove spacings, the greater the surface roughness. In addition, the large oxidation degree of the surface can be seen by the color of the brass surface. The black and dark colors show a high oxidation concentration [31,32]. In the initial tests, large surface energy was connected to a more hydrophilic surface [33–35]. In addition, the increase in annealing temperature was found to have a significant effect on both the increase in the number of nanograins and of the hydrophobicity [36–38].

3.4. Dependence on WCA Evolution by Repetition Rate and Annealing Temperature The effect of fabrication on the laser repetition frequency, in relation to the WCA and micro/nanostructures, was also significant. The higher the laser repetition rate, the smaller the single pulse energy; in addition, the roughness will be lower than the others, and the micro-grooves shallower. It was even difficult, and sometimes not possible, to fabricate the necessary microstructures. Furthermore, the higher the laser frequency, the less likely it was to affect the results after heat treatment. In addition, the higher the heat treatment temperature, the more hydrophilic the surface. Due to the increase in laser repetition frequency, and the decrease in single pulse energy, the copper oxide was shown to be less affected by the heat of the laser processing. Therefore, the WCA of the brass surface that has been heat-treated at 350 °C, became larger than the WCAs for 600 ◦C and 800 ◦C. Figure4 shows the revolution of WCA because of laser repetition rate, on the other hand, Figure5 reveals the existence of nanograins in brass samples that have been textured after different thermal annealing treatments. The CuO nanograins were expected to increase during the higher temperature annealing treatments. In addition, the distance between the nano-clusters became enlarged. As a result, the oxygen concentration on the surface increased, which was mainly due to the huge free space on the surface. The surface Cu atoms could thereby bind to gaseous O2 until they reached their solid solution concentration. Those would lead to a small surface energy and the improvement of water resistance ability. Micromachines 2020, 11, x 9 of 14 Micromachines 2020, 11, x 9 of 14 after heat treatment. In addition, the higher the heat treatment temperature, the more hydrophilic the surface.after heat Due treatment. to the increase In addition in laser, the repetition higher the frequency heat treatment, and the temperature, decrease in singlethe more pulse hydrophilic energy, the the coppersurface. oxide Due was to the shown increase to be in less laser affected repetition by the frequency heat of ,the and laser the decreaseprocessing. in single pulse energy, the copperTherefore, oxide was the WCAshown of to the be brass less affected surface thatby the has heat been of heat the -lasertreated processing. at 350 ℃ , became larger than the WCAsTherefore, for 600 the °C WCA and 800 of the°C. brass surface that has been heat-treated at 350 ℃, became larger than the FigureWCAs 4for shows 600 ° theC and revolution 800 °C. of WCA because of laser repetition rate, on the other hand, figure 5 revealsFigure the existence4 shows the of nanograinsrevolution ofin WCAbrass samplesbecause ofthat laser have repetition been textured rate, on after the differentother hand, thermal figure annealing5 reveals treatments.the existence The of CuOnanograins nanograins in brass were samples expected that to have increase been during textured the after higher different temperature thermal annealingannealing treatments. treatments. In The addition, CuO nanograins the distance were between expected the to nanoincrease-clusters during became the higher enlarged. temperature As a result,annealing the oxygen treatments. concentration In addition, on the the surface distance increased, between which the nano was-clusters mainly became due to the enlarged. huge free As a spaceresult, on the the oxygensurface. concentration The surface Cu on atoms the surface could therebyincreased, bind which to gaseous was mainly O2 until due they to reached the huge their free solidspace solution on the surface.concentration. The surface Those Cu would atoms lead could to therebya small bindsurface to gaseousenergy andO2 until the theyimprovement reached their of Micromachineswatersolid resistance solution2020, 11, concentration. 1057ability. Those would lead to a small surface energy and the improvement9 of of 14 water resistance ability. Brass Water contact angles（WCA） Evolution of Repetition Rate Brass Water contact angles（WCA） Evolution of Repetition Rate Brass Water contact angles（WCA） Evolution of Repetition Rate 160 160 Temperture 800 (a) Temperture 350 (b) Temperture 600 (c) Brass Water contact angles（WCA） Evolution of Repetition Rate Brass Water contact angles（WCA） Evolution of Repetition Rate 160 Brass Water contact angles（WCA） Evolution of Repetition Rate 140 140 160 160 Temperture 800 (a) Temperture 600 140 Temperture 350 (b) (c) 120 160 120 140 120 140 140 100 100KHz 120 WCA1 100 500KHz 100 120 WCA1 100KHz WCA2 120 WCA1 100KHz 80 WCA2 500KHz WCA3 1000KHz 80 WCA2 500KHz 100 WCA1 100KHz WCA3 1000KHz WCA4 2000KHz 100 WCA3 1000KHz 80 500KHz 100 WCA4 WCA1 100KHz 60 WCA5 WCA25000KHz

100KHz WCA1 (degree) 2000KHz WCA1 (degree) WCA1 (degree) 60 WCA4 WCA12000KHz 80 1000KHz WCA5 WCA2 500KHz WCA3 2 80 WCA5 WCA2 500KHz 5000KHz Laser Fluence 11.460J/cm 5000KHz WCA3 1000KHz 40 WCA4 2000KHz 60 2 40 WCA3 1000KHz 80 Laser Fluence 11.46J/cm 60 Periods 100μm Laser Fluence 11.46J/cm2 WCA4 WCA5 5000KHz

WCA1 (degree) 2000KHz WCA1 (degree) WCA1 (degree) 60 WCA4 2000KHz Periods 100μm Scan speed 1mm/s WCA5 2 20 Periods 100μm WCA5 5000KHz 20 Laser Fluence 11.460J/cm 5000KHz 40 Scanspeed 5mm/s 40 40 Scanspeed 5mm/s 60 Laser Fluence 11.46J/cm2 Periods 100μm Laser Fluence 11.46J/cm2 0 Periods 100μm 0 Scan speed 1mm/s -220 0 2 4 6 8 10 12 Periods14 100μm16 -2 0 2 4 6 8 10 12 14 16 -220 0 2 4 6 8 10 12 14 16 40 Scanspeed 5mm/s Times (day) Scanspeed 5mm/s Times (Day) Times (day) 0 0 -2 0 2 4 6 8 10 12 14 16 -2 0 2 4 6 8 10 12 14 16 -2 0 2 4 6 8 10 12 14 16 Times (day) Times (Day) Times (day)

FigureFigure 4. The 4. The graphs graphs showing showing the the WCA WCA transition transition at at annealing annealing temperaturestemperatures of (a) (a) 350 350 °◦CC,, (b) (b )600 600 °C◦,C, Figure 4. The graphs showing the WCA transition at annealing temperatures of (a) 350 °C, (b) 600 °C, andand (c) 800(c) 800◦C. ° ItC.is It alsois also shown shown that that the the brass brass surfaces surfaces are are a affectedﬀected byby didifferentﬀerent repetitionrepetition r rates.ates. and (c) 800 °C. It is also shown that the brass surfaces are affected by different repetition rates.

(a) (b) (c) (a) (b) (c)

Figure 5. Morphologies of laser-textured and (a) 350 C (scale bar 100 nm), (b) 600 C (scale bar 100 nm), Figure 5. Morphologies of laser-textured and (a) 350◦ °C (scale bar 100 nm), (b) 600◦ °C (scale bar 100 and (c) 800 C annealed samples (scale bar 1000 nm). nm)Figure, and◦ 5. (c Morphologies) 800 °C anneal ofed laser sample-textureds (scale and bar (1000a) 350 nm) °C. (scale bar 100 nm), (b) 600 °C (scale bar 100 4. Resultsnm), and (c) 800 °C annealed samples (scale bar 1000 nm). 4. Results The4. Results different annealed samples have been textured by using a nanosecond infrared fiber laser. The The different annealed samples have been textured by using a nanosecond infrared fiber laser. samples have thereafter been optimized with the purpose to speed up the transition wettability (from a The samplesThe different have thereafter annealed been samples optimized have been with textured the purpose by using to speed a nanosecond up the transition infrared wettability fiber laser . hydrophilic surface to a hydrophobic surface). By using a higher fluence of the laser, deeper and broader (fromThe samplesa hydrophilic have thereaftersurface to beena hydrophobic optimized surface with the). Bypurpose using ato higher speed fluenceup the transitionof the laser wettability, deeper groovesand(from broader in a the hydrophilic brass grooves could insurface the be formed.brass to a couldhydrophobic Moreover, be formed surface the. largerMoreover,). By the using annealingthe alarger higher the temperature, fluence annealing of the thetemperature, laser higher, deeper the mechanicaltheand higher broader properties the groovesmechanical of thein the brass.properties brass In could addition, of the be formedbrass. the largerIn. Moreover, addition, the distances thethe largerlarger between thethe distanceannealing the nanograins,s between temperature, the the greaternanothe the grainshigher surface, the the micromechanicalgreater/nanostructure the properties surface roughness. micro of the/nanostructure brass. Moreover, In addition, roughness. the the smaller larger M theoreover, the nanoparticles, distance the ssmaller between the finerthe the thenanoparticles, nanostructurenanograins, the the (which greater finer will the the makenano surfacestructure it easier micro to(which/ formnanostructure willa good make solid–liquid roughness. it easier to contact). M formoreover, a goodWhen the solid thesmaller material–liquid the itselfcontactnanoparticles, is hydrophobic,). When the the amaterial finer large the degree itself nano is ofstructure hydrophobic, pronounced (which a surface willlarge make degree roughness it of easier pronounced will to form induce asurface good a large solidroughness degree–liquid of hydrophobicitywillcontact induce). W a henonlarge the the degree contactmaterial of surfacehydrophobic itself is area. hydrophobic,ity on the contacta large surfacedegree areaof pronounced. surface roughness AswillAs can induce can be be seen a largeseen in degreein Figure Figure 6ofb, hydrophobic6b, d andd and f, whenf, whenity on annealing annealingthe contact the the surface sample sample area toto.350 350 °◦CC,, the the nanostructure nanostructure becamebecame finer,As finer, can and beand the seen the number innumber Figure of of nanograins6b, nanograins d and f, waswhen was smaller smaller annealing andand the moremore sample scattered to 350 (as (as ° Ccompared compared, the nanostructure with with the the resultsresultsbecame of theof finer,the other other and annealing annealingthe number temperatures). temperatures). of nanograins In In was short, short, smaller thethe sizessizes and ofofmore the scatterednanoparticles (as compared did did relate relate with to to the the results of the other annealing temperatures). In short, the sizes of the nanoparticles did relate to the temperaturestemperatures of annealing. of annealing. Moreover, Moreover, the the sizes sizes of theof the Cu 2CuO2 clustersO clusters were were found found to beto similarbe similar to theto the size of thetemperatures nanograins atof anannealing. increased Moreover, annealing the temperature. sizes of the Cu As2O a clusters result, thewere surface found roughnessto be similar became to the related to the microstructure.

Micromachines 2020, 11, x 10 of 14 Micromachines 2020, 11, 1057 10 of 14

(a) (b)

(e) (f)

Figure 6. FigureFESEM 6. picturesFESEM pictures showing showing the micro the micro/nanostructure/nanostructure morphologies morphologies of of hydrophobic hydrophobic surfacessurfaces 2 that havethat been have textured been textured by using by ausing 4.18 a J /4.18cm2 J/cmlaser laser ﬂuence, fluence, in additionin addition to to the the following following annealingannealing temperatures: (a,b) 350 °C, (c,d) 600 °C, (e,f) 800 °C, the scale bar of the SEM Figure in the left row is temperatures: (a,b) 350 ◦C, (c,d) 600 ◦C, (e,f) 800 ◦C, the scale bar of the SEM Figure in the left row is 1 micrometer, the right row is 100 nm. 1 micrometer, the right row is 100 nm. During the initial tests, when the laser energy was enhanced, the degree of hydrophilicity also During the initial tests, when the laser energy was enhanced, the degree of hydrophilicity also increased. It can also be concluded from the measurements that were made after the annealing increased.processing, It can also that bea higher concluded annealing from temperature the measurements is connected that to a were stronger made resistance after thetowards annealing laser processing,processing that a higher on the annealingbrass sample temperature surface. Directly is connected after the toannealing a stronger of the resistance surfaces, towardsa larger laser laser processingfluence on the was brass also sample observed surface. to create Directly a smaller after WCA the annealing(i.e., the degree of the of surfaces, surface hydrophilicity a larger laser ﬂuence wasincreased). also observed However, to createthis changed a smaller after WCA 4 days (i.e., of theevolution, degree when of surface the super-hydrophobicity hydrophilicity increased). grew However,stronger. this changed after 4 days of evolution, when the super-hydrophobicity grew stronger. As can beAs seencan be in seen Figure in Figure7b,d,f, 7b,d, it is and quite f, it obvious is quite thatobvious the that nanograins the nanograins became became smaller smaller in the in the 350 °C annealed sample, as compared with the results for 600 °C and 800 °C annealed samples. 350 ◦C annealed sample, as compared with the results for 600 ◦C and 800 ◦C annealed samples. Higher Higher temperatures resulted in micro-grooves with narrower nanograins. On the other hand, Figure temperatures resulted in micro-grooves with narrower nanograins. On the other hand, Figure7b,f,i 7 b,f, and i show that the nanograin sizes will be reduced when increasing the annealing temperature. show that the nanograin sizes will be reduced when increasing the annealing temperature. An An evolution of the Cu2O nanostructure was also formed, which is shown in Table 3. evolution of theAccording Cu2O nanostructure to the results wasobtained also formed,by Energy which Dispersive is shown Analysis in Table Spectroscopy3. (EDS), the composition of the samples could be identified. In addition, the WCA evolutions were measured with confidence. There were no other impurities sub-quenched into the sample substrates. Micromachines 2020, 11, 1057 11 of 14 Micromachines 2020, 11, x 11 of 14

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

Figure 7. FESEM pictures of the micro/nanostructure morphologies of hydrophobic surfaces where Figure 7. FESEM pictures of the micro/nanostructure morphologies of hydrophobic surfaces where the texturing period was set to 100 µm, and with the following annealing temperatures: (a–c) 350 ◦C, the texturing period was set to 100μm, and with the following annealing temperatures: (a–c) 350 °C, (d–f) 600 C, and (g–i) 800 C. The magniﬁcation in (b,e,h) is 50,000. The scale bar of the SEM Figure in (d–f◦) 600 °C, and (g–i) 800◦ °C. The magnification in (b), (e), and (h) is 50,000. The scale bar of the SEM the left row is 20 micrometers, the scale bar of the right row is 1 micrometer, and the scale bar of the Figure in the left row is 20 micrometers, the scale bar of the right row is 1 micrometer, and the scale middlebar row of the is 100middle nm. row is 100 nm. Table 3. EDS scans of the micro/nanostructure morphologies of hydrophobic surfaces. Table 3. EDS scans of the micro/nanostructure morphologies of hydrophobic surfaces.

ElementsElements Wt% Wt%At% At% ElementsElements Wt% Wt% At% At% Cu Cu51.92 51.92 36.3136.31 CC 5.00 5.00 18.51 18.51 O O6.55 6.55 18.2018.20 NaNa 2.10 2.10 4.05 4.05 Zn Zn33.06 33.06 22.4822.48 FeFe 0.27 0.27 0.21 0.21 PbPb 1.10 1.10 0.24 0.24

5. Conclusions According to the results obtained by Energy Dispersive Analysis Spectroscopy (EDS), the Different annealing treatments of the brass samples have resulted in different wettability composition of the samples could be identified. In addition, the WCA evolutions were measured with transition times, which in turn were caused by the existence of different amounts and types of confidence.nanoparticles There, werenanograins, no other and impurities grain boundaries sub-quenched. In addition into, thethe samplehierarchic substrates. micro/nanostructures played an important role during the reversible transition from super-hydrophilicity to super- 5. Conclusions hydrophobicity. Different annealing treatments of the brass samples have resulted in different wettability transition times, which in turn were caused by the existence of different amounts and types of nanoparticles, nanograins, and grain boundaries. In addition, the hierarchic micro/nanostructures played an important role during the reversible transition from super-hydrophilicity to super-hydrophobicity. Micromachines 2020, 11, 1057 12 of 14

(1) The different distances between the nanograins were observed to affect (a) the reversible wettability time, (b) the nanoparticles, that in turn were related to the progress of the Cu2O evolution, and (c) the surface roughness (that would influence the surface energy). After the initial nanosecond laser ablation, the high laser fluence did increase the distances between the nanograins. More CuO materials were formed, and the surfaces received super-hydrophilic properties. The CuO composition infiltrated the grooves and became more easily embedded into the ablated microstructures. After the evolution of Cu2O, the surfaces were transferred to super-hydrophobic properties. To reduce the laser repeat repetition, it interacts with the H59 brass surface with a higher power in each pulse, and then, the brass oxidation was extended and the distances between the nanograins were increased. Furthermore, while the laser rate was turned down, the laser was removing an increased volume out of the micro-grooves. All of these methods were found to speed up the evolution of Cu2O and WCA. (2) The annealing temperatures were strongly coupled with the evolution of nanoparticles. High temperatures induced much smaller nanoparticles and increased the adhesion of Cu2O to these nanoparticles. Moreover, the number of nanoparticles increased rapidly, which was also the situation with the WCAs. (3) The chemical properties of brass played a fundamental role in the evolution of super-hydrophobicity. For instance, the hydrophilic properties of the brass surfaces were related to the number of CuO nanoclusters. The transformation from a hydrophilic to a hydrophobic surface was then related to the existence of Cu2O nanoclusters in the micro/nanostructures, and to the surface energies.

The more pronounced micro/nanocomposite structures were more easily adsorbed by ions and formed a type of air cushion attached to it. This resulted in liquid–solid bubbles and formed a gas–liquid–solid composite in contact with the surface. With an increase in surface roughness, the solid–liquid contact area did decrease, thereby forming a hydrophobic surface. It has here been assumed that the droplet will be in direct contact with the solid surface, with no air in between. The contact angle increased, and the surface became more hydrophobic. Hence, when the solid surface is made of a hydrophilic material, the increased surface roughness will cause a more hydrophilic surface.

Author Contributions: All the authors have contributed equally to this work, either in the implementation or in the writing of this article. K.J. provided the idea and the total design; L.K. carried out the annealing treatments; B.Y. designed the detailed plan; T.L. contributed to discussion about the surface energy and roughness after heating treatment; H.X. and X.L. designed the manipulation system and the experiments; X.L. analyzed the theoretical model; X.L. constructed the manipulation strategy; G.Z. conducted the SEM observations; J.S. conducted XRD research to test the EDS result. All authors have read and agreed to the published version of the manuscript. Funding: This research was also funded by Huaqiao University Research start-up funds, grant number 605-50Y11277, The authors thank the Natural Science Foundation of Fujian (Grant No. 2019J01059), Open Key Laboratory Foundation (Grant No. 2016JZA002), and National Natural Science Foundation of China (Grant No. 51575198), (Grant No. 51805176), (Grant No. 51475174). Acknowledgments: The authors would like to express their gratitude to EditSprings (https://www.editsprings. com/) for the expert linguistic services provided. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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