Time dependency of the hydrophilicity and

Applied Surface Science 273 (2013) 399–407Contents lists available at SciVerse ScienceDirectApplied SurfaceSciencej o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /a p s u scTime dependency of the hydrophilicity and hydrophobicity of metallic alloys subjected to femtosecond laser irradiationsi-bandoki a ,∗,S.Valette a ,E.Audouard b ,S.Benayoun aaLaboratoire de Tribologie et Dynamique des Systèmes,Ecole Centrale de Lyon,Universitéde Lyon,UMR CNRS 5513,36avenue Guy de Collongues,69134Ecully Cedex,France bLaboratoire Hubert-Curien,UniversitéJean-Monnet,Universitéde Lyon,UMR CNRS 5516,18rue du Professeur Benoît Lauras,42000Saint-Etienne,Francea r t i c l ei n f oArticle history:Received 17October 2012Received in revised form 4February 2013Accepted 12February 2013Available online 18 February 2013Keywords:Femtosecond laser Multi-scale roughness Hydrophilicity Hydrophobicity Metallic alloysChemical modificationsa b s t r a c tSurfaces of metallic alloys were laser-processed with femtosecond laser pulses of 800nm,withpowerdensities.The effect of time on the wettability of these surfaces was investigated.Afrom hydrophilicity to hydrophobicity occurred over time and is dueto surface chemistry modifica-tions.The creation of newhydrophobicfunctional groups on aluminum alloy surface,for example,was proposed to be responsible for the hydrophobic behavior observed on these surfaces.© 2013 Elsevier B.V. All rights reserved.1.IntroductionThe control and optimization of adhesion phenomena present a high interest for applications with a high industrial poten-tiality:the design of anti-icing/de-icing systems,the control of flow in micro/nano-fluidic devices,cell proliferation and super-hydrophobic surfaces for the injection molding of polymers are some of these applications.These different applications can be developed by controlling the affinity of the studied materials with liquids,water in particular.This affinity can be controlled through the surface roughness and chemistry.Numerous techniques were then developed to modify the roughness and the chemistry of mate-rials’surfaces.One of the most promising techniques is the surface treatment by femtosecond laser.It has been largely used with suc-cess for the study of corrosion or thermal/mechanical effects [1–4]for example.It is only recently that the femtosecond laser is used as a technique to modify surface adhesion [5–7].In this paper,we show the efficiency of this technique in modifying and controlling wetting behavior.In most cases,authors focus only on the roughness effect on wettability.But the effect of laser-induced is poorly studied.∗Corresponding author.Tel.:+33472186438.E-mail addresses:delphinbizi@yahoo.fr ,delphinbizi@ (i-bandoki).The Kietzig et al.[8]findings are to the best of our knowledge the only results dealing with the effect of surface chemistry on the wettability of femtosecond laser textured surfaces.However,this work was mainly focused on the effect of the carbon amount on the processed surfaces.In the present work,the investigation is more thorough.The chemical bonds created after the laser irradiations and their influence in the surface wettability are studied.This work also suggests to distinguish both chemical and roughness effects.2.Experimental setup2.1.MaterialsTwo different experimental alloys dedicated to the design of molds for plastics were chosen for the experiments:a martensitic stainless steel (SS)and an aluminum alloy.This martensitic steel is an optimized version of the X40Cr14stainless steel,suitable for polishing and very resistant to corrosion.As this steel is a non-commercial version with an unofficial denomination,it is named X40Cr14Az in this article.The nominal composition of this steel is given in Table 1.The aluminum alloy is also an experimental material with an unofficial denomination.It is named aluminum 7040T6in the fol-lowing.Its nominal composition is given in Table 2.Samples of these materials were mirror-polished and then subjected to laser irradiations.Before and after laser irradiation0169-4332/$–see front matter © 2013 Elsevier B.V. All rights reserved./10.1016/j.apsusc.2013.02.054400i-bandoki et al./Applied Surface Science 273 (2013) 399–407Table 1Nominal composition of SS X40Cr14Az.Elements C N Si Mn Ni Cr Mo V Weight %0.260.100.350.450.35120.350.25Table 2Nominal composition of Aluminum 7040T6.Elements Si Fe Cu Mn Mg Cr Ni Zn Pb Ti Zr Weight %0.0250.0640.60.0031.890.0010.0057.570.0010.0180.111experiments,these samples were cleaned in a 10-min ultrasonicSamples were irradiated with a linearly polarized beam of a titanium-sapphire laser system with a wavelength of =800nm,a repetition rate f =5kHz and a pulse duration of =125fs.Areas of 1cm 2were irradiated on each sample by overlapping the laser scannings as illustrated inFig.1.This processing consists in machin-ing a line at a scanning speed of 7.3mm/s for the stainless steel,and 15mm/s for the aluminum alloy.Then,the laser beam is displaced laterally by a distance .In order to obtain an entire machined surface,this lateral displacements were chosen to be smaller than the spot size .Various lateral displacements (5,10,15and 20␮m)and power densities P D (0.34×1013,2.65×1013and 5.31×1013W/cm 2)were used to study their influence on the sur-face roughness and wettability of investigated materials.The beam was focused to a spot size of =30␮m.Top-hat and Gaussian beams were used.The Top-hat beam was used for the lowest power density P D of 0.34×1013W/cm 2,and the Gaussian beam for the upper values of the power density.The laser power density P D was obtained from Eq.(1):P D =F 0(1)with F 0the laser fluence.The laser fluences for top-hat and Gaussian beams are respectively defined in reference [9,10].2.3.Surface characterizationThe effects of laser processing on surface morphology were investigated by means of scanning electron microscopy (SEM),optical profilometer and atomic force microscopy (AFM)in contactFig.1.Scheme of the laser scanning process using the speed V ,the overlap distance and the spot size .mode.In order to evaluate the wetting properties,the static contact angle was measured using the sessile drop method.3␮L droplets of distilled water were dispensed on the samples surfaces,and the contact angle was determined by analyzing droplets images using the software Drop Shape Analysis.The contact angle hysteresis is determined by comparing the advancing and the receding contact.The advancing contact angle was measured during the growing of the droplet and the receding contact angle was measured during the shrinking of the droplet.These measurements were carried out under atmospheric conditions.X-ray photoelectron spectroscopy (XPS)analysis was used to quantify the elementary composition of the surface.3.Results and discussion3.1.Effect of laser texturing on surface morphologyThe characterization of surface topography clearly shows the effect of laser irradiation on the surface morphology of the mate-rials.After the laser processing,the SS X40Cr14Az surface shows a multi-scale periodic roughness:the first scale of roughness is made of laser-induced ripples with a spatial period 1≈650nm (Fig.2).These ripples are oriented perpendicularly to the laser polariza-tion.They appeared for all values of the lateral displacement and power density of P D used.Ripples are clearly visible at lower power densities or low fluence regime (0.34and 2.65×1013W/cm 2).The laser texturing is “clean and “controllable”.The highest power den-sity of 5.31×1013W/cm 2corresponds to a high fluence regime which is characterized by a “non controllable”laser texturing.Indeed,ripples were clearly produced (they are visible at some locations on the surface)but they were hidden by debris of re-deposited material due to ablation phenomenon.The phenomenon of the material re-deposition has already been observed elsewhere on silicon [11].The second scale of roughness consists in periodic undulations with a spatial period 2equals to the lateral displacement .Fig.3presents 3D optical profilometer images of these undulations at high fluence regime.These undulations originated from ablation processes.When the lateral displacement increases from 5to 10,15and 20␮m,these undulations become coarser and more pro-nounced.For smaller displacements (5and 10␮m),undulations’periodicity is disturbed,probably by the material re-deposition phenomenon.The control of the texturing is then compromised.For higher (15and 20␮m),this periodicity becomes clearer.This shows that the control of such a roughness is dependent on the lateral displacement .But,as shown in SEM images in Fig.2,the microstructures at lower scales (ripples)will always be hidden at this value of power density.Therefore,the multi-scale texturing can be controlled only for low power densities where ripples can be controlled.Fig.4shows SEM and optical profilometer images of aluminum alloy surfaces.As it can be seen,ripples were not produced on this material.Only undulations’scale is observed after the laser tex-turing;which evidences the occurrence of ablation phenomenon.i-bandoki et al./Applied Surface Science273 (2013) 399–407401Fig.2.SEM images of laser-induced surface morphology on SS X40Cr14Az with various and P D.Fig.3.3D images of laser-induced undulations on SS X40Cr14Az surfaces at P D=5.31×1013W/cm2and various lateral displacement of:(a)5␮m,(b)10␮m,(c)15␮m and(d)20␮m.i-bandoki et al./Applied Surface Science 273 (2013) 399–407ser-induced surface morphology on aluminum 7040T6surfaces with =20␮m and power densities of (a)P D =2.65×1013W/cm 2and (b)P D =5.31×1013W/cm 2.The absence of ripples may be explained by the fact that the laser fluences used for this material are too high to generate ripples.3.2.Time effects on surface hydrophilicity/hydrophobicityFig.5shows the dependency of the water contact angle withlaser treatment.P D of0.31×1013W/cm 2where all the values of the contact angle on laser-treated samples are smaller than the contact angle on the pristine surface (Fig.5a).For example,a contact angle lower than 25◦is obtained with =5␮m.For P D =2.65×1013W/cm2,the contact angle is lower than the pristine surface one for all values of ,except for =15␮m which exhibits a slightly higher contact angle (Fig.5b).Beyond one day,contact angle started to increase gradually to reach values above 130◦.Surfaces are then highly hydrophobic.Beyond a certain period (three days here),thisevolution stopped.No major change of the contact angle is then noticed the following days.A similar behavior is observed on aluminum alloy irradi-ated with P D =2.65×1013W/cm 2for various values of .The hydrophilicity of this surface was globally enhanced immediately of the SS X40Cr14Az ser-induced hydrophobic metallic surfaces were reported elsewhere in the literature [8,12–14].Such a dependency of the contact angle with time cannot be explained by the surface topography as the surface topography The chemical compositions of sur-faces before and after laser treatment were compared in order toi-bandoki et al./Applied Surface Science 273 (2013) 399–407403Fig.5.Evolution of contact angle over time.(a)P D =0.34×1013W/cm 2;(b)–(c)P D =2.65×1013W/cm 2.evidence apossible effect of the laser irradiations on the metallic surface chemistry.analysis was made when the contact angle stabilized;namely when surfaces are hydrophobic and chemical reactions stopped.The corresponding results are presented in the following section.4.Surface chemical composition analysisThe chemical analysis was carried out on samples of SS X40Cr14Az and aluminum 7040T6which were subjected to irradi-ations of 2.65×1013W/cm 2.XPS spectra for SS X40Cr14Az before and after laser treatment (>6days after the treatment)are shown in Fig.6.Before the treatment,the SS surface exhibit a carbon con-tent of ≈68%at%.After the laser irradiation,this carbon content decreases to ≈56%.Li et al.[15]reported a similar decrease of carbon content on metallic substrates submitted to femtosecond irradia-tions.This shows that a change in the surface chemistry was clearlythey do not explain the laser induced hydrophobicity observed some days after the samples had been processed.However,let surface had been processed,these molecules disappeared.Table 3Surface chemical composition of a non irradiated X40Cr14Az substrate.Elements/compounds Positions (eV)Atomic %C 1sC H 284.851.9C O286.28.15COOH (Carboxyle)288.27.5Fe 2p3/2Natural element707.20.23FeO ·OH (Hydroxyde)711.52.36N 1s C N400.73.09O 1sMoO 3,SiO 2530.7 5.49O C,H 2O 532.516.92C O,H 2O5344.36i-bandoki et al./Applied Surface Science 273 (2013) 399–407Fig.6.XPS spectra for X40Cr14Az before and after femtosecond laser irradiation with 2.65×1013W/cm 2.Table 4Surface chemical composition of a X40Cr14Az substrate irradiated with 2.65×1013W/cm 2.Elements/compoundsPositions (eV)Atomic %C 1s C H 284.834.66C O 286.613.02C O,CO 3−288.37.88Cr 2p3/2Cr(III)in Cr 2O 3576.91.39Fe 2p3/2Metallic Fe 706.80.05Fe 3+710.5 1.08FeOOH711.62.34N 1s N 2,NH 3400.72.36O 1sCr 2O 3,CrO 3,Fe 2O 3,Fe 3O 4,FeOOH 530.212.65OH 531.912.75C O533.511.82more likely that the femtosecond laser processing removed a thin film of water initially present at the SS X40Cr14Az surface.The dis-appearance of this water film means that the surface became less polar than before the treatment.We suggest that this diminution of surface polarity could have contributed to the observed hydropho-bicity,even if it does not explain it completely.The chemical composition of the aluminum surface is shown in Fig.7.The carbon amount increased from ≈55%before treat-ment to ≈67%after laser treatment.This shows that the laser processing induced an accumulation of carbon concentration atthe 5and 6confirm this result.They show preciselythat this carbon amount increase is in part due to the appearance of new functional groups after laser treatment,especially the methyl group–CH 3and the graphitic carbon.These two groups are known to be hydrophobic.Thesehydrophobic groups are probably respon-sible for the hydrophobic behavior obtained on aluminum surface as their presence is only observed on the hydrophobic sample.5.Topography effects on wetting propertiesThe previous results evidenced to some extent the influence of the surface chemistry on the wettability of metallic surfaces.They explained especially the transition from hydrophilicity to hydrophobicity over time.Now,how to explain the enhancement of hydrophilicity immediately after laser treatment?And what happens when chemical reactions stop beyond three days?The effect of surface topography might be preponderant.To verify this assumption,it is important to search for a correlation between the evolution of the contact angle and the surface roughness of samples.Fig.8presents the evolution of the contact angle and the rough-ness RMS of the investigated materials’surfaces as a function of power density and one day after the laser treatment.Regardless of materials,the contact angle globally decreases with increas-ing the laser power density.The higher the power density,the higher the surface hydrophilicity.Even the super-hydrophilic sur-faces were obtained with the highest value of power density P D of 5.31×1013W/cm 2with contact angle values lower than 10◦.This corresponds to the high fluence regime.At the same time,an increase of the power density leads to an increase of the RMSFig.7.XPS spectra for Aluminum 7040T6before and after femtosecond laser irradiation with 2.65×1013W/cm 2.i-bandoki et al./Applied Surface Science 273 (2013) 399–407405Table 5Surface chemical composition of a non-irradiated Aluminum 7040T6substrate.55% CElements/Co mpounds Positions (eV)Atomic %Al 2p3/2Al2O 3, AlN, Al 2TiO 574.211.27C-H, C-C 284.740.8C=O 286.48.36C 1s C=O, COOH 288.6 5.41N 1s NO x 400.51.05O 1sOH, Al(OH)3531.930.54Table 6Surface chemical composition of an aluminum substrate irradiated with 2.65×1013W/cm 2.Elements/Co mpoundsPositions (eV)Atomic %Al 2p3/2Al-O73.51.03C-H, i-CH 3, graph itic C284.555.4C 1s-C=O, -O-C-O-287.611.98N 1s N-O, N-C-O399.60.79C=O, Al 2O 3531.215.98O 1sOH532.414.7867% CFig.8.Evolution of contact angle Âm and roughness RMS measured on X40Cr14Az and Aluminum 7040T6sustrates,as a function of laser power density.i-bandoki et al./Applied Surface Science 273 (2013) 399–407Fig.9.Contact angle as a function of lateral displacement measured on SS X40Cr14Az subjected to irradiations of P D :(a)0.34×1013W/cm 2and (b)2.65×1013W/cm 2.parameter of roughness.The enhancement of the hydrophilicity can then be linked to the increase in the surface roughness.The dif-ferences observed in the evolution curves of the contact angle and the RMS roughness could mean that the chemical modifications are all the same activated even if they are less important.The evolution of the contact angle as a function of the lateral displacement is presented in Fig.9.The reported results were measured beyond three days when the chemical modifications are supposed to be stopped.By increasing ,that is equals to the undulations periodicity,the contact angle slightly decreases.This denotes an effect of the surface topography.The previous results suggest that the effect of the surface topography is preponderant the first moments following the laser treatment and some days after the laser treatment when the chem-ical reactions are stopped.According to the literature,this influence of the surface topography can be explained either by Wenzel’s model [16]or by Cassie-Baxter’s model [17].When the Wenzel’s state is induced by a surface roughness,the intrinsic wetting behav-ior of the surface is enhanced:a hydrophobic surface becomes more hydrophobic with an increase of the roughness,and a hydrophilic surface becomes more hydrophilic with an increase in roughness.In the case of the Cassie–Baxter state,equation predicts only an enhancement of the hydrophobicity of the surface with the rough-ness,thus the contact angle increases with the roughness increase (its value is between the contact angle on the flat surface and the one in the air,that is 180◦).In this work the hydrophilicity of the surfaces increases with the RMS roughness immediately after the laser treatment.That means that a Wenzel’s state is more likely.This can be verified by measuring the contact angle hysteresis on the corresponding surfaces.Different values of contact angle hysteresis H measured on SS X40Cr14Az and aluminum surfaces are summarized in Table 7.Before the laser treatment,the SS X40Cr14Az surface exhibited a contact angle hysteresis of H ≈31.2±2.After irradiation with P D =0.34×1013W/cm 2pulses,H slightly decreased but remained larger than 20◦.This characterizes a Wenzel’s state [18].Moreover,uncertainty on H reported values is relatively large.This means that values can be greater than reported ones.A Wenzel’s state is then more likely.In the case of a SS X40Cr14Az irradiated with P D =2.65×1013W/cm 2,H is much larger after laser processing than before (H 20◦).A maximum value is obtained at ≈54◦±3.This trend is characteristic of a Wenzel’s state.An aluminum sur-face irradiated with the same pulses exhibited a significant contact hysteresis and the same Wenzel’s state.Indeed,this contact angle hysteresis goes from H =29◦±2.4to values of H between 35◦and 48◦.These results also suggest that one can have a very markedTable 7Contact angle hysteresis H before and after femtosecond laser texturing of surfaces.Matéria ux Δ(µ m) H (°)X40Cr14Az P D =0.34×1013W/cm²031.5 ± 2525 ± 181021 ± 121516.5 ± 42026 ± 17X40Cr14Az P D =2.65×1013W/cm²031.5 ± 2548 ± 3.81054 ± 3.21545 ± 82034.4 ± 11.2Aluminium 7040T6P D =2.65×1013W/cm²029 ± 2.4439 ± 21035 ± 16.31545 ± 112048 ± 8effect on the wettability due to high contact angle hysteresis (which corresponds to some important chemical effects),as shown in our results,while having less rough surfaces than hose reported by Kietzig et al.[8].6.ConclusionSamples of a stainless steel and an aluminum alloy were subjected to femtosecond laser pulses in order to modify their surface topography and wettability.The femtosecond laser tex-turing of these materials with a laser fluence in the range of 0.34×1013W/cm 2and 5.31×1013W/cm 2is found to create a 2-scale roughness consisting of Laser-Induced Periodic Surfaces Structures (so-called ripples)and undulations emerging fromi-bandoki et al./Applied Surface Science273 (2013) 399–407407ablation phenomenon.This multi-scale topography can be con-trolled at lowfluence regime but not at highfluence regime.The contact angle measurement results showed that the modification of the wettability of metallic surfaces through femtosecond laser texturing is a function of time and the result of combined effects of laser-induced surface topography and chemistry.The effect of topography was explained based on Wenzel’s state.The effect of surface chemistry is explained by chemical modifications such as the appearance of new functional groups.In the case of the aluminum,the hydrophobic behavior can be explained by the creation of new non-polar carbon groups(–CH3and graphitic carbon)after laser treatment.On stainless steel surfaces,no hydrophobic functional groups were created after laser treatment. 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