We report on periodic, homogeneous nanoripples fabricated on stainless steel (SS), copper (Cu), and aluminum (Al) substrates using an ytterbium pulsed femtosecond laser. These structures called laser induced periodic surface structures (LIPSS) are processed at a relatively high-speed and over large areas. This paper investigates the effect of LIPSS on a wettability behavior of SS, Cu, and Al surfaces. It is shown that nanoripples significantly influenced the wettability character of these metals turning them from hydrophilic to hydrophobic behavior.
Due to the high intensity and the short pulse duration of the femtosecond laser sources, some peculiar behaviors were observed in their interaction with the material surface, in particular the generation of ablated structures with periodicity shorter than the laser wavelength, which is called LIPSS. These structures attracted the attention of researchers given the possibility to break the diffraction limits that otherwise limits the minimum dimension of the features that could be obtained by ablation on the surface. Many models were proposed to explain the formation of these structures as in Refs. [1,2]. The phenomenon is not fully explained, but the most accredited models are based on electrodynamic, in particular the interaction between the incoming electromagnetic field and the surface plasmon polariton.
It is well known that the surface wettability can be controlled by varying the roughness of the surface, in particular by obtaining a multimodal distribution in the periodicity and height of the surface morphology. Many methods have been proposed to obtain the surface nano‐texturing but many of them involve the use of vacuum chambers, creation of lithographic masks and multiple passes replication steps. The first direct applications of femtosecond laser to vary the wettability of material were proposed and tested on some materials. In particular, femtosecond laser was used to change wettability of titanium alloy and selectively induce the carbon nanotubes growth . In Ref. , authors generated LIPSS on Ti6Al4V surface for biomedical application and evaluated the variations in the wettability. Superhydrophobicity behavior on copper has been produced in Ref.  while an application on AISI316L stainless steel was presented in Ref. . The aim of the present work is to evaluate the application of the method in modifying the wettability of different materials, on large areas and on surfaces mechanically polished like in the most part of industrial applications.
Copper (99.9% purity), aluminum alloy, and stainless steel AISI 316L samples with a dimension of 20 × 20 × 5 mm were mechanically polished and cleaned in ethanol before testing. The initial roughness of the samples was evaluated by means of atomic force microscopy (AFM) analysis and the results are shown in Table 1.
LIPSS were performed by using a commercial Yb-doped femtosecond laser system “Pharos” light conversion. It provides 213 fs pulses at full width at half maximum (FWHM) with a central wavelength of 1030 nm. Nanostructures were obtained by scanning the materials with a fluence slightly higher than the surface damage thresholds, between 1.32 and 5 J/cm2. The surfaces were scanned with a lateral step size between 2 and 4 μm. In all the scanning electron microscope (SEM) images presented in this work, scan line direction will appear vertical. The pulse repetition rate was set to 600 kHz. Laser output was then coupled into a galvanometer scanner system Cambridge Technology ProSeries equipped with an F-theta lens of 55 mm focal length. Using this configuration, areas of 10 × 10 mm were treated with a speed of 1.5 m/s. The surface morphology was observed in secondary electrons imaging mode using a FEI Nova NanoSEM 450. Water contact angle measurements were carried out by using a DataPhysics OCA 20 apparatus through the sessile drop method. To avoid any surface contamination, all specimens were repeatedly washed with ethanol and carefully dried just before each measurement. Static contact angle determinations were carried out on drops having a volume of 3 μl and an average value of contact angle was determined on the basis of at least five measurements. The wettability measurements were carried out in a “parallel” direction that means observing along the direction of the generated LIPSS and measuring the angles of the drops while they are spreading across the ripples. “Perpendicular” measurements were performed observing perpendicularly to the LIPSS direction and measuring the contact angles while drops spread along the ripples.
Results and Discussion
All the experiments were conducted maintaining a pulse duration of 213 fs, a repetition rate of 600 kHz, a scanning speed of 1500 mm/s, and a lateral step of 3 μm.
The laser parameters set used in this work are coded in Table 2.
On all the materials, the laser treatment with linear polarized light resulted in the generation of nanostructures with a certain degree of regularity. The direction of LIPSS for Cu, Al, and SS appears to be perpendicular to polarization of incident light. Figure 1 shows the nanostructures differently oriented, the scanning path is always directed from left to right. The codes in the upper row define the direction of the polarization plane with respect to the scanning direction. The mechanism of LIPSS creation is not completely clarified but is clearly based on superfast heating and ablation. The periodicity of nanostructures can be explained by the Sipe model, which involves the interactions of incoming laser beam with a surface electromagnetic wave (SEW) .
Surface Morphology and Topography.
SEM micrographs of the treated surfaces aluminum, copper, and stainless steel AISI 316L are presented in Fig. 1. While scanning speed, repetition rate, and pulse duration were maintained similar for all the materials, the influence of the polarization angle θP that is the angle between the polarization plane and the scanning direction and the step between laser ablation lines were investigated. In Fig. 1, the symbols related to the different columns ⊥, ⧸⧸, and ∠ indicate angles θP of 90 deg, 0 deg, and 45 deg.
The surface morphology of aluminum samples was observed depending on both laser treatment and scanning parameters. Figure 1 shows the differently oriented nanostructures with respect to scanning direction. In perpendicular mode (θP = 90 deg), the average period of structures is Λ = 891.2 ± 50 nm that is slightly smaller than the wavelength of laser beam.
In a parallel mode, where θP = 0 deg, the generated ripples are perpendicular to the scanning direction and a period Λ = 866.8 ± 50 nm is shown. In an angular treatment, with θP = 45 deg the average period of grooves is Λ = 782.5 ± 40 nm.
At a laser fluence lower than 2.2 J/cm2, surface nanostructures formation is not observed while at fluences between 2.5 J/cm2 and 2.8 J/cm2 ripples periodic bifurcations are observed. It proves a strong correlation between morphology and fluence.
Treatment TCU1 was performed in a perpendicular mode that is θP = 90 deg. The SEM images shows nanostructure with a period of Λ = 839.2 ± 30 nm and the orientation of the LIPSS structures is perpendicular to the laser beam polarization. Structures appear to be smooth without any bifurcations and they cover homogeneously all 5 × 5 mm square. It can be outlined that the threshold for the creation of na-ogrooves is significantly higher for Cu than for Al or stainless steel, two and five times, respectively.
Figure 1 represents the SEM images taken at on AISI 316L surface treated in TSS2 and TTS5 conditions. The nanostructures created on stainless steel are substantially different from that generated on Al or Cu.
It is possible to see the presence of bifurcations and joining of different ripples. It is not easy to indicate a proper periodicity of this phenomenon that may occur only in alloyed materials. It was not observed in pure metal, only in AISI 316L that present at least 25% of noniron elements, mainly chromium and nickel.
The presence of the high spatial frequency LIPPS (HSFL) for TTS2 condition is clearly shown in Fig. 2. These fine structures present a period between 60 nm and 100 nm that means ten times less the laser wavelength and are parallel to the polarization plane in accordance with previous works [7,8]. HSFL are located between low spatial frequency LIPSS (LSFL) and are perpendicular to them. Observations suggest that HSFL are created before LFSL, they appear in some condition were intensity/fluence are not enough to generate LSFL. LSFL have period close to the irradiation wavelength; in our case, period between single ripples is Λ = 337.5 ± 50 nm, and between double ripples is Λ = 725.5 ± 50 nm. The results of the treatment TSS5 on AISI 316 is shown in Fig. 3. In this case, the polarization plane is perpendicular to the scanning direction and consequently the generated LSFL are parallel to scanning. The measurement of the period on LSFL results in Λ = 697.7 ± 50 nm for single ripple and Λ = 470.3 ± 50 nm in case of twinned ripple. The HSFL appear in the order of 100 ± 20 nm.
Energy-Dispersive X-ray Spectroscopy Analysis.
The material properties of the nanostructures on Al, Cu, and SS that are created in this process have been characterized through EDS. The EDS analysis in Figs. 4–6 clearly show the presence of oxygen after the laser treatment. For example, in Al it arises from 2.21% on untreated surface to 14.17% for treated ones as indicated in Tables 3 and 4. For Copper, an increasing of oxygen part was observed from 2.79% to 5.19% in Tables 5 and 6. The same behavior appears for stainless steel, where for nonirradiated surface oxygen is absent, while for irradiated it increases up to 3.54% in Tables 7 and 8.
Atomic Force Microscopy Analysis.
The observed morphology of Al, Cu, and SS (Fig. 7) using AFM shows that surface is composed by parallel and periodic, homogeneous nanoripples for all materials. For Al, the height of profile is 270 ± 30 nm. The average roughness is Ra = 44 nm. For SS, the height of profile is essentially higher, 410 ± 20 nm, and the average roughness is Ra = 50.1 nm. For Cu, the height of profile is 93 ± 50 nm while the average roughness is Ra = 17.8 nm much lower respect than the other two materials.
The wettability behavior of materials can be controlled either by changing its surface chemistry or by modifying its surface morphology. Hydrophilicity (wetting) or hydrophobicity (nonwetting) character of a material is commonly defined by the static contact angle θ between a liquid (water or oil) droplet and the material's surface. Hydrophilicity and hydrophobicity are defined by the θ below and above 90 deg, respectively. In this study, the effect of surface chemistry and surface morphology on wettability is with respect to the periodicity and topology of nanoripples by changing laser polarization and fluence. Figure 8 summarizes the contact angle measurements performed on three aforementioned metal surfaces with slightly modified morphologies. As can be seen from the figure, all bare (untreated) metal surfaces display the hydrophilic behavior that has changed to the hydrophobic behavior after laser treatment. This can be explained by the surface roughness that is introduced due to LIPSS formation. As Wenzel reported for the first time, the water does not wet roughened surfaces because it cannot enter inside the pores but sits on top of them, leading to a hydrophobic behavior. However, the real-world examples also showed that if a material is initially hydrophilic, surface roughness further enhances this behavior. This is due to the fact that the Wenzel equation is only valid when the surface (roughness) is homogeneous, which is not always true for real world examples since the roughness can be nonuniform or heterogeneous or there can be chemical inhomogeneities on the surface, which again translates into heterogeneous surfaces. The wetting behavior for such surfaces is generally explained by the Cassie–Baxter model [9–11]. According to this model, regardless of the initial wetting behavior of the material, if the surface roughness allows air or gas to trap inside the cavities, air or gas also acts as a surface, where the water droplet can behave as if it is on a smooth, flat surface leading to a hydrophobic behavior. This means that the shift from hydrophilicity to hydrophobicity via induced nanoripples observed in this work should better be explained by the Cassie–Bexter model, rather than the Wenzel model. Furthermore, it is also shown that this behavioral shift is more prominent for SS and Al, where a dramatic change of θ from ∼70 deg to ∼110 deg is observed for SS and from ∼80 deg to ∼110 deg is observed for Al when the surface is nanopatterned with laser. This is due to the fact that the periodicity and nanoripples on these two metal surfaces are more prominent compared to that of Cu, which means the surface roughnesses of these two metals are significantly larger than that of Cu, Fig. 1.
Figure 8 also shows that the polarization and laser fluence also influence the wettability behavior, specifically for Al. It is shown that the highest θ value is measured when the polarization is 45 deg, whereas parallel and perpendicular polarizations do not have a large effect on the hydrophobicity behavior since the structures are isotropic on average. If the structures were anisotropic (directional), then we would have observed a significant difference in wettability measured observing in parallel and perpendicular directions. In particular, the surface wettability behavior along the LIPSS direction appears less hydrophobic than perpendicularly to it.
This means that the wettability not only depends on the surface roughness but also dramatically influenced by its homogeneity, periodicity, uniformity, anisotropy, and the length scale (dimensions of the structures and their distances to each other) of introduced roughnesses . We also note that the laser fluence did not dramatically affected the overall behavior of the treated SS surface (Table 2 and Fig. 1), where the θ varied between ∼100 deg and ∼110 deg.
Experiments carried out using Yb-fiber femtosecond laser showed that it was possible to generate nano textures on both aluminum and copper pure metals and stainless steel metallic alloys. The obtained structures are periodic and homogeneous; they can be obtained for indefinitely large area and the process time appears very interesting for industrial applications. In its simple form, this method does not require expensive equipments, environmental conditioning system, or vacuum chamber. The process can be scaled in order to increase the productivity. This can be done by proportionally increasing the power and the scanning speed while the spot diameter cannot be increased above few microns. With a scanning speed of 1.5 m/s typical of many galvanometric systems, it is possible to expect values of production rate between 450 and 1200 mm2/min. Results on the surface wettability appear very interesting. LIPSS treatments permit to change the surface behavior from hydrophilic to hydrophobic behavior. In particular, on aluminum the surface morphology results in very high contact angles. The last interesting remark is the fact that LIPSS treatment could be easily and selectively applied on the surface in order to control the interaction with liquids.
The authors would like to thank Massimo Tonelli and Mauro Zapparoli (CIGS-University of Modena and Reggio Emilia) for the SEM characterizations.