1 Introduction

Laser processing of solids by femtosecond pulses, resulting in the formation of extended areas covered with Laser-Induced Periodic Surface Structures (LIPSS) [1,2,3,4,5,6], has been proven to be a versatile and efficient tool to modify the surface topography of irradiated targets and, as a consequence, to affect their roughness and wettability [7,8,9,10]. Controllable variation of these properties is of growing interest for many technical, biological and medical applications [2, 11, 12].

Depending on the applied irradiation dose, a large variety of topographies at both nano- and microscales with the corresponding roughness features ranging from several nanometers up to several tens of micrometers can be obtained at the ablated area [13]. It means that by systematical changing of the applied irradiation dose, the surface wetting properties can be tuned in a wide range from superhydrophobic to superhydrophilic.

In this study, we investigate the wetting behavior of structured stainless steel and silicon targets in dependence on the micro- and macroscopic laser-induced surface patterns and resulting statistical roughness.

Taking into account that the wettability of stainless steel is a rather complex phenomenon that not only depends on the surface topography, but also on surface chemistry [14, 15], we analyze the role of chemical changes and aging of structured stainless steel surfaces. For silicon targets, we assume that the natural thin oxide layer growing immediately after ablation is protecting the surface and resulting in a chemically stable surface with wetting properties depending only on the topography.

2 Experimental

To generate extended (10 × 10 mm2) areas of LIPSS on polished stainless steel (AISI 304) and silicon (100) surfaces, linearly polarized pulses (≈ 790 nm; ≈100 fs; 1 kHz) from an amplified Ti: Sapphire laser system were focused at normal incidence to a spot of ≈ 1.13 × 10− 4 cm2. Extended areas were achieved by scanning the target, mounted on an XY-translation stage, under the laser spot, as shown in Fig. 1.

Fig. 1
figure 1

Formation of a large structured area by LIPSS writing. v is the scanning speed of the translation stage; the arrow indicates scan direction; Δy is the lateral overlap between scan lines; E with double arrows indicates direction of laser polarization

Full size image

To obtain structured surfaces with different morphological features, the applied irradiation dose was systematically varied by varying the laser fluence and the effective pulse number (Neff). The effective pulse number, Neff, and the number of laser shots per unit area can be controlled via the scanning speed of the translation stage, vx, and the lateral overlap between the scan lines, Δy. Additionally, a multiple over-scanning of the same area can increase the effective pulse number. Changing the scanning speed between 0.1 mm/s and 100 mm/s, and using the constant scan line overlap of 50%, the effective pulse number per unit area was changed in the range from 2 to 2000 (2 ≤ Neff ≤ 2000) for silicon targets. For stainless steel, using the same 50% overlap between adjacent scan lines and varying the scanning speed between 0.5 and 10 mm/s, we change the effective pulse number per unit area in the range from 20 to 800 (20 ≤ Neff ≤800). One should be mentioned here, that the highest irradiation dose (Neff = 800) was obtained by a double scan of the same area. In all experiments, direction of laser polarization E, was perpendicular to the scan velocity v.

After irradiation, the surface morphology of ablated areas was analyzed by means of scanning electron microscopy (SEM). The statistical roughness parameters of the structured surfaces were determined by a confocal optical microscope. To study the dependence of surface wetting properties on the topography, the dynamic and static contact angles were measured on the structured areas by the sessile drop method. The liquid used was distilled water and the volume of the measuring droplets was between 2 and 5 µL.

X-ray photoelectron spectroscopy (XPS) was used to examine changes in the chemical composition on laser-structured stainless steel targets in comparison with a non-irradiated surface.

3 Results and discussion

3.1 Extended LIPSS areas on stainless steel

For structuring of stainless steel, the laser fluence, F, was varied in the range of 0.35–2.1 J/cm2 and the effective pulse number, Neff, between 20 and 800.

The dependence of surface topography and surface roughness on the applied irradiation dose is shown in Fig. 2. Structured surfaces were produced at F = 1.15 J/cm2. The effective pulse number was increased from 20 to 800 pulses/spot by variation of scanning speed from 10 mm/s (panels a–c) to 0.5 mm/s, and a double scan of the surface (panels d–f). The observed topographies are ranging from very regular linear periodic ripples at the low irradiation dose, Neff = 20 pulses/spot, (Fig. 2a–c) to complex hierarchical multi-scale 3D patterns at the higher irradiation dose, Neff = 800 pulses/spot (Fig. 2d, e). In the overview of structured areas (Fig. 2a, d), one can distinguish traces written by the 100-µm laser beam with 50% overlap between the scan lines, resulting in a width of about 50 µm. Despite the visible contrast between the scan lines in Fig. 2a, the structured area generated at low irradiation dose (1.15 J/cm2 × 20 pulses/spot) exhibits a pattern of very homogeneous and regular ripples (Fig. 2b), oriented perpendicular to the laser polarization, with a spatial period about of 565 nm. The high statistical homogeneity and regularity of this structured area, exhibiting the total height of the roughness profile well below 1 µm (Rt ≈ 0.88 µm), is reflected again in a 3D mapping (Fig. 2c).

Fig. 2
figure 2

Dependence of LIPSS patterns generated on stainless steel at F = 1.15 J/cm2 on the irradiation dose. a–c Neff = 20 pulses/spot and d–f Neff = 800 pulses/spot. a, d are SEM microphotos of overview of structured areas; V is a scan velocity and double arrows indicate scan direction. SEM images in b, e exhibit magnified details of LIPSS-structured areas highlighted by white squares in overviews. Here, E with double arrows is the direction of laser polarization. c, f are confocal microscope 3D mappings of structured areas, Rt is the total height of the roughness profile

Full size image

At high irradiation dose (1.15 J/cm2 × 800 pulses/spot), the surface is covered with complex multi-scale patterns (Fig. 2e) consisting of coarse 5-µm wide segments, oriented along the polarization direction, which are over-structured by fine ripples of periods about of 600 nm. The fine ripples are perpendicular to the laser polarization (cf. Fig. 2b) and to the coarse features. The pronounced scanning traces are clearly visible in a 3D roughness mapping of this area (Fig. 2f). The total height of the roughness profile is about of 10 µm (Rt ≈ 9.54 µm), ten times higher than at low dose.

3.2 Chemical analysis of ablated stainless steel samples

Additionally to the topography, we studied the chemical changes on the ablated stainless steel samples compared to the non-irradiated material (Table 1). XPS analyses were performed on two different structured areas irradiated at high (1.15 J/cm2 × 200 pulses/spot) resp. low irradiation dose (1.15 J/cm2 × 20 pulses/spot) and, as a reference, on a non-irradiated stainless steel surface. We found that the content of the oxygen (O), carbon (C) and main metallic components (Fe, Cr, Ni) is changing in the laser-affected area with the applied irradiation dose. At low irradiation dose (1.15 J/cm2 × 20 pulses/spot), the oxygen and carbon amounts are increasing 10 and 2 times in comparison with the non-irradiated surface, correspondingly, while the amount of iron, chrome and nickel slightly decreases. The increase of oxygen and carbon, and the decrease of the metallic components are more distinct at the higher irradiation dose (see Table 1). These changes may indicate oxidation of structured surfaces irradiated by multiple femtosecond laser pulses in air. The elemental distribution on the LIPSS-patterned area does not depend on the modified topography, as we could prove by 2D XDS mapping.

Table 1 Elemental composition of stainless steel surface (in %) dependent on the applied irradiation dose
Full size table

3.3 Wetting properties of structured stainless steel surfaces

To characterize the wetting properties of structured surfaces, the dynamic sessile drop method was applied, where the droplet volume is changed by pumping distilled water in and out. Thus, first the advancing and then the receding contact angles can be determined. Since it is known that the full equilibrium wettability properties are only obtained after some latency [14], the contact angle was measured on samples that have reached their steady state (35-day-old samples). The dependence of the dynamic (advancing and receding) contact angles on the total height of the optically measured roughness is presented in Fig. 3. Compared to the wettability of non-irradiated surface (dashed lines), we observe an increase of hydrophobicity in Region I, and hydrophilic behavior in Region II.

Fig. 3
figure 3

Plot of advancing (-adv) and receding (-rec) contact angles (CA) of distilled water droplets on microstructured stainless steel surfaces as a function of surface roughness. The dashed lines give a reference CA of the non-irradiated surface. Region I and Region II are hydrophobic and hydrophilic regions, correspondingly

Full size image

From the diagram, one can see that the hydrophilic behavior is observed for surface topographies with high roughness values, Rt > 5 µm. In Region II, the typical patterns consist of nanostructured micro-islands separated by a very deep trenches (grooves) (Fig. 2e). The strong hydrophilicity of these surfaces can be attributed to typical Wenzel behavior [16] with the roughness leading to an increase in surface and related binding forces. At first sight, this seems to contradict previous observations that such multi-scale hierarchical structures should result in even increased hydrophobicity [9, 10, 15]. In our case, only after a very long ripening, the expected behavior was observed as shown in Fig. 4: formerly hydrophilic stainless steel surfaces (Fig. 2e) can turn over to become superhydrophobic (the static CA is more than 145°). This behavior suggests that it is not only the pattern which determines the wettability, but there should be additional chemical evolution, possibly easy oxidation of the rough surface (see Table 1). This could establish a reduction of the steel–water binding forces, favor water droplet cohesion, and thus induce repulsion.

Fig. 4
figure 4

Water droplet on a hierarchically structured 17-month-old stainless steel target (at the top). At the middle panel, SEM micrograph exhibits topography of the target structured at 2.1 J/cm2 × 10 pulses/spot with the surface profile (at the bottom) along the white line. The double arrow indicates direction of laser polarization

Full size image

Surfaces exhibiting hydrophobic properties (Region I in Fig. 3) are structured with fine ripples of lateral periods in the range between 500 and 600 nm (cf. Fig. 2b); their roughness is below 2 µm (Rt <2 µm). This observation points to a typical Cassie–Baxter scenario [17]: the droplet cannot penetrate inside the surface roughness, thus reducing the surface–liquid binding forces.

3.4 Wetting properties of structured silicon targets

Similar to stainless steel structuring, extended LIPSS-covered surfaces were prepared on silicon targets. Here, we changed fluence, F, and the effective pulse number, Neff, in the range from 0.85 to 1.18 J/cm2 and from 2 to 2000, correspondingly. Depending on irradiation dose (F × Neff), a variety of surface patterns with total heights of roughness profiles, ranging from one to several tens of micrometers, was obtained in the laser-modified area. For a statistical description of roughness effects on wetting properties of structured targets, the static sessile drop contact angle (CA) was measured as a function of the mean roughness value (Fig. 5.) For a non-irradiated silicon surface, the static CA is about of 55°. Measurements of CAs on surfaces structured at different irradiation doses reveal that directly after laser irradiation all surfaces become more hydrophilic (Fig. 5a). With coarsening of patterns at increasing irradiation dose (Fig. 5b–d), the CA decreases very fast and its value approximates to zero. Extreme superhydrophilic behavior was observed on the textured surface irradiated at the highest irradiation dose (1.12 J/cm2 × 2000 pulses/spot). The corresponding topography of superhydrophilic surface is shown in Fig. 5d: large microcones of a size up to 25 µm, which are connected in the long, more than 100 µm, chains. The total height of the surface profile is about 52.13 µm.

Fig. 5
figure 5

a Static contact angle (CA) measured on structured silicon targets as a function of the mean roughness. The dashed lines gives a reference CA of a non-irradiated surface. Points highlighted by circles present measured data for b–d topographies. b–d SEM images of surface topographies generated at F ≈ 1.12 J/cm2 with 5 pulses/spot, 400 pulses/spot, and 2000 pulses/spot, correspondingly. Double arrows indicate direction of laser polarization

Full size image

The enhanced hydrophilicity of silicon after surface micro-structuring can be related to the superhydrophilicity of porous silicon due to a capillary effect. It has been shown that, already after first few pulses at threshold fluence, surface modification on silicon results in formation not only of ripples but also of deep, round holes with a diameter and depth in the range of a several tens of nanometers [18]. Formation of such laser-induced nano-porous and—by accumulation—very deep trenches can enhance the hydrophilicity of structured silicon. As known from the water attraction of porous silicon, water seems to be able to penetrate even into very tiny surface structures, thus increasing the adherence. This suggests that for silicon the Wenzel situation [16] even holds for surface structures in the sub-micron range.

By measuring of the static contact angle, we found that not only spherical droplets, but also an asymmetrical water spreading on the structured area can occur, as shown in Fig. 6a, b. Closer investigation of laser-induced topographies (Fig. 6c–h) has revealed that despite of almost identical nano-/microscopical features of these areas (Fig. 6g, h), the additional macroscopic roughness in the form of laser scan lines [cf. (c, e) and (d, f) in Fig. 6] can also influence wetting behavior on the structured surface.

Fig. 6
figure 6

Photograph of isotropic (a) and anisotropic (b) water spreading on structured silicon surfaces. c, d are low magnified SEM images of laser-scanned areas shown in a and b, respectively. e, f reveal details of laser scan lines highlighted in c and d by rectangles. d, h are enlarged details of LIPSS-covered areas from regions indicated by rectangles in e and f. Block-arrows in a and b show scan direction. Double arrows indicate direction of laser polarization

Full size image

4 4. Conclusion

In summary, controlled modifications of wetting properties of stainless steel (AISI 304) and silicon (100) targets by femtosecond LIPSS writing were investigated in this study.

We have demonstrated that it is possible to tune LIPSS-related surface wettability on stainless steel by carefully controlling the specific LIPSS patterns. A very sensitive parameter for this pattern control is the local irradiation dose, adjustable by laser fluence or by scanning parameters viz. the dwelling time of the laser spot. However, it has been shown that the wetting behavior of laser-structured stainless steel surfaces depends not only on the topography, but it is the complex effect of surface morphology and surface chemistry: initially hydrophilic behavior of hierarchically structured stainless steel surfaces turn out to hydrophobic, even to super hydrophobic, after long ripening.

In comparison with the LIPSS-covered stainless steel surfaces, the laser-induced micro-/nanopatterns on silicon result in the enhanced hydrophilicity of the structured area. The hydrophilic properties are nonlinearly increasing with the laser-induced surface roughness. Moreover, the surface morphology/roughness seems to be a main control parameter for the wetting properties of laser-structured silicon targets. Considering the generation of extended surfaces covered by LIPSS that can be easily prepared by 2D scanning of the target, one should take into account additionally to the microscale LIPSS-modifications, macroscopic rough features in the form of laser traces/scan lines that can also influence a wettability on the structured surface.