PSI - Issue 74

Kipkurui Ronoh et al. / Procedia Structural Integrity 74 (2025) 77–84 Kipkurui Ronoh / Stru ctural Integrity Procedia 00 (202 5 ) 000 – 000 the apparent contact angle on a rough surface, increases with the roughness factor, as shown by (2): cos = − = cos (2) where r is the roughness factor, , and are solid-vapour, solid-liquid and liquid-vapour interfacial tensions respectively and is the intrinsic contact angle (Wenzel, 1936). The roughness factor r is defined as the ratio of the rough surface’s real surface area to the geometric projected area and ≥ 1 , where r = 1 for a perfectly planar surface and r > 1 for a rough surface (M. A. Khan et al., 2024). Fig. 5 shows that contact angles increase with increasing laser fluence, indicating enhanced hydrophobicity (Biffi et al., 2023; Wu et al., 2009). Although the increase is modest, it correlates with changes in the morphology of the LIPSS which becomes less smooth and shorter at higher fluences. Supporting data from Table 3 and Table 5 confirm that increased laser fluence leads to higher Sa, which directly contributes to the observed increase in hydrophobicity of the laser-ablated samples. An exception to the general trend was observed in 699 XA, where the surface ablated with a laser fluence of 4 J/cm 2 exhibited a higher contact angle than at 8 J/cm 2 . This deviation may be attributed to the greater Sa at 4 J/cm² than at 8 J/cm² . Fig. 6 shows that increasing the hatching distance during laser ablation results in reduced contact angles, indicating decreased hydrophobicity. The trend is supported by the surface topographical images shown in Fig. 3, which reveal that different hatching distances lead to varying surface textures. These topographical changes affect the solid-liquid contact area under a droplet. As the proportion of flat, non-ablated surface increases with larger hatching distances, the laser-structured surface area decreases, reducing hydrophobicity. Conversely, smaller hatching distances increase surface roughness, promoting partial wetting and greater hydrophobicity (Chun et al., 2014; Raja et al., 2020). Hydrophobicity is also influenced by the surface chemistry of the alloys. Immediately after laser ablation, the surfaces are typically hydrophilic but become hydrophobic over time due to adsorption of airborne hydrocarbon contaminations (Lin et al., 2025). The contact angle measurements in this study were taken after one-month post ablation, when stable hydrophobicity had likely developed. The presence of most non-polar hydrophobic functional groups like C–C and C–H in the carbonaceous layer repels polar water molecules, increases contact angle, and reduces surface free energy by coating the underlying hydrophilic oxides. The carbon layer may coat the rough micro/nanostructures, and this may increase the contact angle through the Cassie-Baxter effects, where the hydrophobic carbon layer will act as air pockets under the droplet in the usual Cassie-Baxter superhydrophobic state. In summary, the resulting hydrophobicity is attributed to combined effects of rough surface structures and the adsorbed carbonaceous materials (Lin et al., 2025). As shown in Table 5, with increasing laser fluence increases, carbon content decreases while contact angle increases. This relationship shows that both surface roughness and carbon content have a significant influence on hydrophobicity.

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Table 5: Effect of laser fluence on carbon contents, contact angles and surface roughness on the surfaces of the alloys 699 XA Kanthal® AF

MONEL® alloy 400

Laser Fluence (J/cm²) Polished

C (at. %)

Surface Roughness, Sa, (μm)

Contact Angle, θ (°)

C (at. %)

Surface Roughness, Sa, (μm)

Contact Angle, θ (°)

C (at. %) Surface

Contact Angle, θ (°)

Roughness, Sa, (μm)

70.62 48.50 35.10 38.91 28.07

0.03 0.07 0.19 0.12 0.19

93 ± 1.23 136 ± 1.17 141 ± 0.75 138 ± 0.42 141 ± 0.64

56.45 44.11 31.99 24.21 21.93

0.02 0.05 0.14 0.30 0.42

91 ± 1.38 137 ± 0.74 140 ± 0.97 144 ± 0.66 146 ± 0.71

69.88 44.12 30.93 25.88 26.19

0.02 0.07 0.23 0.19 0.25

90 ± 1.92 141 ± 0.63 142 ± 0.88 144 ± 1.36 144 ± 0.78

1 4 8

10

4. Conclusion Ultrafast laser ablation was used to modify the wetting properties of alloys, followed by wettability testing. The study found that increasing laser fluence produced distinct LIPSS, increased surface roughness, and decreased surface carbon content. While polished samples showed low hydrophobicity (contact angles of 90 – 93°), laser-ablated