## 3 Results and Discussion
### Morphology of the Laser-Processed Surfaces
The ridges of the LIPSS are decorated by cloudy-like nanostructures and a few nanodroplet materials as seen in the Inset on B1 region in Figure 2 and can be considered nanostructured-covered LIPSS (NC-LIPSS) [41]. At the region C1 region, the ablated region is covered by less smooth and shorter NC-LIPSS than that in the B1 region, as seen in Figure 2). The valleys of these NC-LIPSSs are much narrower than those in the B1 region and their average periodicity is 727 nm and hence considered as LSFL. The NC-LIPSSs are decorated by a few nanodroplet materials. Irregular micro-ripples appear to be starting to form on the surface along the scanning direction [25]. At regions D1 and E1, the NC-LIPSS are poorly formed, decorated by a few nanodroplet materials and irregular micro-ripples are formed as seen in Figure 2. The average periodicity of these NC-LIPSSs is 735 nm and 764 nm for regions D1 and E1, respectively, hence considered as LSFL. Micro-ripples in the E1 region are more visible than in region D1. In these five regions, the ablated zones show no signs of molten material, according to the SEM images in Figure 2, showing that equilibrium vaporisation (gentle ablation phase) was the ablation mechanism in these regions [42]. The ablation rate d\({}_{\text{a}}\) was low due to the low laser fluence employed, and it was dependent on the optical penetration depth, as shown in (2) [28; 43]:  
\[\text{d}_{\text{a}}=\alpha^{-1}\text{ln}\big{(}\frac{\text{F}_{\text{p}}}{ \text{F}_{\text{t,g}}}\big{)} \tag{2}\]  
where \(\alpha^{-1}\) is the optical penetration depth, F\({}_{\text{p}}\) is peak laser fluence and F\({}_{\text{t,g}}\) is the single-pulse ablation threshold fluence. From the SEM images in Figure 2, the LIPSS formed have different morphology with a different periodicity. This is because they were processed by different laser fluences which depend on the pulse energy of the laser pulse. Laser fluence increases with pulse energy, according to (1). As a result, more energy is deposited to the substrate, causing different LIPSSs with different morphologies to form. In general, HSFLs are formed at low fluence regimes, whereas LSFLs become more noticeable at higher fluence levels. Additionally, the periodicity of the LIPSS increases as the laser fluence increases. Additionally, it was observed that, as laser fluence rises, LIPSSs become discontinuous and are decorated by nanodroplets. The SEM micrographs of the laser-structured surfaces at 100, 150, 200, and 400 mm/s scanning velocities in the regions indicated on each image are displayed in Figure 3.  
In these regions, there is the formation of LIPSSs in a perpendicular direction to the laser polarization. In Figure S1, it can be noted that the LIPSSs are formed over the whole surface of region A1, while in regions A2, A3, and A4, the surface is made up of the laser-structured areas covered with LIPSS and unstructured square areas in an alternating manner due to the hatching distance of 25 \(\upmu\)m, 40 \(\upmu\)m, and 50 \(\upmu\)m, respectively. The average periodicity of the LIPSSs was 476 nm, 456 nm, 449, and 439 nm for regions A1, A2, A3, and A4, respectively, hence considered as HSFLs [39]. It can be noted in Figure 3 that, as scanning velocity or hatching distance increases, the formation of threadlike nanostructures along the valleys of the LIPSS that are perpendicular to the LIPSS becomes more evident. This can be noted in the A4 region, where LIPSSs are decorated by threadlike nanostructures along the valleys. In the C1, C2, C3, and C4 regions in Figure 3, there is also the formation of LIPSSs. In Figure S1, C1 and C2 regions are covered by LIPSS while C3 and C4 regions have some unstructured areas while structured areas are covered by LIPSS. At the C1 region, LIPSS is poorly formed and is nanostructured, hence referred to as NC-LIPSS and irregular micro-ripples are seen. In the C2 and C3 regions, the LIPSSs formed are more visible than in the C1 region.