## 3 Results and Discussion
### Wettability Analysis on the Laser-Treated Surfaces
The roughness of the surface structures could have played a role where wettability is enhanced by roughness as modelled by the Wenzel equation as given by (5) [67]:  
\[\cos\theta_{\text{W}}=\text{r}\frac{\gamma_{\text{SV}}-\gamma_{\text{SL}}}{ \gamma_{\text{LV}}}=\text{r}\cos\theta \tag{5}\]  
where \(\theta_{\text{W}}\) is the apparent contact angle of a rough homogeneous surface, r is the roughness factor (r \(\geq\) 1, where r = 1 for a perfectly planar surface and r > 1 for a rough surface), \(\gamma_{\text{SV}}\), \(\gamma_{\text{SL}}\), and \(\gamma_{\text{LV}}\) are the solid--vapour, solid-liquid, and liquid--vapour interfacial tensions, respectively [67, 68]. The Wenzel equation (since r > 1 as shown in (5)) shows that the hydrophilic surfaces become more hydrophilic when roughness increases [29]. It is assumed that water penetrated the valleys/grooves of the rough surface and completely wetted it. It was also noted during measurement after immersion that the wetting behaviour was a little anisotropic, and this shows that wettability was dependent on the morphological directions or surface chemistry. After immersion, the sample was stored in ambient conditions for ten days. Then, contact angle measurements were performed again. It was noted that CAs were increasing again in some regions and transitioning to hydrophobic as shown in column blue (CA 3) in Figure 8. As explained, the transition is due to changes in the surface chemistry as a result of the adsorption of the non-polar carbon material on the surfaces of the laser-structured regions and the roughness of the laser-structured surfaces as per the modified Cassie-Baxter model. Laser surface structuring alters the chemical composition, surface structure, and surface free energy (SFE) of the surface of a material. The SFE of the polished surface and the laser-structured surfaces of the CP Ti were determined by utilizing the Li-Neumann equation after measuring the contact angles of distilled water. The SFE of the polished CP Ti surface was 44.08 mJ/m\({}^{2}\). The values of the laser-structured surfaces obtained were plotted as shown in Figure S3 (Supplementary Information). From Figure S3, it can be noted that the SFE of the laser-structured surfaces before immersion was the lowest. Reduction of the SFE led to an increase in contact angles of the laser-structured surfaces of the CP Ti. The SFE of different regions of the laser-structured sample immediately after immersion is highest as seen from Figure S3. This increase is responsible for the highly hydrophilic behaviour of the laser-structured surfaces. The SFE at the surfaces determined ten days after immersion shows that the SFE is reducing, and the hydrophobicity is increasing. By considering the A1, B1, C1, D1, and E1 regions processed by different laser fluence but the same scanning velocity, hatching distance, and number of scanning passes, it can be noted from Figure S3 that, as the laser fluences increase, the values of the SFE reduces. By considering the A1, A2, A3, and A4 regions processed by the same laser fluence but different scanning velocities, hatching distance, and the number of scanning passes, it can be noted from Figure S3 that, as the scanning velocity and/or hatching distance increases, the values of the SFE increases. It is also noted that the plotted graph of the SFE is opposite to that of the plotted graph of the CA measurements. As the SFE increases, the CA reduces, and the opposite is true [28]. From the observations made, it can be stated that surface wettability is related to the changes to the surface chemistry and surface topography after laser structuring. The presence of rough surfaces after laser structuring enhances the hydrophobicity and the presence of hydrophobic carbon materials amplifies the attainment of the hydrophobicity. The changes in the transition of wettability show that laser structuring can be a promising tool to modify CP Ti surfaces to obtain desirable properties.