## 3 Results and Discussion
### Wettability Analysis on the Laser-Treated Surfaces
It was also noted during measurement before immersion that the wetting behaviour was isotropic, and this shows that wettability was independent of the morphological or ablations directions. wetting transition of the laser-structured CP Ti surfaces from a hydrophilic to a hydrophobic state is due to surface morphology/topography and surface chemistry [5, 29, 50, 60]. Topography can be described by the shape profiles and roughness of a surface and the laser-processed surfaces have increased surface roughness caused by laser ablation [5], as noted in Table 2. The wettability transition due to roughness can be explained by the Cassie-Baxter theory [17, 61, 62] through a modified Cassie-Baxter equation, as shown in (3) [29, 63, 64, 65]:  
\[\cos\theta_{\text{CB}}=\text{rf}_{\text{SL}}\cos\theta-\text{f}_{\text{LV}}= \text{r}(1-\text{f}_{\text{LV}})\cos\theta-\text{f}_{\text{LV}}=\text{r}\cos \theta-\text{rf}_{\text{LV}}\cos\theta-\text{f}_{\text{LV}} \tag{3}\]  
where \(\theta_{\text{CB}}\) is the apparent contact angle of a rough surface, r is the roughness factor defined as a ratio of the actual surface area to the projected area, \(\text{f}_{\text{SL}}\) is the total area of the solid surface under the droplet per unit projected area under the droplet with \(\theta\) as the contact angle on the surface of material 1 (\(\theta\) is Young's contact angle of an ideal smooth homogeneous surface), and \(\text{f}_{\text{LV}}\) is the total area of air under the droplet per unit projected area under the droplet. The Cassie-Baxter model is based on the presence of air pockets on the roughened surface which will prevent water from wetting the entire surface [60]. According to (3), the contact angle of a hydrophilic surface can also increase to become a hydrophobic state as \(\text{f}_{\text{LV}}\) increases and meets the specific requirement shown in (4) [29, 63]:  
\[\text{f}_{\text{LV}}\geq\frac{\text{r}\cos\theta}{\text{r}\cos\theta+1}\text{ for }\theta<90^{\circ} \tag{4}\]  
Chemical compositions on a surface determine the surface free energy (SFE) and by lowering the SFE, hydrophobicity is achieved [29]. The storage period of the samples in ambient conditions plays an important in changing the surface chemistry of the laser-structured regions. After two months under ambient conditions, the hydrophobic carbonaceous materials (e.g., -CH\({}_{3}\)) were adsorbed on the laser-irradiated surfaces [9, 50, 53]. These carbonaceous materials are known to facilitate the achievement of hydrophobicity by the laser-irradiated surfaces over a period of time [5, 17, 61]. After the immersion of the laser-structured sample in seawater, the wetting behaviour of all regions changed from being a hydrophobic to a hydrophilic state, as shown in column red (CA 2) in Figure 8. This could be explained by a chemical state at the surface of the processed regions, where some Ti oxides (e.g., Ti\({}_{2}\)O\({}_{3}\)) present are known to have higher polarity and hence hydrophilic [5, 33]. The photo-responsive nature of titanium oxide creates highly unstable oxygen vacancies which attract water and provide the functional -OH polar groups [66] which are known to have strong wettability [51, 60]. The carbonaceous elements, which are hydrophobic, that had adsorbed on the surfaces initially (Table 3) might have desorbed, leaving the material to be hydrophilic. This can be confirmed from Table 3, where the C element is zero immediately after immersion.