## 3 Results and Discussions
### Wettability Behavior
However, for the A_WS, the liquid fully penetrates in the surface cavities, forming a continuous water/substrate contact line resulting in a very high sliding angle (\(>\)90\({}^{\circ}\)) following the Wenzel state. On the other hand, A_NS, with a sliding angle of about SA \(\approx\) 25\({}^{\circ}\), evidenced an intermediate state between Wenzel and Cassie-Baxter. Therefore, the adhesion behavior depends on the quantity of air entrapped on the substrate and, thus, on the morphology of the surface. The self-assembled layers were grown by using toluene solution as an apolar reaction medium. Several solvents have been utilized in the literature, such as ethanol [41], toluene [25], and hexane [31], for self-assembled silane monolayers. However, only the polar solvents have been extensively used under the consideration that they dissolve the molecules well. Recently, [PERSON] [42] demonstrated experimentally that the dielectric constant of the solvent has a great role in producing the self-assembled monolayers because it measures the ability of dissolving or separating charged particles. Thus, not only the polar solvent but also the apolar solvent with an appropriate dielectric constant is able to align and concentrate the polar head-groups of the molecules on its surface. Then, when physical contact between the hydrophilic substrate and the apolar medium occurs, the aligned silane molecules spontaneously transfer onto the hydrophilic surface, forming a self-assembled monolayer that makes the dip-coating suitable for industrial application [31; 43; 41]. In our case, the interaction between the octadecyltrimethoxysilane and the toluene probably renders a situation where the S18 molecules are well dispersed and oriented. These molecules spontaneously transfer and align on the aluminum alloy hydrophilic surface, forming a monolayer with oriented hydrophobic organic alkyl chains on the topside of this layer, as shown in Figure 6. The formation of the silane layer induced the reduction in surface energy and, thus, the reduction in the affinity of the surface with the polar liquids [44]. However, the difference in contact angles between the three superhydrophobic surfaces suggest that the polarity of the solvent is not the only factor that influences the orientation of the amphiphilic  
Figure 6: Illustration of octadecyltrimethoxysilane (S18) molecules, with wavy lines representing the hydrocarbon chain and yellow circles representing the head groups dissolved in toluene. molecules but also the morphology of the surface. The silane functional long alkyl chains formed a monolayer on the aluminum surface thanks to the high reactivity of the Si-OH silanol groups with the Al-OH aluminum hydroxyl groups [45]. However, the silane monolayer structure, which strongly depends on the surface profile, cannot be optimized when the silanol groups interact only with limited surface sites. On the surface of A_W, the flower-like structure overlapped and connected petals that may limit the silane deposition on the cavities. Indeed, the reaction between the silanol groups (Si-OH) of the silane and the hydroxyl groups (Al-OH) of the aluminum surface is limited on these narrow cavities that may induce a partial saturation of the -OH sites, resulting in a high water adhesion. However, the silane is mainly deposited in the A_N surface cavities due to the large number of valleys in this structure, as shown by the skewness factor, inducing the reduction in the surface roughness. This limits the contribution of the surface hierarchical structure on the superhydrophobic behavior where an intermediate water adhesion (\(\approx\)26\({}^{\circ}\)) and quite low contact angle (164\({}^{\circ}\)), compared to the other surfaces, were observed.