*3.5. Contact Angle Measurement*

Examining the wettability of the uncoated SS 316L sample (Figure 9a), with 20 ◦C of DIW of pH 7, showed that the surface had an average contact angle (ACA) of 131.7◦, which illustrates a hydrophobic behaviour. The high ACA value is believed to be linked to the substrate Cassie–Baxter state via its surface roughness, as reported by other authors [30,48]. In general, there are two common texture states that explain the relationship between surface wettability and roughness, which are the Cassie-Baxter [49] and Wenzel [50] states. In the Cassie-Baxter state, the surface pores and valleys tend to trap the air, which leads to a reduction in the degree of liquid-surface interaction. On the other hand, in the Wenzel state, the liquid fully occupies the pores, thus improving the surface wettability. Applying DIW's of pH 4 and 9 have led the surface ACA to reduce to 124.9◦ and 117.4◦, respectively, without additional surface modifications. Furthermore, it was found that raising the temperature of the as-prepared DIW's tends to weaken the hydrophobic nature of the uncoated surface, as demonstrated by the obtained data in Figure 9b–d. The grey dashed line in the plots (Figure 9b-d) illustrates the transition point between the surface hydrophobic (top) and hydrophilic (bottom) regions. Moreover, the deposition thickness was seen to be inversely related to the CA, where increasing the film thickness caused the ACA to reduce. For example, when examining DIW, of 20 ◦C and pH of 4, the ACA of the uncoated, 50 nm, 100 nm, and 150 nm film gave angles of 124.9◦, 119.5◦, 116.7◦, and 110.9◦, correspondingly. This can be attributed to the reduction in surface micro-roughness and air pockets formation at the interface between the substrate and liquid as a result of the deposited particles occupying the surface structure, thus enhancing the substrate surface energy to attract the liquid towards the surface (i.e., reducing the CA) [49,51]. In addition, the level of decrease in CA is seen to correspond to the liquid temperature, pH value, and fabricated film thickness due to their ability to modify the surface mode from a Cassie–Baxter state to a Wenzel state, and vice versa. For instance, the ACA of DIW of pH 4, 7, and 9, at 50 ◦C, showed a decrease from 110.1◦, 114.8◦, and 112.3◦ (uncoated) to 93.1◦, 95.5◦, and 97.0◦ (150 nm film), respectively. It was also possible to change the substrate surface wettability nature from hydrophobic to hydrophilic by manipulating the three aforementioned parameters, as shown in Figure 9b when investigating the 150 nm coated substrate with DIW of 60 ◦C and pH 4. Such findings are very attractive for heat transfer applications, as lowering the CA can enhance the heat transfer efficiency of SS by providing larger contact area between the liquid and the surface. On the other hand, the fluctuation in the data trend of DIW of pH 4 and 9 across the examined temperature range is believed to be caused by the free ions hosted by the liquid. Since both fluids are considered ionically unstable, attempting to change the surface functional group of the substrate from hydrophobic toward hydrophilic by inducing the transformation in surface charge, in what is known as the Hofmeister series reversal effect, could be the reason behind this kind of behaviour in the data trend [52]. In addition to the previously mentioned reason, there is a possibility that traces of hydrocarbon contamination from the surrounding atmosphere on the substrate surface are strongly interacting with the two aforementioned liquids free ions, since the DIW of pH 7 did not show such fluctuation in its data trend. Usually, such contamination is unavoidable in open atmospheric experiments, and would have some sort of influence on all the conducted measurements [53,54]. Supplementary Table S1 summarises the testing parameters and obtained contact angles of the characterised samples.

**Figure 9.** Effect of DIW temperature and pH value on the wettability behaviour of SS 316L surface, where (**a**) illustrates the contact angles between the 20 ◦C DIW's, of pH 4, 7, and 9, and the uncoated SS 316L substrate surface, and (**b**–**d**) demonstrates the average contact angle measurements of the uncoated and coated samples using the DIW's, at 20–60 ◦C, as the testing fluids.
