*2.5. Measurement Protocol*

After degassing the working fluid, five experimental runs were performed for each combination of the sample and working fluid. During each experimental run, the heat flux was continuously increased at a rate of 2 kW m−<sup>2</sup> s −1 . The chosen methodology of a slow continuous increase in the heat flux can be considered as quasi-stationary, as shown by Može et al. [45]. The boiling curve was recorded until the CHF was reached or, when that was not possible, until a surface superheat was reached (~100 K, representing a safety limit of the experimental setup). When an experimental run was finished, cartridge heaters were turned off, and the sample was left to cool down on its own. The duration of each run was approximately 45 min, while all 5 runs were completed in a 5-hour period.

### **3. Results and Discussion 3. Results and Discussion**

#### *3.1. Boiling Heat Transfer with Water 3.1. Boiling Heat Transfer with Water*

To establish a baseline for experiments with nanofluids, the boiling performance of the bare copper surface and the laser textured surface was first evaluated utilizing twice-distilled water. The results are shown in Figure 3. To establish a baseline for experiments with nanofluids, the boiling performance of the bare copper surface and the laser textured surface was first evaluated utilizing twicedistilled water. The results are shown in Figure 3.

Može et al. [45]. The boiling curve was recorded until the CHF was reached or, when that was not possible, until a surface superheat was reached (~100 K, representing a safety limit of the experimental setup). When an experimental run was finished, cartridge heaters were turned off, and the sample was left to cool down on its own. The duration of each run was approximately 45 min, while all 5 runs were completed in a 5-hour period.

*Nanomaterials* **2022**, *12*, x FOR PEER REVIEW 9 of 22

**Figure 3.** Boiling curves (**a**) and heat transfer coefficients (**b**) on the reference surface; boiling curves (**c**) and heat transfer coefficients (**d**) on the laser-textured surface. **Figure 3.** Boiling curves (**a**) and heat transfer coefficients (**b**) on the reference surface; boiling curves (**c**) and heat transfer coefficients (**d**) on the laser-textured surface.

Figure 3a,c shows the boiling curves (i.e., heat flux as a function of surface superheat) recorded on the bare surface (REF) and the laser textured surface (LT), respectively. In both cases, the boiling curves were recorded until the incipience of the CHF. It is evident that the boiling curves on the REF surface are slightly shifted towards lower superheats after each experimental run. The same shifting of the boiling curve also occurs on the LT surface during the first two runs, but then boiling curves stabilize, with a slight reversal of the trend. This is also consistent with the wettability of the examined surfaces. The REF surface is transitioning from a hydrophilic state (before boiling) towards the hydrophobic state (after boiling), and the LT surface is transitioning from a super hydrophilic state to a hydrophobic state. This is in correlation with the previous research, where it was shown that the shift in the boiling curves ensues due to changes in surface morphology and Figure 3a,c shows the boiling curves (i.e., heat flux as a function of surface superheat) recorded on the bare surface (REF) and the laser textured surface (LT), respectively. In both cases, the boiling curves were recorded until the incipience of the CHF. It is evident that the boiling curves on the REF surface are slightly shifted towards lower superheats after each experimental run. The same shifting of the boiling curve also occurs on the LT surface during the first two runs, but then boiling curves stabilize, with a slight reversal of the trend. This is also consistent with the wettability of the examined surfaces. The REF surface is transitioning from a hydrophilic state (before boiling) towards the hydrophobic state (after boiling), and the LT surface is transitioning from a super hydrophilic state to a hydrophobic state. This is in correlation with the previous research, where it was shown that the shift in the boiling curves ensues due to changes in surface morphology and chemistry [49,53]. The HTC of the REF and LT surfaces at selected heat fluxes are shown in Figure 3b,d, respectively.

The highest HTC values were recorded at the point of CHF incipience, where the LT surface provides a significant improvement compared to the untreated reference surface. Secondary boiling effects were detected on the LT surface, leading to a significant decrease in wall superheat temperature near the CHF, which in turn leads to the remarkable en-

hancement of the HTC. The reduction in surface superheats is caused by the higher density of active nucleation sites at high heat flux, when boiling also starts to take place on the peaks of the surface morphology instead of just within the cavities [46,55]. the peaks of the surface morphology instead of just within the cavities [46,55]. SEM images of the laser-textured sample (LT) after exposure to the boiling of water are shown in Figure 4. It is evident that the microtopography remains the same, but

chemistry [49,53]. The HTC of the REF and LT surfaces at selected heat fluxes are shown

The highest HTC values were recorded at the point of CHF incipience, where the LT surface provides a significant improvement compared to the untreated reference surface. Secondary boiling effects were detected on the LT surface, leading to a significant decrease in wall superheat temperature near the CHF, which in turn leads to the remarkable enhancement of the HTC. The reduction in surface superheats is caused by the higher density of active nucleation sites at high heat flux, when boiling also starts to take place on

SEM images of the laser-textured sample (LT) after exposure to the boiling of water are shown in Figure 4. It is evident that the microtopography remains the same, but changes are present on the submicron scale, where the transition of oxide species from copper(II) oxide to copper(I) oxide is evident and perfectly matches previous observations [49,53]. In [49], the appearance of copper(I) oxide in the form of sub-micron sized cubes or (truncated) octahedra was shown to take place after CHF incipience as a result of low temperature annealing, resulting in a transition of needle-shaped copper(II) oxide in the reduced form. changes are present on the submicron scale, where the transition of oxide species from copper(II) oxide to copper(I) oxide is evident and perfectly matches previous observations [49,53]. In [49], the appearance of copper(I) oxide in the form of sub-micron sized cubes or (truncated) octahedra was shown to take place after CHF incipience as a result of low temperature annealing, resulting in a transition of needle-shaped copper(II) oxide in the reduced form.

*Nanomaterials* **2022**, *12*, x FOR PEER REVIEW 10 of 22

in Figure 3b,d, respectively.

**Figure 4.** SEM images of the laser-textured sample LT after exposure to the boiling of water*.* **Figure 4.** SEM images of the laser-textured sample LT after exposure to the boiling of water.

#### *3.2. The Effect of Concentration on Boiling of Nanofluid with Small and Large TiO2 3.2. The Effect of Concentration on Boiling of Nanofluid with Small and Large TiO<sup>2</sup> Nanoparticles*

*Nanoparticles*  The boiling of nanofluid with small (4–8 nm) TiO2 nanoparticles on the copper laser textured surface was evaluated at two different mass concentrations of 0.001 and 0.1 wt.%. The corresponding boiling curves and HTCs are shown in Figure 5a,b for 0.001 wt.% nanofluid, while the data for the 0.1 wt.% nanofluid is shown in Figure 5c,d. The boiling curves obtained using 0.001 wt.% nanofluid are very stable, while the curves for the 0.1 wt.% nanofluid are shifted towards higher superheat temperatures with each consecutive run. This shifting can be explained through a decrease in the active nucleation site density [33,35] due to the deposition of nanoparticles onto the surface. At 0.1 wt.%, many microcavities on the laser textured surface become filled with nanoparticles during the first experimental run, which leads to a decrease in active nucleation site density and an increase in surface superheat during the next boiling runs. With increasing the number of runs, the The boiling of nanofluid with small (4–8 nm) TiO<sup>2</sup> nanoparticles on the copper laser textured surface was evaluated at two different mass concentrations of 0.001 and 0.1 wt.%. The corresponding boiling curves and HTCs are shown in Figure 5a,b for 0.001 wt.% nanofluid, while the data for the 0.1 wt.% nanofluid is shown in Figure 5c,d. The boiling curves obtained using 0.001 wt.% nanofluid are very stable, while the curves for the 0.1 wt.% nanofluid are shifted towards higher superheat temperatures with each consecutive run. This shifting can be explained through a decrease in the active nucleation site density [33,35] due to the deposition of nanoparticles onto the surface. At 0.1 wt.%, many microcavities on the laser textured surface become filled with nanoparticles during the first experimental run, which leads to a decrease in active nucleation site density and an increase in surface superheat during the next boiling runs. With increasing the number of runs, the surface superheat increases to unacceptable levels for practical use and the boiling performance is deteriorated. The highest HTC value, recorded during the first run (approx. 1 h of boiling on the surface), was 79,4 kW m−<sup>2</sup> K−<sup>1</sup> near CHF incipience. After the 5th run, the HTC value was 83% lower at same heat flux. During the second boiling run, the high surface temperature prevented the CHF incipience from being recorded, as the setup was turned off due to safety reasons. The highest CHF of 1457 kW m−<sup>2</sup> was recorded at 0.1 wt.%, which

*Nanomaterials* **2022**, *12*, x FOR PEER REVIEW 11 of 22

represents an enhancement of 35% compared to the highest recorded CHF at 0.001 wt.% (1082 kWm−<sup>2</sup> ). The enhancement of CHF is attributed due to a reduction in static contact angle and an improvement in surface wettability. wt.%, which represents an enhancement of 35% compared to the highest recorded CHF at 0.001 wt.% (1082 kWm−2). The enhancement of CHF is attributed due to a reduction in static contact angle and an improvement in surface wettability.

surface superheat increases to unacceptable levels for practical use and the boiling performance is deteriorated. The highest HTC value, recorded during the first run (approx. 1 h of boiling on the surface), was 79,4 kW m−2 K−1 near CHF incipience. After the 5th run, the HTC value was 83% lower at same heat flux. During the second boiling run, the high surface temperature prevented the CHF incipience from being recorded, as the setup was turned off due to safety reasons. The highest CHF of 1457 kW m−2 was recorded at 0.1

**Figure 5.** Boiling of TiO2-water nanofluid with small size nanoparticles at 0.001 wt.%: boiling curves (**a**) and heat transfer coefficients (**b**). Boiling of TiO2-water nanofluid with small size nanoparticles at 0.1 wt.%: boiling curves (**c**) and heat transfer coefficients (**d**). **Figure 5.** Boiling of TiO<sup>2</sup> -water nanofluid with small size nanoparticles at 0.001 wt.%: boiling curves (**a**) and heat transfer coefficients (**b**). Boiling of TiO<sup>2</sup> -water nanofluid with small size nanoparticles at 0.1 wt.%: boiling curves (**c**) and heat transfer coefficients (**d**).

SEM images of the laser-textured sample after exposure to boiling of 0.1 wt.% nanofluid with smaller nanoparticles are shown in Figure 6. It is observable that a thick nanoparticle deposit has formed on the surface, obscuring the shape of the laser-induced microstructure below. However, the deposited layer is not homogeneous and uniform, as evident from the missing patches, under which the laser-textured copper surface is visible. This was confirmed through EDS analysis, which detected a mixture of titanium and oxygen stemming from the TiO2 deposits on most of the surface, while a lower percentage of these two elements was detected, along with a notable percentage of copper on the SEM images of the laser-textured sample after exposure to boiling of 0.1 wt.% nanofluid with smaller nanoparticles are shown in Figure 6. It is observable that a thick nanoparticle deposit has formed on the surface, obscuring the shape of the laser-induced microstructure below. However, the deposited layer is not homogeneous and uniform, as evident from the missing patches, under which the laser-textured copper surface is visible. This was confirmed through EDS analysis, which detected a mixture of titanium and oxygen stemming from the TiO<sup>2</sup> deposits on most of the surface, while a lower percentage of these two elements was detected, along with a notable percentage of copper on the flakedoff patch.

flaked-off patch. The boiling of nanofluid with larger nanoparticles (490 nm) was performed at 0.001 and 0.1 wt.%. Boiling curves at 0.001 and 0.1 wt.% are shown in Figure 7a,c, respectively, while the HTCs are shown in Figure 7b,d, respectively.

*Nanomaterials* **2022**, *12*, x FOR PEER REVIEW 12 of 22

**Figure 6.** SEM images of the laser-textured sample LT after exposure to boiling 0.1 wt.% nanofluid with small nanoparticles (i.e., S-0.1). **Figure 6.** SEM images of the laser-textured sample LT after exposure to boiling 0.1 wt.% nanofluid with small nanoparticles (i.e., S-0.1). The boiling of nanofluid with larger nanoparticles (490 nm) was performed at 0.001 and 0.1 wt.%. Boiling curves at 0.001 and 0.1 wt.% are shown in Figure 7a,c, respectively, while the HTCs are shown in Figure 7b,d, respectively.

**Figure 7.** Boiling of TiO2-water nanofluid with large size nanoparticles at 0.001 wt.%: boiling curves (**a**) and heat transfer coefficients (**b**). Boiling of TiO2-water nanofluid with large size nanoparticles **Figure 7.** Boiling of TiO2-water nanofluid with large size nanoparticles at 0.001 wt.%: boiling curves (**a**) and heat transfer coefficients (**b**). Boiling of TiO2-water nanofluid with large size nanoparticles at 0.1 wt.%: boiling curves (**c**) and heat transfer coefficients (**d**). **Figure 7.** Boiling of TiO<sup>2</sup> -water nanofluid with large size nanoparticles at 0.001 wt.%: boiling curves (**a**) and heat transfer coefficients (**b**). Boiling of TiO<sup>2</sup> -water nanofluid with large size nanoparticles at 0.1 wt.%: boiling curves (**c**) and heat transfer coefficients (**d**).

at 0.1 wt.%: boiling curves (**c**) and heat transfer coefficients (**d**).

At 0.001 wt.%, a significant shift in the boiling curves towards a lower surface superheat was observed, while at 0.1 wt.%, the boiling curves are shifted toward higher superheat values. The general observations match those made for the smaller nanoparticle size. However, it was observed that after prolonged boiling of highly concentrated nanofluid (0.1 wt.% for 4+ h), the boiling curves became very unstable (e.g., Figure 7c) due to the very thick deposited layer on the surface, which also flakes off locally. SEM images of the laser-textured sample after exposure to the boiling of 0.1 wt.% nanofluid with larger nanoparticles are shown in Figure 8. Here, the underlying laser-induced microstructure is evident, but the deposited layer is again heterogeneous. It is estimated that significant flaking of the deposited layer occurred during the boiling process and after removal of the sample due to the low adhesion strength. The EDS analysis confirmed a nearly perfect atomic ratio of oxygen to titanium (2:1) for the TiO<sup>2</sup> deposit. heat was observed, while at 0.1 wt.%, the boiling curves are shifted toward higher superheat values. The general observations match those made for the smaller nanoparticle size. However, it was observed that after prolonged boiling of highly concentrated nanofluid (0.1 wt.% for 4+ h), the boiling curves became very unstable (e.g., Figure 7c) due to the very thick deposited layer on the surface, which also flakes off locally. SEM images of the laser-textured sample after exposure to the boiling of 0.1 wt.% nanofluid with larger nanoparticles are shown in Figure 8. Here, the underlying laser-induced microstructure is evident, but the deposited layer is again heterogeneous. It is estimated that significant flaking of the deposited layer occurred during the boiling process and after removal of the sample due to the low adhesion strength. The EDS analysis confirmed a nearly perfect atomic ratio of oxygen to titanium (2:1) for the TiO2 deposit.

At 0.001 wt.%, a significant shift in the boiling curves towards a lower surface super-

*Nanomaterials* **2022**, *12*, x FOR PEER REVIEW 13 of 22

**Figure 8.** SEM images of the laser-textured sample LT after exposure to boiling 0.1 wt.% nanofluid with large nanoparticles (i.e., L-0.1). **Figure 8.** SEM images of the laser-textured sample LT after exposure to boiling 0.1 wt.% nanofluid with large nanoparticles (i.e., L-0.1).

Interestingly, the boiling performance increase detected with larger nanoparticles at 0.001 wt.% was not observed at the same concentration using the smaller nanoparticle size. It is likely that the small nanoparticles (two orders of magnitude smaller than the large nanoparticles used in this study) form a thin, compact deposited layer without additional nucleation sites (i.e., the HTC is not enhanced at comparable heat flux values), while the increased wettability is able to raise the CHF value. On the other hand, much larger nanoparticles (490 nm) form a more porous deposit, which offers additional cavities for bubble nucleation, thus enhancing the HTC, but the overall effect on increasing the surface wettability (and with that, the CHF value) is lower. A comparison of the SEM images taken after boiling of 0.001 wt.% nanofluid, with both small and large nanoparticle size, is shown in Figure 9. Only a thin layer of small nanoparticles remains on the laserinduced structured after boiling the 0.001 wt.% nanofluid and is only evident at higher magnifications. On the contrary, larger nanoparticles deposited at the same concentration Interestingly, the boiling performance increase detected with larger nanoparticles at 0.001 wt.% was not observed at the same concentration using the smaller nanoparticle size. It is likely that the small nanoparticles (two orders of magnitude smaller than the large nanoparticles used in this study) form a thin, compact deposited layer without additional nucleation sites (i.e., the HTC is not enhanced at comparable heat flux values), while the increased wettability is able to raise the CHF value. On the other hand, much larger nanoparticles (490 nm) form a more porous deposit, which offers additional cavities for bubble nucleation, thus enhancing the HTC, but the overall effect on increasing the surface wettability (and with that, the CHF value) is lower. A comparison of the SEM images taken after boiling of 0.001 wt.% nanofluid, with both small and large nanoparticle size, is shown in Figure 9. Only a thin layer of small nanoparticles remains on the laser-induced structured after boiling the 0.001 wt.% nanofluid and is only evident at higher magnifications. On the contrary, larger nanoparticles deposited at the same concentration are more clearly visible and seem to fill a part of the laser-induced surface channels.

are more clearly visible and seem to fill a part of the laser-induced surface channels.

*Nanomaterials* **2022**, *12*, x FOR PEER REVIEW 14 of 22

**Figure 9.** (**a**) SEM images of the laser-textured sample LT after exposure to boiling 0.001 wt.% nanofluid with small nanoparticles (i.e., S-0.001) and (**b**) after exposure to boiling 0.001 wt.% nanofluid with large nanoparticles (i.e., L-0.001). **Figure 9.** (**a**) SEM images of the laser-textured sample LT after exposure to boiling 0.001 wt.% nanofluid with small nanoparticles (i.e., S-0.001) and (**b**) after exposure to boiling 0.001 wt.% nanofluid with large nanoparticles (i.e., L-0.001). nanofluid with large nanoparticles (i.e., L-0.001). The HTC is enhanced by 49% after 5 h of boiling at the concentration of 0.001 wt.%, and both the CHF and the HTC enhancements are stable. The CHF enhancement is more

The HTC is enhanced by 49% after 5 h of boiling at the concentration of 0.001 wt.%, and both the CHF and the HTC enhancements are stable. The CHF enhancement is more pronounced with larger nanoparticles at 0.1 wt.%, with the highest value reaching 2021 kW m−2, which represents an 86% enhancement over the highest CHF recorded for the 0.001 wt.% Enhancement of the CHF was recorded for all performed runs, which was attributed to better wettability characteristics of the surface exposed to the boiling of the concentrated nanofluid. Increased wettability compared to the results for the boiling of pure water on the LT surface was confirmed by the measurement of the static contact angle after boiling. The contact angle after boiling with the 0.1 wt.% nanofluid was 13.5°, The HTC is enhanced by 49% after 5 h of boiling at the concentration of 0.001 wt.%, and both the CHF and the HTC enhancements are stable. The CHF enhancement is more pronounced with larger nanoparticles at 0.1 wt.%, with the highest value reaching 2021 kW m−<sup>2</sup> , which represents an 86% enhancement over the highest CHF recorded for the 0.001 wt.% Enhancement of the CHF was recorded for all performed runs, which was attributed to better wettability characteristics of the surface exposed to the boiling of the concentrated nanofluid. Increased wettability compared to the results for the boiling of pure water on the LT surface was confirmed by the measurement of the static contact angle after boiling. The contact angle after boiling with the 0.1 wt.% nanofluid was 13.5◦ , and 29.8◦ for the 0.001 wt.% nanofluid. pronounced with larger nanoparticles at 0.1 wt.%, with the highest value reaching 2021 kW m−2, which represents an 86% enhancement over the highest CHF recorded for the 0.001 wt.% Enhancement of the CHF was recorded for all performed runs, which was attributed to better wettability characteristics of the surface exposed to the boiling of the concentrated nanofluid. Increased wettability compared to the results for the boiling of pure water on the LT surface was confirmed by the measurement of the static contact angle after boiling. The contact angle after boiling with the 0.1 wt.% nanofluid was 13.5°, and 29.8° for the 0.001 wt.% nanofluid. A further investigation was performed by mixing 0.05 wt.% of small and 0.05 wt.%

and 29.8° for the 0.001 wt.% nanofluid. A further investigation was performed by mixing 0.05 wt.% of small and 0.05 wt.% A further investigation was performed by mixing 0.05 wt.% of small and 0.05 wt.% of large size nanoparticles, respectively. The boiling curves and HTCs are shown in Figure 10. of large size nanoparticles, respectively. The boiling curves and HTCs are shown in Figure 10.

**Figure 10.** Boiling of TiO2-water nanofluid with 0.05 wt.% of small size and 0.05 wt.% large size nanoparticles, respectively: boiling curves (**a**) and heat transfer coefficients (**b**). **Figure 10.** Boiling of TiO<sup>2</sup> -water nanofluid with 0.05 wt.% of small size and 0.05 wt.% large size nanoparticles, respectively: boiling curves (**a**) and heat transfer coefficients (**b**).

**Figure 10.** Boiling of TiO2-water nanofluid with 0.05 wt.% of small size and 0.05 wt.% large size nanoparticles, respectively: boiling curves (**a**) and heat transfer coefficients (**b**). The boiling curves are again shifted toward a higher surface superheat, which matches the behavior when 0.1 wt.% nanofluid was boiled with either small or large nanoparticles. After the first experimental run, the surface superheat increased significantly, The boiling curves are again shifted toward a higher surface superheat, which matches the behavior when 0.1 wt.% nanofluid was boiled with either small or large nanoparticles. After the first experimental run, the surface superheat increased significantly, which was likely caused by the presence of small size nanoparticles that filled the microcavities and surface channels, thus decreasing the active nucleation site density. As the latter decreases, so does the HTC. CHF was again not recorded after the first hour of The boiling curves are again shifted toward a higher surface superheat, which matches the behavior when 0.1 wt.% nanofluid was boiled with either small or large nanoparticles. After the first experimental run, the surface superheat increased significantly, which was likely caused by the presence of small size nanoparticles that filled the microcavities and surface channels, thus decreasing the active nucleation site density. As the latter decreases,

which was likely caused by the presence of small size nanoparticles that filled the micro-

so does the HTC. CHF was again not recorded after the first hour of boiling due to very high surface temperatures and the danger of damage to the setup, but the achieved maximal heat flux values were notably lower than for the 0.1 wt.% nanofluid with large particles, but higher than for the same concentration of nanofluid with small nanoparticles. boiling due to very high surface temperatures and the danger of damage to the setup, but the achieved maximal heat flux values were notably lower than for the 0.1 wt.% nanofluid with large particles, but higher than for the same concentration of nanofluid with small nanoparticles. boiling due to very high surface temperatures and the danger of damage to the setup, but the achieved maximal heat flux values were notably lower than for the 0.1 wt.% nanofluid with large particles, but higher than for the same concentration of nanofluid with small nanoparticles. SEM images of the surface after exposure to the boiling of nanofluid with mixed par-

SEM images of the surface after exposure to the boiling of nanofluid with mixed particle size (i.e., MIX-0.05) are shown in Figure 11. SEM images of the surface after exposure to the boiling of nanofluid with mixed particle size (i.e., MIX-0.05) are shown in Figure 11. ticle size (i.e., MIX-0.05) are shown in Figure 11.

*Nanomaterials* **2022**, *12*, x FOR PEER REVIEW 15 of 22

*Nanomaterials* **2022**, *12*, x FOR PEER REVIEW 15 of 22

**Figure 11.** SEM images of the laser-textured sample LT after exposure to boiling of nanofluid with a mixture of 0.05 wt.% of small nanoparticles and 0.05 wt.% of large nanoparticles (i.e., MIX-0.05). **Figure 11.** SEM images of the laser-textured sample LT after exposure to boiling of nanofluid with a mixture of 0.05 wt.% of small nanoparticles and 0.05 wt.% of large nanoparticles (i.e., MIX-0.05). a mixture of 0.05 wt.% of small nanoparticles and 0.05 wt.% of large nanoparticles (i.e., MIX-0.05). It is noticeable that the surface is rather uniformly covered with a deposited layer,

It is noticeable that the surface is rather uniformly covered with a deposited layer, and the laser-induced structures are not visible. However, the deposit is again inhomoge-It is noticeable that the surface is rather uniformly covered with a deposited layer, and the laser-induced structures are not visible. However, the deposit is again inhomogeneous. and the laser-induced structures are not visible. However, the deposit is again inhomogeneous.

#### neous. *3.3. Contact Angle Measurments 3.3. Contact Angle Measurments*

*3.3. Contact Angle Measurments*  The static contact angle of water was measured on each surface to determine its wettability and help explain the observed boiling behaviors, especially in terms of the effect of nanoparticle deposition onto the surface during the experiments. The recorded values, The static contact angle of water was measured on each surface to determine its wettability and help explain the observed boiling behaviors, especially in terms of the effect of nanoparticle deposition onto the surface during the experiments. The recorded values, obtained through measurements at different points in time, as previously explained in Section 2.1, are depicted and compared in Figure 12. The static contact angle of water was measured on each surface to determine its wettability and help explain the observed boiling behaviors, especially in terms of the effect of nanoparticle deposition onto the surface during the experiments. The recorded values, obtained through measurements at different points in time, as previously explained in Section 2.1, are depicted and compared in Figure 12.

**Figure 12.** Static contact angle measurements on all surfaces before boiling, after boiling, and after storage. **Figure 12.** Static contact angle measurements on all surfaces before boiling, after boiling, and after storage.

**Figure 12.** Static contact angle measurements on all surfaces before boiling, after boiling, and after storage. Laser textured surfaces were initially superhydrophilic immediately after processing, Laser textured surfaces were initially superhydrophilic immediately after processing, and the contact angle of the bare copper surface was 82° before the boiling experiments. After the boiling experiments with water on the laser textured surface and the reference surface, the contact angles increased, as was shown in previous studies [51]. After the Laser textured surfaces were initially superhydrophilic immediately after processing, and the contact angle of the bare copper surface was 82◦ before the boiling experiments. After the boiling experiments with water on the laser textured surface and the reference surface, the contact angles increased, as was shown in previous studies [51]. After the boiling experiments, the wettability of the surfaces changed due to the deposition of

and the contact angle of the bare copper surface was 82° before the boiling experiments.

nanoparticles. The increase in the contact angles of surfaces is slightly higher after boiling with low-concentration nanofluids than for the contact angles measured after boiling with high-concentration nanofluids. This could be due to the higher deposition rate at higher concentrations, which causes a thicker layer of deposited TiO<sup>2</sup> nanoparticles on the surface. TiO<sup>2</sup> nanoparticles are (super)hydrophilic, and a thick layer of them on the surface leads to the surface staying in a hydrophilic state. At lower concentrations and for the small-sized nanoparticles, the deposited layer on the surface is thinner. On the other hand, the larger nanoparticles form more porous deposits, resulting in lower contact angles compared to those for the smaller nanoparticles used in this study.

Additionally, all surfaces were exposed to ambient conditions for several days after the boiling experiments were finished and then cleaned with a UV/ozone cleaner to remove volatile organic compounds (VOC) and other carbon-based impurities. Contact angle measurements were performed before and immediately after cleaning with the UV/ozone cleaner. Afterward, the surfaces were again exposed to ambient air for three days, and the contact angles were measured again. The results of the measurements show the contact angles of all surfaces gradually increased over time, which was observed in many previous studies [56,57], and this was confirmed by previous research [58,59]. The change in the wettability is attributed to the adsorption of hydrophobic contaminants from the air. The adsorption of polymeric organosilicon compounds was recently found to be important as the most probable reason for the wettability transition of such samples in the laboratory environment [60]. The air-exposed surfaces were then cleaned with a UV/ozone cleaner for 30 min. This removes most typical contaminants, such as oils and greases, and other contamination adsorbed during prolonged exposure to air [61]. The wettability of all surfaces increased dramatically after the UV/ozone cleaning, but after exposing them to the ambient air conditions for a further 3 days, the wettability again decreased.
