*4.4. Wettability of the Nanoparticles*

The nanoparticle wettability can affect the deposited nanoparticle layer morphology. On the one hand, as it can be seen in Figure 10, the suspended nanoparticles with moderate hydrophilicity are adsorbed to the liquid and vapor phases interface avoiding the fluid drainage among the vapor bubbles. This phenomenon hinders the coalescence concern of the bubbles and decreases their diameter at departure, resulting in a nucleate pool-boiling HTC and CHF amelioration. In contrast, the nanoparticles with high hydrophilicity will not adsorb onto the heating surface and, consequently, the bubble coalescence remains unchanged. As illustrated in Figure 10, the wettability of the nanoparticles influences the deposition layer morphology in which the highly hydrophilic layers are relatively smooth having a uniform nanoparticle dispersion onto the heat transfer interface, while the deposited layers produced with moderate to medium hydrophilicity possess higher roughness and more irregularities.

**Figure 10.** Nanoparticle hydrophilicity effect on the deposited layer morphology. **Figure 10.** Nanoparticle hydrophilicity effect on the deposited layer morphology.

### *4.5. Base Fluid 4.5. Base Fluid*

The differences in the base fluid nature lead to great differences in the thermophysical characteristics of the nanofluid. For instance, if the viscosity of the base fluid is considerable, the viscosity of the nanofluid results higher. Moreover, the disturbance due to the nucleation and departure of the vapor bubbles is the main rationale behind the intense heat transfer. Moreover, the varying viscosity of the nanofluid is critical for the impact on the boiling bubbles because of the differences in the base fluid. It can be found in the published scientific articles that the inclusion of nanoparticles into a base fluid with relatively high viscosity strongly improves the boiling heat transfer. The main features arising from this fact are that the nanoparticles are suspended in a high viscosity fluid, which assures good stability over time. Moreover, the addition of nanoparticles has a negligible impact on the viscosity of the nanofluid that results in only a small alteration in the bubble growth and departure. In summary, the stability of the nanofluid is improved, the nanoparticle deposition is decreased, the number of gasification points together with The differences in the base fluid nature lead to great differences in the thermophysical characteristics of the nanofluid. For instance, if the viscosity of the base fluid is considerable, the viscosity of the nanofluid results higher. Moreover, the disturbance due to the nucleation and departure of the vapor bubbles is the main rationale behind the intense heat transfer. Moreover, the varying viscosity of the nanofluid is critical for the impact on the boiling bubbles because of the differences in the base fluid. It can be found in the published scientific articles that the inclusion of nanoparticles into a base fluid with relatively high viscosity strongly improves the boiling heat transfer. The main features arising from this fact are that the nanoparticles are suspended in a high viscosity fluid, which assures good stability over time. Moreover, the addition of nanoparticles has a negligible impact on the viscosity of the nanofluid that results in only a small alteration in the bubble growth and departure. In summary, the stability of the nanofluid is improved, the nanoparticle deposition is decreased, the number of gasification points together with the surface wettability is enhanced, and, hence, the pool-boiling heat transfer is improved.

the surface wettability is enhanced, and, hence, the pool-boiling heat transfer is im-

#### proved. *4.6. Surfactants*

*4.6. Surfactants*  The effect of the addition of a surfactant and clustering on the thermal conductivity of titanium oxide and alumina dispersed in water was experimentally studied by the authors [98]. The obtained results showed that the cluster size increased with increasing concentration of nanoparticles, while the thermal conductivity of the nanofluids decreased with increasing cluster size. The CTAB surfactant was effective in improving the dispersion of the nanoparticles and stability of the nanofluids. The added surfactant also contributed to the thermal conductivity of the nanofluids' improvement. Although the surfactant proved to be a benefit in the stability and thermal conductivity, the effect of the nanoparticle clustering on the same features was found to be negative. The authors Zhou et al. [99] studied the influence of the nanoparticle deposition and interfacial characteristics on the pool boiling using nanofluids and n-butanol as the surfactant over a platinum microwire. The researchers stated that the inclusion of n-butanol altered the liquid/vapor interface properties and it intensified the nanoparticle deposition at low heat fluxes. The obtained results confirmed that the CHF of the nanofluid became en-The effect of the addition of a surfactant and clustering on the thermal conductivity of titanium oxide and alumina dispersed in water was experimentally studied by the authors [98]. The obtained results showed that the cluster size increased with increasing concentration of nanoparticles, while the thermal conductivity of the nanofluids decreased with increasing cluster size. The CTAB surfactant was effective in improving the dispersion of the nanoparticles and stability of the nanofluids. The added surfactant also contributed to the thermal conductivity of the nanofluids' improvement. Although the surfactant proved to be a benefit in the stability and thermal conductivity, the effect of the nanoparticle clustering on the same features was found to be negative. The authors Zhou et al. [99] studied the influence of the nanoparticle deposition and interfacial characteristics on the pool boiling using nanofluids and n-butanol as the surfactant over a platinum microwire. The researchers stated that the inclusion of n-butanol altered the liquid/vapor interface properties and it intensified the nanoparticle deposition at low heat fluxes. The obtained results confirmed that the CHF of the nanofluid became enhanced when the n-butanol was added to the nanofluid. The experimental data showed that the hindered bubble growth and increased nanoparticle agglomeration in the fluid wedge region were the

reasons behind the degradation of the heat transport deterioration when the amount of the added surfactant was increased. Additionally, it was already proven that the impact of the surfactant on the heat transfer and CHF was greater than the unstable settlement of the nanoparticles onto the heating surface. The deposition profiles were significantly influenced by the n-butanol addition since the low concentration of the self-rewetting fluid produced higher surface tension and, consequently, attracted a greater amount of nanoparticles onto the microwire surface. However, the unstable settlement of the nanoparticles generated a less-uniform deposit as compared with that of the nanofluid alone, which is likely due to the greater disturbance derived from the surfactant addition. In conclusion, when the n-butanol was added to the nanofluid, this surfactant promoted the clustering of the nanoparticles at low heat fluxes. The nanoparticle deposition pattern indicated that this one modified the surface roughness and, thus, accelerated the emergence of the sweeping mechanism of the vapor bubbles. Moreover, it was found that the nanoparticle deposition is more intense with the further addition of a surfactant. The addition of the surfactant will increase the CHF and such an increase is higher for high-nanoparticle concentrations. Moreover, the CHF enhancement ratio decreased when more surfactant was added. In addition, the overall mechanism of the heat transfer enhancement in which the Marangoni flows imposed by the surfactant pushes the nanoparticles to the heating surface to produce the deposition layer and conducts the nanoparticles to penetrate the confined wedge of the vapor bubbles. Furthermore, in the work conducted by the authors Jung et al. [100], the addition of nitric acid as an ionic surfactant promoted the formation of self-assembled layers and structures of nanoparticles on the heat transfer surface. This alternative method builds a more uniform and smoother surface structure, reducing the CHF improvement.
