2.3.8. Peripheral Regions of the Nucleation Sites and Deposition Points

During the initial part of the growing of the vapor bubbles in the nucleate boiling regime, the viscous effect can be large enough as compared withthe surface tension to hinder the working fluid motion and trap a very thin liquid layer underneath the vapor bubbles beneaththe growing bubbles. This layer is usually designated by a microlayer and has a thickness smaller than ten micrometers and a length up to one millimeter. Its lifetime is only a few milliseconds but it contributes to the major part of the heat transfer enhancement between the heating surface and the bubbles. For longervapor bubble growing times, the formation of a thin liquid layer designated by a macrolayer occurs, wherein the meniscus of the bubbles changes curvature suddenly. In terms of nanoparticle deposition, the particles deposited in the microlayer and macrolayer are usually nano-scaled particles [68]. Another aspect of the boiling-induced nanoparticle deposition process is related to what happens in the peripheral regions of the nucleate boiling points and microparticle deposition points. The periphery of the deposited micro-scaled particles corresponds to the maximum value of peripheral spreading of the area affected thermally by the nucleation sites, beyond which only negligible changes in the temperature are to be observed in the heating surface during bubble growth. This maximum peripheral region corresponds to the macrolayer of the bubbles. Large agglomerated microparticles up to 20 µm have been found in the peripheral region in the form of a ring on the deposited pattern, as it can be seen in Figure 6. Moreover, these micro-scaled particles are considerably larger than the particles deposited in high-density areas near the center of the deposited structure. Considering the associated complexities, it is challenging to interpret whether these large agglomerated microparticles migrated from some region of the liquid bulk to the periphery or, alternatively, whether these particles were formed at the periphery of the nucleation site. In this sense, the work carried out by Kangude and Srivastava [41] made a serious attempt to reveal the most likely phenomena responsible for the presence of the micro-sized particles at the periphery of the active nucleation sites. By AFM imaging observation of the nanoparticle deposition, the researchers confirmed that the nanoparticle deposition during the boiling process led to the formation of a porous deposited structure. They also concluded that the hydrophilicity of the porous deposited layer augmented the capillary flows by offering a chain of hierarchical nano/micro-scaled paths for the fluid to flow. The authors obtained a nanoparticle-deposited structure using the 0.005% nanofluid that was found to exhibit capillary flows.The presence of the capillary flows was ascertained through high-speed camera imaging by observing the motion of the contact line of a sessile droplet deposited onto the nanoparticle deposition layer. It was observed that the droplet achieved static equilibrium first and then the apparent contact line of the sessile droplet moved through the porous layer until it attained a very small contact angle as compared with that of the substrate. However, no such movement of the contact line was verified when the droplet was deposited onto regions of the substrate surface away from the nucleation sites. The authors concluded that the contact line motion confirmed the presence of capillary flows through the deposited porous layer of nanoparticles. Those flows had a strong impact

on the microlayer evaporation by continuously replenishing the fluid to the evaporating front of the microlayer. As the microlayer starts evaporating at the central portion of the nucleation site, the capillary pressure forces the fluid to flow toward the contact line of the evaporating front of the microlayer from the periphery through the porous surface of the deposited layer. Furthermore, it should be stated that the thickness and density of the deposit increased from the periphery to the center of the nucleation site. When enough deposition occured, only the working fluid penetrated through the porous layer by capillary forces and the dispersed nanoparticles filtered out toward the peripheral region. The filtered-out nanoparticles agglomerated to form micrometer-sized particles at the periphery over time. Thus, the existence of the micrometer-sized particles in the peripheral region of the nanoparticle deposition points can be reasonably attributed to the presence ofcapillary-assisted radially-inward flows through the pores of the deposition layer during the microlayer evaporation. flows had a strong impact on the microlayer evaporation by continuously replenishing the fluid to the evaporating front of the microlayer. As the microlayer starts evaporating at the central portion of the nucleation site, the capillary pressure forces the fluid to flow toward the contact line of the evaporating front of the microlayer from the periphery through the porous surface of the deposited layer. Furthermore, it should be stated that the thickness and density of the deposit increased from the periphery to the center of the nucleation site. When enough deposition occured, only the working fluid penetrated through the porous layer by capillary forces and the dispersed nanoparticles filtered out toward the peripheral region. The filtered-out nanoparticles agglomerated to form micrometer-sized particles at the periphery over time. Thus, the existence of the micrometer-sized particles in the peripheral region of the nanoparticle deposition points can be reasonably attributed to the presence ofcapillary-assisted radially-inward flows through the pores of the deposition layer during the microlayer evaporation.

deposited structure. They also concluded that the hydrophilicity of the porous deposited layer augmented the capillary flows by offering a chain of hierarchical nano/micro-scaled paths for the fluid to flow. The authors obtained a nanoparticle-deposited structure using the 0.005% nanofluid that was found to exhibit capillary flows.The presence of the capillary flows was ascertained through high-speed camera imaging by observing the motion of the contact line of a sessile droplet deposited onto the nanoparticle deposition layer. It was observed that the droplet achieved static equilibrium first and then the apparent contact line of the sessile droplet moved through the porous layer until it attained a very small contact angle as compared with that of the substrate. However, no such movement of the contact line was verified when the droplet was deposited onto regions of the substrate surface away from the nucleation sites. The authors concluded that the contact line motion confirmed the presence of capillary flows through the deposited porous layer of nanoparticles. Those

*Nanomaterials* **2022**, *12*, x FOR PEER REVIEW 23 of 45

**Figure 6.** Schematic illustration of the nanoparticle deposition process and top view of the deposited structure showing the size distribution of the nanoparticles. **Figure 6.** Schematic illustration of the nanoparticle deposition process and top view of the deposited structure showing the size distribution of the nanoparticles.
