**1. Introduction**

The boiling process has been used to dissipate energy in the form of heat generated by macroscale operating equipment. As miniaturized, faster and more powerful electronic devices and systems are developed and implemented on a day-to-day basis, the operating thermal fluids need to be improved to augment the heat removal capability. Pool-boiling heat transfer depends on factors closely linked with the fluid and solid surface properties such as thermal conductivity, surface tension, viscosity, enthalpy, specific heat, and roughness, structure, and homogeneity of the heating surface. It also depends on the hydrodynamic state near the heating surface determined by the bubble departure frequency and diameter and hot/dry spots dynamics [1], among other factors. The suspension of nanoparticles into the base fluid, or in other words using a nanofluid, is one of the most followed routes to enhance the pool-boiling heat-transfer coefficient (HTC) and critical heat flux (CHF) and can modify many of the aforementioned factors and properties. The use of nanoparticles may have considerable influence on the thermophysical properties of the working thermal fluids and on the behavior of the triple solid–liquid–vapor contact line. The alteration of the liquid/vapor and solid surface tensions changes the acting force balance and length of the contact line [2], wettability, and number of active nucleation sites. The focus of the research studies on the field should be to achieve an optimized combination of the base fluid, concentration, shape, and dimensions of the nanoparticles and solid surface substrate to increase, at the same time, the CHF and HTC of the pool-boiling scenarios. Nevertheless, there are still features of vital importance that should be further studied such

**Citation:** Pereira, J.; Moita, A.; Moreira, A. The Pool-Boiling-Induced Deposition of Nanoparticles as the Transient Game Changer—A Review. *Nanomaterials* **2022**, *12*, 4270. https://doi.org/10.3390/ nano12234270

Academic Editor: Henrich Frielinghaus

Received: 10 October 2022 Accepted: 24 November 2022 Published: 1 December 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

as the impact and control of the transient boiling-induced nanoparticle deposition [1]. In the course of the nucleate boiling process with nanofluids, the nanoparticles deposit onto the heating surface over time and can modify its properties including the wettability, water angle hysteresis, roughness, capillary wicking [3,4], and nucleation site density. Particularly, the surface roughness after the nanoparticle deposition is influenced by the fraction and intrinsic thermal properties of the nanoparticles, heating surface roughness, and on the thickness and morphology of the nanoparticle deposition layer. Moreover, the active nucleation site's density has a relevant role in promoting superior boiling heat-transfer performance, and can be controlled by factors such as the surface wettability and roughness, surface superheat value, and thermal properties of the fluid [5]. There is an intricate connection between the number of nucleation sites and the roughness and wettability of the heating surface: as the wettability improves, the likelihood of the liquid filling the cavities increases and, consequently, the number of active nucleation points decreases. In the work carried out by the authors Shoghl et al. [6], after the boiling process, using a alumina nanofluid as operating fluid, the deposited nanoparticle layer decreased the heating surface roughness in the case where the size of the nanoparticles was smaller than the roughness of the heat transfer surface. Moreover, Das et al. [7] reported that the smaller nanoparticles fill the cavities of the surface and reduce its original roughness. Since the nanoparticles agglomerate and deposit, the heating surface roughness decreases when the size of the clusters is smaller than the size of the surface cavities. Moreover, in the study conducted by Chopkar et al. [8], it was reported that after pool-boiling experiments using a 0.005 vol. % zirconia nanofluid, the heating surface roughness decreased. Moreover, the researchers revealed that the surface roughness decreased with the increasing number of successive pool-boiling experiments. Additionally, the authors Narayan et al. [9] employed machine vision to determine the nucleation site density after the completion of pool-boiling experiments with alumina nanofluids. The authors found that the number of active nucleation core points increased when the size of the nanoparticles of 47 nm was smaller than the roughness of the heating surface of 524 nm. In this case, the alumina nanoparticles penetrated into large surface cavities and enlarged the number of sites by splitting one active nucleation point into many points. On the other hand, if the nanoparticle size was similar to the average surface roughness of 48 nm, the number of nucleation sites was found to decrease considerably. Furthermore, the atomic force microscopy (AFM) technique was employed by White et al. [10] to measure the heating surface roughness after multiple pool-boiling tests with deionized water and with a 40 nm zinc oxide nanofluid. The research team observed that the surface roughness increased to 440 nm after seven nanofluid pool-boiling experiments. In addition, the heating surface roughness was reported by Bang and Chang [11] to increase after the nanofluid pool-boiling process if the alumina nanoparticles were larger than the surface roughness. Figure 1 shows the intertwined nanoparticle deposition time-dependent features at play in the pool-boiling heat transfer. Furthermore, the published results in the literature showed that the nanoparticle deposition layer affected the wettability more than the surface roughness [12]. In this direction, Buongiorno et al. [1] conducted pool-boiling experiments with nanofluids on smooth stainless-steel surfaces and found that the contact angle was modified because of the alterations in the solid surface tension provoked by the different chemistry and morphology of the deposited layer of nanoparticles.

It was previously reported in the literature that the CHF is usually enhanced by the alteration of the heat transfer surface characteristics [13,14]. Nevertheless, the nucleate boiling heat transfer of nanofluids is a complex phenomenon that depends on many factors such as the operating conditions [15,16]. Moreover, the experimental results so far published on the boiling heat transfer parameters of the nanofluids are not consistent with each other, even under similar experimental conditions [17]. In addition, the demanding time variation of the boiling heat transfer of nanofluids in which the most representative example is the nanoparticle deposition would be one of the main reasons behind such inconsistency of results [18]. To make matters worse, there is no available systematic experimental information or database regarding the transient nanoparticle deposition and

its effects on pool-boiling heat transfer using nanofluids. Hence, further in-depth research works are recommended to achieve a better understanding of the mechanisms responsible for the conflicting trends. The main objective of the current work is to provide, given the circumstances, the most possible complete and accurate experimental and theoretical information about the nanoparticle deposition onto the heat transfer surfaces, in an effort to minimize the severe shortage of understanding of such deposition during pool boiling of nanofluids. Table 1 summarizes some of the main recent experimental works on the boiling-induced nanoparticle deposition and its fundamental effects on the poll boiling heat transfer characteristics. *Nanomaterials* **2022**, *12*, x FOR PEER REVIEW 3 of 45

**Figure 1.** Nanoparticle deposition time-dependent features at play. **Figure 1.** Nanoparticle deposition time-dependent features at play.


It was previously reported in the literature that the CHF is usually enhanced by the **Table 1.** Main recent experimental works on boiling-induced nanoparticle deposition.

teration was the main factor influencing the HTC. Because the surface particle interaction parameter was more than the unity, only the increment of HTC was observed. At low concentration, the deposited

of the boiling duration, was caused by the higher concentration of the nanoparticles.

[19] Raveshi et al./2013 Aluminum oxide Copper






