**3. Preparation of Nanofluids**

Nanofluids can be formed by dispersion particles made of single elements (e.g., Cu and Fe), single element oxides (e.g., CuO and Al2O3), alloys (e.g., stainless steel), metal carbides (e.g., silicon carbide and zirconium carbide), metal nitrides (e.g., silicon nitride and titanium nitride), or carbon-based materials in a none dissolving base fluid such as water, methanol, glycol, ethylene glycol (EG), transformer oil, kerosene, and/or different types of refrigerants with or without the use of surfactant/s [13,113]. The nanosuspension is given the name 'nanofluid' when one type of nanoparticles is used in the fabrication process; in contrast to the previous category, dispersions formed by employing two or more types of nanoparticles are classified as 'hybrid nanofluids' [114,115]. To the best of the authors knowledge, unlike the previous two nanofluids categories that are subjected to the number of different particles used in the process, there does not exist a specific classification for nanofluids made of more than one type of base fluid. However, researchers could have used the terms 'Bi-liquid nanofluid' or 'Tri-liquid nanofluid' to refer to their nanofluid that is made from two or three base fluids, respectively. Figure 8 shows an illustration of the conventional nanofluid and the hybrid nanofluid. In addition, the homogeneity and physical stability of the dispersion depend significantly on the implemented preparation approach, which can substantially influence the effective thermophysical properties of the as-prepared suspension. Knowing the aforementioned is essential when selecting the appropriate type of nanofluid for any targeted application [116]. In general, two known fabrication processes are currently used for producing nanofluids, namely, the one-step (also referred to as the single-step) method and the two-step approach [37]. It is important to note that some researchers prefer to classify the one-step production processes into two categories, which are the one-step physical technique and the one-step chemical approach, resulting in three types of methods of nanofluid fabrication for these groups [117,118]. A summary of the two fabrication schemes (i.e., the one-step and two-step methods) is presented in the following subsections.

**Figure 8.** Schematic demonstration to compare between conventional (**a**) and hybrid (**b**) nanofluids that uses the same base fluid. **Figure 8.** Schematic demonstration to compare between conventional (**a**) and hybrid (**b**) nanofluids that uses the same base fluid.

#### *3.1. One-Step Method 3.1. One-Step Method*

The production of nanofluids by the one-step method is conducted by simultaneous synthesizing and dispersing the nanoparticles in the base fluid. Thus, the storage, drying, and transportation of nanoparticles are unnecessary [119]. Furthermore, the dispersed particles in this bottom-up process avoid oxidization from their surrounding environment. In addition, this technique is well known to highly eliminate clustering and agglomeration of dispersed particles within the hosting fluid, and hence coagulation of nanoparticles in real-life applications that uses microchannels can be minimized with an increase in the level of the physical stability of the colloidal compared to the two-step production approach. Moreover, this method allows greater control over the size and shape of the dispersed nanoparticles during the fabrication process. Nevertheless, the presence of residual reactants as a result of uncompleted reactions has always been a major drawback of such a production route. Other disadvantages can also be experienced when following the single-step synthesis approach, such as the inconsistency of the scale for industrial applications, which can only be used with base liquids of low pressure, high production cost, and limitation in the types of nanofluids that can be fabricated compared to the twostep route [120–123]. The production of nanofluids by the one-step method is conducted by simultaneous synthesizing and dispersing the nanoparticles in the base fluid. Thus, the storage, drying, and transportation of nanoparticles are unnecessary [119]. Furthermore, the dispersed particles in this bottom-up process avoid oxidization from their surrounding environment. In addition, this technique is well known to highly eliminate clustering and agglomeration of dispersed particles within the hosting fluid, and hence coagulation of nanoparticles in real-life applications that uses microchannels can be minimized with an increase in the level of the physical stability of the colloidal compared to the two-step production approach. Moreover, this method allows greater control over the size and shape of the dispersed nanoparticles during the fabrication process. Nevertheless, the presence of residual reactants as a result of uncompleted reactions has always been a major drawback of such a production route. Other disadvantages can also be experienced when following the single-step synthesis approach, such as the inconsistency of the scale for industrial applications, which can only be used with base liquids of low pressure, high production cost, and limitation in the types of nanofluids that can be fabricated compared to the two-step route [120–123].

One of the most common one-step approaches is the one that was established by Eastman et al. [21]. In this method, nanofluids are synthesized by evaporating a bulk material, after which the evaporated particles get deposited then condensed in a thin film of base fluid attached to a vessel wall due to centrifugation. Figure 9 demonstrates the aforementioned one-step approach. Many researchers have continuously worked on developing the one-step fabrication approach through physical and/or chemical means. Today, different methods have been acknowledged to be in the one-step nanofluid production category [36,120,124]. Figure 10 shows some of the commonly known one-step nanofluid fabrication routes in the field where their method of conduct can be found fully explained in the published work of Ali et al. [13] and Mukherjee et al. [36]. One of the most common one-step approaches is the one that was established by Eastman et al. [21]. In this method, nanofluids are synthesized by evaporating a bulk material, after which the evaporated particles get deposited then condensed in a thin film of base fluid attached to a vessel wall due to centrifugation. Figure 9 demonstrates the aforementioned one-step approach. Many researchers have continuously worked on developing the one-step fabrication approach through physical and/or chemical means. Today, different methods have been acknowledged to be in the one-step nanofluid production category [36,120,124]. Figure 10 shows some of the commonly known one-step nanofluid fabrication routes in the field where their method of conduct can be found fully explained in the published work of Ali et al. [13] and Mukherjee et al. [36].

**Figure 9.** Eastman et al. [21] one-step method of evaporation and centrifugation for nanofluids fabrication [37]. **Figure 9.** Eastman et al. [21] one-step method of evaporation and centrifugation for nanofluids fabrication [37]. **Figure 9.** Eastman et al. [21] one-step method of evaporation and centrifugation for nanofluids fabrication [37].

**Figure 10.** Different nanofluid production approaches that fall under the one-step method. **Figure 10.** Different nanofluid production approaches that fall under the one-step method.

**Figure 10.** Different nanofluid production approaches that fall under the one-step method.
