Industries with cooling solution requirements have focused on the use of modified fluids with various additives
[7][1] to obtain improved thermal properties. Nanofluids are colloidal mixtures of nanosized particles (10–100 nm) suspended in base fluids
[8][2]. They possess good physical or chemical properties and thermal or rheological properties
[9,10][3][4]. Hybrid nanofluids are suspensions of a mixture of dissimilar nanoparticles or nanocomposites infused in the conventional base fluid, which yield better thermal conductivity and heat transfer characteristics due to hybridization
[11][5]. They are used in phase change materials, heat exchangers, solar energy, electronics, agriculture, chemical, manufacturing, and automobile industries
[12,13,14,15,16,17,18,19,20,21,22,23,24,25][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. The two-step method is used for preparing hybrid nanofluids. Different nanoparticles are prepared and mixed in the primary liquid through magnetic or mechanical stirring. The solution is sonicated and characterized to ensure stable and homogeneous mixing, providing improved heat transfer characteristics
[26][20]. Enhanced heat transfer is due to increased surface area, collision, interactive effect, and proper mixing of nanoparticles in base fluids (causing micro turbulences). Hybrid nanofluids play four roles (used as refrigerant, lubricant, absorbent, and secondary refrigerant) in improving the thermal system performance in low-temperature applications. The effects of nanoparticle concentration and size on the performance of water-based CuO nanofluids were investigated in
[27][21]. The synthesis, stability, and thermo-physical properties of hybrid nanofluids were studied in
[28][22]. Zaynon and Azmi
[29][23] presented the influence of nanoparticle type, concentration, temperature, shape, and size on the nanofluid properties. The amount of grapheme required in the base fluid to improve thermal performance was suggested in
[30][24].
2. Preparation of Mono/Hybrid Nanofluids
Nanofluids are organized according to their preparation using one- or two-step methods (
Figure 21). In a one-step approach, nanoparticles are prepared and mixed directly in a base fluid using physical or chemical processes. In the two-step method, nanoparticles are obtained using physical or chemical methods and then effectively infused in an essential base liquid
[33][25]. Several investigators have reviewed the preparation of different mono/hybrid nanofluids based on various base fluids
[34,35,36,37,38,39,40,41,42,43,44][26][27][28][29][30][31][32][33][34][35][36]. Spherical ZnO particles were synthesized using a sol–gel annealing process at 500–600 °C in
[45][37]. The ball milling process was used to grind aluminum nitride carbon nanocomposite (a nontoxic ceramic) for heat transfer experiments
[46][38]. Making nanofluids through a single-step method is expensive and time-consuming. The control of particle agglomeration is the primary problem in the two-step method. Ultrasonication minimizes nanoparticle sedimentation and improves nanofluid stability
[47][39]. Due to simplicity, 95% of researchers used a two-step method when preparing nanofluids (see
Table 1).
Figure 21. Flowchart for producing hybrid nanofluids [48]. Flowchart for producing hybrid nanofluids [40].
Table 1.
Examples of hybrid nanofluids adopting two-step preparation.
99].
Ultrasonic mills, baths, stirrers, and high-pressure homogenizers are used for the dispersion of nanoparticles. Baby and Sundara
[59][51] used an economical method to synthesize hydrogen-functionalized, exfoliation-induced silver-decorated graphene (Ag/HEG) and prepared nanofluids. Ag/HEG was distributed in a mixture of deionized water/ethylene glycol using ultrasonic agitation without surfactant. The hybrid nanofluid was observed to be stable for more than 3 months. Aravind and Ramaprabhu
[62][54] prepared MWCNT nanocomposites with graphene shells and synthesized them by chemical vapor deposition. The prepared hybrid nanofluid was stable for an extended period. Megatif et al.
[80][72] prepared a CNT–TiO
2 hybrid nanocomposite and dispersed it in water to obtain a hybrid nanofluid. The surfactant SDBS was added to the suspension for proper dispersion. They sonicated the solution for 15 min and tested its stability. The solution was stable for 2 days.
Although most (95%) of the researchers adhered to the two-step method, nanofluids synthesized by the expensive and complex one-step method improve the stability of nanoparticle suspensions in base oils due to high sedimentation rates with short sonication times
[108][100]. Ultrasonication lessens the sedimentation of nanoparticles, resulting in enhanced nanofluid stability. A better understanding of the mechanisms of nanofluids at the atomic level is required to address particle transport, aggregation, and stability issues with minimal experimentation.
No sophisticated equipment is required to produce nanofluids using a simple two-step method. Dispersion of nanoparticles requires sonication times of 3–10 h
[109][101]. Amin et al.
[110][102] critically reviewed the properties of single and hybrid nanofluids based on organic and synthetic materials. Malika and Sonavan
[111][103] used a two-step method to prepare CuO–ZnO/water hybrid nanofluids. Ultrasonication provided nanofluid stability. FESEM/EDS, dynamic light scattering, and zeta potential measurements provide insight into nanoparticle morphology, shape, and size. The stability of Al
2O
3–CuO/(50/50) EG/W (ethylene glycol/water) hybrid nanofluids at 60 °C was confirmed by zeta potential measurements
[112][104].
The stability of trihybrid nanofluids was tested by mixing three types of nanoparticles (i.e., Al
2O
3, TiO
2, and SiO
2 with volume concentrations of 0.05–0.3%) in a water/ethylene glycol-based fluid
[113][105] and a recommended sonication time of 10 h at a zeta potential of 25.1 mV. To improve the stability of nanofluids, Afshari et al.
[114][106] highlighted properties such as the acidity degree of the nanofluid, ultrasonication, nanoparticle material, base fluid type, nanoparticle concentration, surfactants, and surface modification of nanoparticles. Arora and Gupta
[115][107] reviewed stability evaluation techniques (spectral absorbance, sedimentation, zeta-potential, and electron microscopy) and enhancement techniques (ultrasonication, surfactant addition, particle surface modifications, and pH change). Future research should focus on industrial applications to minimize pressure losses, the concentration of nanoparticles, and the long-term stability of hybrid nanofluids.
The stability characteristics of mono and hybrid nanofluids have been studied using zeta potential measurements and vibrating sample magnetometry (VSM) analysis
[116][108]. To maintain nanofluid stability, Zainon and Azmi
[29][23] recommend analysis by sonication, pH modification, surfactant, TEM, field emission scanning electron microscopy (FESEM), XRD, zeta potential, and UV/visible spectroscopy techniques. Bumataria et al.
[117][109] used single and hybrid nanofluids to study heat transfer consider in heat pipe technologies. The use of dispersing agents and sonication increases the stability of nanofluids
[118][110]. Excellent suspension stability could be obtained by adding small amounts of SDBS and PEG to DW (hybrid nanofluid 25% Al
2O
3 + 75% TiO
2)
[119][111]. The hybrid nanofluid’s stability was high, as the zeta potential value (i.e., the electrostatic repulsive force between the nanoparticle and the base fluid) was 42.6 mV compared to the reference value of 30 mV. Said et al.
[120][112] investigated the stability of carbon nanofibers (CNF), functionalized carbon nanofibers (F-CNF), reduced graphene oxide (rGO), and F-CNF/rGO nanofluids. Hybrid nanofluids (FCNF/rGO) showed higher stability than CNF, F-CNF, and rGO nanofluids.
Muthoka et al.
[121][113] investigated the stability of hybrid nanofluids with two nanoparticles in PCM/DI water. The stability of surfactant-free MgO and 24 wt.% primary liquid was poor after 24 h, whereas the functionalized MWCNT solution showed no separation after 24 h. It was confirmed that the nanofluid’s low-temperature stability was increased using a surfactant. Acid treatment with CNF was used to test stability
[122][114]. The zeta potential of 0.02 vol.% F–CNF nanofluids measured after 2 and 90 days was −42.9 and −41.8 mV, indicating improved stability compared to the −16.3 and −15.5 mV UNV zeta potentials, which were characterized by relatively unstable dispersion. Alawi et al.
[123][115] synthesized aqueous nanofluids PEG–GnP, PEG–TGr, Al
2O
3, and SiO
2. The dispersion stability of the carbon-based nanofluid and the metal oxide nanofluid was observed for 30 days, and the high dispersibility of PEG–HNP and PEG–TGr in an aqueous medium with low sedimentation was confirmed. Compared to GnP/DW nanofluids, TiO
2/DW nanofluids showed superior stability
[124][116]. The addition of CTAB surfactant showed excellent stability of ternary hybrid nanofluids
[125][117]. Uysal
[126][118] used a 500 rpm homogenizer to mix and stabilize nano-graphene in vegetable oil. Al-Waeli et al.
[127][119] demonstrated high nanofluid stability (over 80 days) with CTAB and tannic acid + ammonia solution. The stability of Al
2O
3/water nanofluids using CTAB and SDBS surfactants was investigated for various pH values
[128][120]. Kazemi et al.
[129][121] visually observed the stability of SiO
2/water and G/water nanofluids. SiO
2/water nanofluids were found stable at all pH values (see
Table 2 for the stability of various nanofluids).
Table 2.
Stability of different nanofluids with surfactants.