Introduction

Heating and cooling by the fluid are the most important challenge in industrial application, and it is applied in many areas: cooling of electronic devices [1], solar water heating [2, 3], manufacturing [4], and many more. Heat removal is a major issue for any technology which deals with high power and small size. An electronic device that operates at high speed with high power and heat fluxes requires a high-quality cooling medium. The selection of a convenient heat transfer (HT) fluid for heat dissipation is necessary to design the heat exchanging system. In the last two decades, researchers and scientists have attempted to use conventional fluids (ethylene glycol (EG), water or a mixture of water and EG, kerosene, engine oil (EO), paraffin oil and vegetable oil), but due to the low thermal conductivity (TC) of these fluid is often a matter of concern for industrial application. By improvising the fluid, a new type of fluid was developed, which provides better heating and cooling performance for different thermal systems. Choi [5] invented a new concept of fluid known as nanofluid has improved its thermal properties and shown great promise in enchanting the indifferent area of HT application. The application of nanofluids appears promising in the future, but the use of nanofluids has shown serious problems which need to be overcome. Few of the applications and challenges of single nanofluids and hybrid nanofluids are demonstrated: Gupta et al. [6] measured the TC of metal oxides (CuO, MgO, Fe2O3) with water/EG at different temperatures (30–80 °C) and concentrations (0.05%, 0.1%, 0.2%). They identified that the enhancement in the TC of metal oxide nanofluids as the increment of temperature and concentration. Alfellag et al. [7] investigated the optimal nanoparticle mixing ratio of clove-treated MWCNTs/TiO2 nanomaterials at a fixed concentration (0.1 mass%). TC and viscosity measurements were done at different temperatures (30–50 °C). They found that ideal mixing ratio with the highest TC and lowest possible viscosity was found to be 60:40 based on the thermophysical performance factor and also identified that hybrid nanofluids shown higher performance than single nanofluids. Bao et al. [8] applied MWCNTs–SiO hybrid nanofluids on direct absorption solar collector and observed that the photothermal conversion efficiency improved (64.7%) by applying the hybrid nanofluid and TC of hybrid nanofluids is also enhanced. Ajeena et al. [9] measured TC of ZrO2–SiC/DW hybrid nanofluids at different concentrations (0.025–0.1%). They found that TC of hybrid nanofluid enhanced up to 25.75% at concentration of 0.1% and at temperature of 60 °C. Rudyak et al. [10] measured TC of two nanofluids (SWCNTs and MWCNTs) with different host fluids at various concentration (0.01–1.0%). They found that TC of SWCNTs was more than MWCNTs and at a fixed concentration, the TC of nanofluids increased with a decrease in the nanotube length. The challenge of nanofluid includes increases in pressure drop, stability of nanoparticle dispersion for a long period, clogging, high erosion, thermal performance in turbulent flow, troubles in production process, cost of nanoparticles, high viscosity, low specific heat, etc. Most of the experimental studies are to determine the heat transfer coefficient (HTC) of nanofluid based on metallic oxide nanoparticles. Designing a thermal system using a properly prepared stable nanofluid requires the determination of its thermophysical properties, such as viscosity, TC, density and heat capacity. Nanofluids can be applied in various devices and systems, so more studies have been performed to inspect the physical properties of nanofluids. Hybrid nanofluids shows higher TC, chemical stability, mechanical resistance and physical strength compared to their respective base fluids and single nanofluids. So the efficiency of the hybrid nanofluid is better than the single nanofluid. Hybrid nanofluids also reduce drag and transfer heat more efficiently compared to base fluids and single nanofluids. Table 1 shows the difference between single nanofluid and hybrid nanofluid.

Table 1 Difference between single and hybrid nanofluids
Full size table

This article is demonstrated an extensive literature review of single nanofluid and hybrid nanofluid. The main aim of the present research is to give a detailed review of the preparation of various types of single and hybrid nanofluids. The preparation of nanofluids includes different types of base fluids and surfactants. Different methods of preparation techniques (single/one step method, two step method), processes of stabilization, algorithm of preparation, and characterization of single and hybrid nanofluids are demonstrated. The applications of nanofluids are reviewed and identified the research gaps for the future research. Moreover, a few challenges are identified that researchers must overcome before implementing the industrial application. Hence, acute research is required for the excerpt of convenient hybrid nanoparticles, their preparation, characterization and long period of stability. Very few research works have been reported regarding this scenario in the literature.

Preparation and stabilization of single and hybrid nanofluids

The most important part of the experimental study on nanofluids is the preparation of stable nanofluids, free from agglomeration and no sedimentation for longer. Two methods were used to prepare nanofluids, i.e.:

  • Single/one-step method

  • Two-step method

Single/one-step method

Single-step method is a process where the production of nanoparticles and their dispersion in the host fluid occurs simultaneously. Nanoparticles are prepared by the physical vapour deposition (PVD) method, i.e. condensation of the metallic vapour into a flowing low vapour pressure fluid, which is called vacuum evaporation onto a running oil substrate (VEROS) invented by Yatsuya et al. [12]. In this technique, drying processes, dispersion and storage of nanoparticles are not necessary, so the accumulation of nanofluids can be optimized, and nanofluid stability can be increased. Direct Evaporation technique is the modified version of the VEROS technique; in this process, the vapour metal condensed to particles and mixed into the host fluid. This technique provides good control over the particle size and forms stable nanofluids without adding surfactants [13]. The laser ablation (LA) method is another one which has been used to form alumina nanofluids [14]. A pure chemical synthesis method developed copper nanofluid mixed in EG [15].

The advantages of the single-step method are less particle accumulation, enhanced nanofluid stability, drying cost, and averted dispersion. The disadvantages of this method are: residual reactants are left in the nanofluids, due to incomplete reaction or stabilization (difficult to remove), difficult to scale it up due to the cost of production, compatible low vapour pressure base fluid [16]. Some other examples of single-step processes are described in Table 2. Figures 1–7 show the image of different types of single-step method innovated by various researchers, such as VEROS, direct evaporation technique, pulsed laser ablation in liquid (PLAL), microwave synthesis, submerged arc nanoparticle synthesis system (SANSS), polyol process, and phase transfer method, respectively.

Table 2 Different types of single-step methods
Full size table
Fig. 1
figure 1

Schematic diagram of vacuum evaporation onto a running oil substrate method [12]

Full size image
Fig. 2
figure 2

Schematic diagram of direct evaporation technique [18]

Full size image
Fig. 3
figure 3

Schematic diagram of pulsed laser ablation in liquid method [24]

Full size image
Fig. 4
figure 4

Schematic diagram of microwave synthesis method [15]

Full size image
Fig. 5
figure 5

Schematic diagram of polyol process [31]

Full size image
Fig. 6
figure 6

Schematic diagram of submerged arc nanoparticle synthesis system (SANSS) technique [33]

Full size image
Fig. 7
figure 7

Schematic diagram of the phase transfer process [35]

Full size image

Two-step method

This process is mostly used to prepare single and hybrid nanofluids, employed by several researchers [37, 38]. Initially, the nanoparticles were produced by either chemical (sol–gel and vapour phase methods) or physical (milling, grinding, etc.) processes in dry powder. In the next step, prepared nanoparticles were mixed into base fluid with the help of a mixing device such as an intensive magnetic stirrer [39], high-shear mixing, homogenizer [40], ball milling [41], or by employing ultrasonic devices [42]. Stirring process decreases particle agglomeration and sedimentation [43] because agglomeration is a major problem in preparing nanofluids [44]. Due to large scalability and cost-effectiveness, this method is adopted for preparing the nanofluids. Manna [45] and Eastman et al. [19] suggested that the two-step method is more relevant for preparing nanofluids with oxide nanoparticles than metallic nanoparticles. Nanoparticle tends to aggregate due to its high surface area and surface activity. Therefore, nanoparticles are to be stabilized to prevent agglomeration [46] with the support of sonication. Stability is the main problem for this method, as the nanopowder mixes comfortably due to strong van der Waals force among the particles. Despite this issue, two-step methods are the most economical for preparing nanofluids. The algorithm and nanofluid preparation by the two-step method are described in Figs. 8 and 9, respectively.

Fig. 8
figure 8

Algorithm for preparation of nanofluids

Full size image
Fig. 9
figure 9

Nanofluids preparation by two-step method

Full size image

Stabilization

Initially, Xuan and Li [47] used the basic method for stabilization of the prepared suspension by (a) altering the pH value of the suspension, (b) employing surfactants and (c) applying ultrasonication. The above methods can modify the surface properties of suspension. The use of this technique of stabilization depends upon the application of nanofluid. The properties of the solution and particles will decide the selection of surfactants. Surfactants lower the surface tension between two liquids or between a solid and a liquid. Various surfactants are available; it depends on the charge of the head group of surfactants. A few examples of surfactants are mentioned in Table 3.

Table 3 Different types of surfactants
Full size table

Preparation of different types of single and hybrid nanofluids

The difference between single and hybrid nanofluids is already explained in Table 1. Hybrid nanomaterial shows exceptional physiochemical properties not present in the single component. A single material may have good thermal and rheological properties but not all favourable characteristics. Several methods were used to prepare hybrid nanofluids, which have been discussed later.

Water-based single nanofluids

Researchers and scientists mostly used water for preparing the single nanofluids. A few of the examples are described here. Kathiravan et al. [48] prepared Cu nanoparticles (size of 10 nm) by sputtering method. Different nanofluids were prepared with water and water with 9.0% surfactant (SDS) and sonicated in an ultrasonicator for about 10 h. Few particles were found to agglomerate and form clusters after 10 h of ultrasonication. Kole and Dey [49] prepared stable Cu (40 nm)–water nanofluids by ultrasonication (power at 200 W) and followed by a magnetic stirring process (10 h) without using any surfactant. The suspension stability of Cu–water nanofluid was stable for more than two weeks without any sedimentation. Still, due to the small cluster formation, the average diameter was between 164 and 122 nm. Li et al. [50] prepared two types of Cu–water nanofluids (25 nm and 100 nm) by two-phase method. The suspension was vibrated for some time of 4 h in an ultrasonicator and observed that the sample was stable in the stationary state. Li et al. [51, 52] mixed Cu nanoparticles (25 nm) in water, with SDBS and CTAB as surfactants, to prepare nanofluid. They found that the nanofluid with CTAB surfactant can be stable for up to one week without any sedimentation, while the suspension without surfactant occurs aggregation immediately. Riehl and Santos [53] mentioned that the Cu–water nanofluids (< 40 nm) were prepared by using an ultrasonicator and observed no sedimentation even after 4 h of observation. Yang et al. [54] prepared Cu (50 nm) nanofluids by using an aqueous solution of cetyl trimethyl ammonium chloride (CTAC) and sodium salicylate (NaSl). The same mass concentration of NaSl and CTAC was added to the solution, whereas water was used as host fluid. An ultrasonicator dispersed the aqueous solution and the nanoparticles in the host fluid. It has also been reported that prepared nanofluid showed a stable property using 3-h of sonication.

Parametthanuwat et al. [55] prepared Ag (< 100 nm) deionized (DI) water nanofluids and sonicated them for 5 h. They found that the nanofluid was stable for 48 h, at φ = 0.5%. Hari et al. [56] mentioned that the Ag seed nanoparticles were prepared by chemical reduction of Ag nitrate by sodium borohydride in the presence of tri-sodium citrate to stabilize the particles. Silver nitrate was reduced by ascorbic acid in the presence of seed (mediated chemical synthesis), the micellar template CTAB and sodium hydroxide to prepare nanorods. CTAB was used for capping the silver nanorods. They identified that prepared nanofluids were stabilized for 1 week. Hajian et al. [57] prepared Ag water-based nanofluid by chemical method consisting of reduction of Ag ions, and before the experiment (injection of nanofluids into the heat pipe) started, nanofluids were kept in an ultrasonicator for 15 min to break apart nanoparticles masses and homogenize the nanofluids. Lo et al. [34] prepared Ag (6–25 nm) nanoparticles by SANSS method and observed that nanoparticles were properly mixed in host fluid (DI water) by using an ultrasonicator for 15 min. Moreover, Ag (29 nm) DI water-based nanofluids were prepared using the multi-beam laser ablation method and were stable for several months without adding surfactants [23]. Kang et al. [58] prepared Ag nanoparticles 35 nm in size formed by a catalytic CVD method using an ultrasonicator, where water was considered the base fluid. Asirvatham et al. [59] mixed the Ag nanoparticles (< 100 nm) with DI water without any stabilizer by using an ultrasonicator for 12 h. They mentioned that the nanofluids were uniform, but little accumulation formed.

Kim et al. [60] dispersed Au nanopowder (7.1–12.1 nm) in water with an ultrasonicator for up to 6 h. They suspended the Au nanoparticles in the host fluid by pulsed laser ablation. Despite 6 h of ultrasonication, they reported that the maximum quantity of nanoparticles was precipitated in the host fluid, and the Au nanoparticles stabilized for more than 1 month. Kambli and Mane [61] prepared Au-water nanofluids by reducing an aqueous hydrogen tetrachloroauric acid (HAuCl4) using 1% tri-sodium citrate. The colloidal suspension of different φ was ultrasonicated at room T for 10 min to disperse particles, and optical measurements made using UV–vis Spectrophotometer and suspensions remained stable for 2 weeks. Mondragon et al. [62] synthesized the Au nanoparticles (18.5 nm and 20 nm) with water as host fluid, PBS and citrate buffer solution as a stabilizer by PLAL and photo-fragmentation (PF) technique. The nanofluids prepared by the PLAL-PF technique remain stable over at least 3 months without using any stabilizer. Stakenborg et al. [63] prepared Au nanoparticles of various sizes by adding different quantities of aqueous citrate solution to boiling aqueous Au salt solution. A thiolated DNA probe (1 µM) was mixed with the Au nanoparticles solution (OD ~ 1) in the presence of NaCl, and the suspension was shaken at 800 rpm for ~ 12 h. Paul et al. [64] prepared Au–water nanofluid and informed that uniform distribution, purity of Au nanoparticles and colour of the nanofluids remain stable without any settlement even after 2 days. Patel et al. [65] prepared the Au and Ag nanoparticles (10–20 nm) by citrate reduction route method. The prepared samples were stable for a few months, and no sedimentation was found during storage during the experiment. Balasubramanian et al. [66] synthesized Au nanofluids such as 5 mg of Au (20 nm) nanoparticles added with 95 mL of HAuCl4 boiled at 100 °C with mechanical stirring for 20 min. Then, 50 mg of trisodium citrate dihydrate (5 mL of 1% solution in water) was added and boiled for 20 min before the sample was gradually cooled to single temperature. Storage in the dark at 4 °C prolongs the stability of purified Au–NP suspensions for up to 20 days. Singh and Soni [67] suspended Al nanoparticle in DI water and polymeric solution ablated for about 20 min. Different types of aqueous stabilizing agent solutions like SDS, PEG, PVA, and PVP are used. For PVA, the particles were stable against oxidation for more than 1 week for all φ. Nanoparticles grown in PVP solution exhibited long-term stability over 3 weeks against oxide formation.

Priya et al. [68] synthesized the CuO nanoparticle in water using an ultrasonicator ~ 6 h; iron was used as a dispersant for better stability. The CuO:Iron ratio was considered as 2.5:1 and the nanofluid was visually seen to be stable. Suresh et al. [69] followed the same route. Lee et al. [24] prepared CuO–water nanofluid by two-step method with sonication of 6 h and by one-step method with PLAL. They found that the particle size of the nanofluids prepared by the single-step method is less than the two-step method. Yang and Liu [70] mixed CuO nanoparticles into water using an ultrasonicator for 12 h, and the nanofluid was in stable condition for several days. Chang et al. [71] prepared CuO nanoparticles and then synthesized CuO–water nanofluid using a spinning disc reactor (SDR), where NaHMP was used as a dispersant. They found that φ more than 0.40%, the solution was unstable. Kannadasan et al. [72] prepared CuO nanoparticles by using the chemical precipitation method, and CuO nanofluid was produced by mixing the nanoparticle with water surfactant was added to the solution, and a slight settlement was noticed in the suspension even after 25 days of preparation. Fotukian and Esfahany [73] mixed CuO (30–50 nm) and water to prepare nanofluid at different φ. The prepared solution used an ultrasonic mixer for 10 h and showed no sedimentation after 5 h. William et al. [74] prepared CuO water nanofluids using a two-step method, where xanthan gum (XG) was used as a dispersant and an ultrasonic tank was used for sonication of the nanofluid for 1 h. Bhanvase et al. [75] prepared nanofluid by dispersing dry polyaniline (PANI) or PANI–CuO composite nanoparticles into DI water at different φ. The nanofluid was prepared by magnetic stirring and sonication of 30 min, where SLS was the dispersant. Liu et al. [76] mixed the CuO nanoparticles (size of 50 nm, prepared by the gas-condensation method) in the DI–water to prepare the nanofluid and sonicated it for about 10 h using an ultrasonicator. Selvakumar and Suresh [77] prepared CuO nanofluids by mixing the nanoparticles in DI–water with an ultrasonicator for 6 h. They observed stable nanofluids even after a week with little sedimentation. Byrne et al. [78] produced CuO–water nanofluids, with and without surfactant (CTAB), with the help of an ultrasonicator ~ 7–8 h. They observed that at φ = 0.1%, the surfactant-based nanofluid had a particle size of ~ 200 nm and was stable for 7 days. They mentioned that using a surfactant decreases the nanoparticle size and enhances the dispersion of the nanoparticles. Michael and Iniyan [79] synthesized CuO nanoparticles from Cu acetate by the aqueous precipitation method. Cu acetate monohydrate is a precursor, and glacial AA prevents hydrolysis. This was taken in a flat-bottom beaker and heated with constant stirring. CuO–water nanofluids were prepared with Triton X-100 with and without surfactant (SDBS). After 3 days of preparation, CuO–SDBS water nanofluid showed better stability than Triton X-100.

Anoop et al. [80] used two different types of Al2O3 nanoparticles (size of 45 nm and 150 nm), developed by laser evaporated physical method to prepare the Al2O3–water nanofluid with the help of ultrasonicator and found that the suspension was stable for several weeks. Esmaeilzadeh et al. [81] prepared Al2O3 (15 nm) water nanofluid using magnetic stirring and ultrasonication for 4 h; no sedimentation was found during the experiment. Hegde et al. [82] stirred the Al2O3 particle (80 nm in size) with water in a sonicator for 3 h to produce an Al2O3–water nanofluid. The prepared nanofluid showed no agglomeration even after the sonication process of 2 h. Mahbubul et al. [83, 84] suspended an Al2O3 nanoparticle (13 nm) in water by a two-step method, and the suspension was sonicated for a period of 1–5 h by using a sonic dismembrator machine (20 kHz and 500 W) and nanofluid found to be stable for long days. Ghanbarpour et al. [85] and Das et al. [86] prepared Al2O3–water nanofluid by two-step method using surfactants, stirrer and ultrasonicator. Chandrasekar et al. [87] dispersed Al2O3 nanoparticles (43 nm) in water using an ultrasonicator for 6 h to prepare nanofluid and noticed that the prepared solution was stable for a few months. Hung et al. [88] produced Al2O3 (20 nm)–water nanofluid with φ less than 5% using a homogenizer, an electromagnetic agitator and an ultrasonicator where chitosan was used as a cationic dispersant, and the nanofluid remained stable for 2 weeks. Gharagozloo and Goodson [89] mixed Al2O3 (φ of 20%) in water with a maximum of 1% of nitric acid to produce nanofluid. The prepared solution was sonicated for 4 h, and minor settling was observed after a week. Soltani et al. [90] prepared host liquid by adding carboxymethyl cellulose aqueous solutions at φ = 0.5% in water. The dry nanoparticles were dispersed into the host fluid to prepare nanofluid, and before the experiment, the suspension was sonicated for 1 h. Sharma et al. [91] mixed Al2O3 nanoparticles (47 nm) in water, whereas SDBS was used as a surfactant, and the suspension was stirred up to 12 h. They identified that φ < 3%, was stable for more than a week, and some sedimentation was identified at maximumφ. Teng et al. [92] prepared Al2O3–water nanofluid, chitosan used as a surfactant, with the help of ultrasonicator and electromagnetic agitation. They found that the prepared suspension was stabilized for 1 month. Qu et al. [93] used Al2O3 nanoparticles (56 nm) with water by a two-step method, and the dispersion was sonicated for 4 h in an ultrasonicator; and they noticed that the nanofluid could be stable for ~ 72 h. Jung et al. [94] prepared different types of water nanofluids, with and without PVA, using a horn-type ultrasonicator ~ 2 h. They found that suspension was stable for more than 1 month.

Several researchers [95,96,97,98] prepared TiO2 water nanofluids by using a mixer and sonicator and noticed that the prepared nanofluids stable for long periods. Abbasian Arani and Amani [99] diluted TiO2 nanoparticles in water, and CTAB was used as a surfactant to prepare nanofluids with the support of an ultrasonicator and stirrer to break the accumulation and noticed that the suspension stable for several hours. Mo et al. [100] mixed rutile and anatase nanoparticles (TiO2) in water (pH adjusted to 8 by using NH3), and SDS was used as a surfactant. The suspension was mixed using a stirrer for 10 min and sonicated for 40 min and noticed that the nanofluid was stable for 15 days. Bobbo et al. [101] prepared TiO2 nanofluid using various surfactants (SDS and PEG) and physical treatment methods. They observed that the homogenization method is suitable for improving suspension stability. Nanoparticles are dispersed into water by a high-pressure homogenizer. For TiO2–water nanofluid, the ratio between nanoparticles and surfactant mass was 1:2 for all φ, and TiO2–water-PEG solution was less stable. Fedele et al. [102] prepared 35 mass% of TiO2 nanofluid with bidistilled water. AA is presently used as a dispersant (φ = 1–5%), and the composition is prepared by 1 h sonication to enhance the sedimentation rate in the nanofluid. Das et al. [37] prepared TiO2-water-based nanofluid with dispersants like CTAB and AA. The prepared nanofluid mixture was dispersed in the ultrasonic bath for about 2–3 h, and the nanofluid was found stable for several days with negligible disturbance. Duangthongsuk and Wongwises [39] prepared TiO2-SDBS water nanofluid using an ultrasonicator for 3–4 h and found that the sample was stable up to 3 h of sonication. After that, sedimentation started. Similarly, Murshed et al. [103] mixed TiO2 nanoparticles in water, where OA and CTAB were used as stabilizers, using 8–10 h ultrasonication. Utomo et al. [104] synthesized TiO2–water nanofluid (pH constant) by sonicating the suspension for 3 min, where ammonium polyacrylate was used as a surfactant. Kayhani et al. [105] prepared TiO2 nanoparticles by a chemical process and mixed them in C6H19NSi2 (hexamethyldisilazane) using a sonicator for 1 h. Then, the nanoparticles were added to water using a sonicator for 3–5 h, marking the nanofluid stable for several days. A similar kind of study was found by He et al. [106], Longo and Zilio [107]. Fedele et al. [108] considered different nanoparticles SWCNHs (100 nm), TiO2 (21 nm), and CuO (30–50 nm) for preparing the nanofluids by different dispersion techniques; ball milling and high-pressure homogenization, including sonication (0–2 h). They found that the high-pressure homogenization method is the best one. Sen et al. [109] mixed an aqueous alkaline solution (containing mM LiOH and 30 mM KOH) in water for all the nanofluid preparations, and representative salt and additives were used in industrial processes and alkaline. Dry nanopowder was mixed extensively with the host fluid to achieve a homogeneous suspension by magnetic stirring for at least 24 h, and suspensions were resonicated for 4−6 h. Before any testing, nanofluids were sonicated for at least 1 h. Similarly, some other researchers [110,111,112] prepared TiO2 and Al2O3 nanofluids by mixing the nanoparticles in water, sonicating for 30 min and stirring for 20 min.

Anoop et al. [112] mixed SiO2 nanoparticles (~ 20 nm) in DI water and deagglomerated the clusters using an ultrasonicator (35 kHz) for 30 min and noticed that the nanofluid was stable for a long time. Bhuiyan et al. [113] applied a two-step method to prepare SiO2 nanofluids with distilled water. The prepared mixture was vibrated vigorously by a mechanical shaker for 1 h at 150 rpm; subsequently, an ultrasonic homogenizer (frequency of 20 kHz with 50% amplitude) was used for two hours to produce high amplitude to break down particle clusters. Fazeli et al. [114] added SiO2 nanoparticles with a particle size of 18 nm in water (without surfactant), and the prepared solution was indicated for 90 min. They observed that the nanofluid was stabilized for 3 days without any sedimentation. Bolukbasi and Ciloglu [115] synthesized SiO2 nanofluid with a two-step procedure at different nanoparticles φ using a stirrer and ultrasonicator for 2 h. They described no sedimentation was found at the time of the experiment. Sulaiman et al. [116] mixed SiO2 nanoparticles (20 nm) in water and placed them in an ultrasonic bath with different ultrasonication hours (1–5 h). Qu and Wu [117] dispersed two different nanoparticles of SiO2 (30 nm) and Al2O3 (56 nm) in water. The nanofluid was synthesized by a two-step method. No surfactants were added during the synthesis process, and the prepared solution was sonicated for about 4 h. Shahrul et al. [118] mixed SiO2 nanoparticles (10–20 nm) in water without surfactant and sonicated for 90 min. The sedimentation was observed after 21 days of preparation. Mahian et al. [119] mixed SiO2 nanoparticles (7 nm) in water, stirrer it for 30 min, and then it was kept in an ultrasonic bath (600 W of power and 40 kHz of frequency). They identified that nanofluids were stable for more than a week. Yang and Liu [120] prepared SiO2 nanofluid by dispersing silica nanoparticle powders and silane of trimethyoxysilane into the water. In contrast, the mass ratio of reacting silane and nanoparticles was taken as 0.115, and the prepared nanofluid was stable for 12 months, even at φ = 10% without any sedimentation.

Lee et al. [121] dispersed SiC nanoparticles (< 100 nm) in DI water to prepare nanofluid under various pH and characterized with the zeta potential (ZP) values. SiC–water mixture was sonicated for 12 h for proper dispersion of nanoparticles in the base fluid. Kim et al. [122] prepared SiC–water nanofluid by dispersing the nanoparticles into water and sonicated it for 3 h. From the literature review of SiC nanofluid, it has been confirmed that by adjusting the pH value of nanofluid, SiC nanofluids can be stable for more than 30 days. Ghanbarpour et al. [123] dispersed α-SiC nanoparticles in distilled water to prepare nanofluid at different φ. Then, the suspension was sonicated, and the pH of the nanofluid was adjusted. To study the stability region and identify the optimum pH value of nanofluid, ZP analysis was done in the pH region of 2–10. To know about the real effect of α-SiC nanoparticles on heat pipe performance, surfactant/surface modifiers were avoided. Zhang et al. [124] prepared SiC (30 nm) water-based nanofluids at different φ by using a two-step method, where CTAB was used as the dispersant. The suspension was ultrasonicated for 3–5 h, and the nanofluid remained stable for a week without any sedimentation. Suganthi and Rajan [125] prepared ZnO nanoparticles by chemical precipitation method using zinc nitrate hexahydrate. The nanoparticle was added to sodium hexametaphosphate (surfactant) solution, and the nanoparticle/surfactant ratio was taken as 5:1. Raykar and Singh [126] prepared ZnO nanoparticles by using (CH3COO)2Zn·H2O, NaOH, water, TEA, stirrer, and, microwave oven. Various types of nanofluids were prepared by dispersing 75 mg (type I), 250 mg (type II) and 500 mg (type III) each in 100 mL of DI water. The solution was indicated for 1 h, and subsequently, 3, 5 and 7 mL of ACAC was added in all types of solutions. All solution was sonicated for 10 min and noticed that nanofluids were stable for up to 12 months. Chung et al. [43] mixed two types of ZnO particles (prepared by sol–gel and physical vapour synthesis) in water; ammonium polymethacrylate was used as a surfactant. They used several sonication systems: a solenoid-actuated bath, a single piezo-actuated bath, and a static bath. They observed that the dispersion by ultrasonic horn was more effective than others. Jeong et al. [127] used two types of ZnO nanopowder for preparing the nanofluids: rectangular-shaped (90–210 nm) and spherical-shaped (20–40 nm). They dispersed ZnO nanopowder in DI water, using ammonium polymethacrylate as a dispersant, and stirred it at 25 °C for different φ nanoparticles from 0.05 to 5.0%. From the above literature study for the ZnO-nanofluid preparation method, long-term stability was observed when ACAC was used as a surfactant, and the minimum sonication was considered for 10 min.

Phuoc et al. [128] produced a nanofluid by adding chitosan in water having 0.5% AA and stirring it for 24 h. Three different mass % (0.1, 0.2, and 0.5) of chitosan and four different mass % (0.5, 1, 2, and 3) of MWCNTS were used for this experiment. Nanofluid was prepared by mixing MWCNTs into the prepared host fluid and sonicated for 10 min, followed by 20 mins of stirring. They found that the solution prepared without chitosan was unstable but stabilized by 0.2% of chitosan-stable for a month. Garg et al. [129] used water, GA and MWCNT to prepare nanofluids. Initially, GA was mixed in water using a stirrer, and the nanoparticle was added to the solution. The prepared samples were sonicated for 5 min by an ultrasonication probe with different sonication times (20 mins to 80 mins). The sonication and stirring process was alternated every 5 minutes and finally noticed that the sample was stable for 1 month. Babu and Prasanna Kumar [130] mixed CNT powder in water to prepare nanofluid. The chemical treatment of CNTs involved soaking of CNTs in an acid mixture for 5 h. The prepared sample was sonicated for 5 min and observed that TCNTs-based nanofluid exhibited better stability than PCNTs-based nanofluid. Ding et al. [131] first sonicated the CNT sample in a bath sonicator for 24 h and dispersed sonicated CNTs into a pre-set amount of water containing GA (surfactant). Finally, the nanofluid was mixed with a high-shear homogenizer for 30 min and found stable for months without visual sedimentation. Sadri et al. [132] used GA, SDBS, and SDS as surfactants in distilled water to prepare CNT nanofluid. The suspension was dispersed using a magnetic stirrer and then sonicated for 20 min using an ultrasonic probe until a homogeneous suspension was achieved. Sarafraz et al. [133] dispersed the desired masses of CNTs (mass% of 0.1–0.3) with a size diameter of 10–20 nm in water by using an ultrasonic process (at 40 kHz, 300 W) for 10 min. Then, nonylphenolethoxylate (alkylphenol organic surfactant) was added dropwise, showing that the nanofluid remained stable for more than 21 days. Su et al. [134] dispersed aligned multi-wall carbon nanotubes (A-MWCNTs) in water, and the solution was agitated for 2 h by a sonicator. It was found that the suspension was stable for up to several days without any agglomeration. Abareshi et al. [135] prepared nanofluids which contain different φ Fe3O4 nanoparticles by dispersing the nanoparticles in water. Tetramethyl ammonium hydroxide was used as a surfactant and was stable for a few days. Harikrishnan and Kalaiselvam [136] prepared the CuO–OA nanofluid by precipitation method using a sonicator. They found CuO–OA nanofluid dispersion stabilization stable for more days. Hwang et al. [137] prepared several types of nanofluids: MWCNT, CuO and SiO2 in water and CuO in EG, using an ultrasonic disruptor for 2 h. They obtained a stable suspension for SiO2 and CuO nanoparticles where SDS was used as a surfactant to prepare MWCNT nanofluids. Chen et al. [138] produced MWCNTs (15 nm) by CVD method to prepare nanofluids. They chemically treated CNTs into host fluid (distilled water, EG and glycerol), where potassium hydroxide was used as surfactant and observed that all the nanotubes were sedimented after 5 min. From the above literature study, we can conclude that all nanoparticles can mix with water for better stabilization; low φ, appropriate surfactant ratio and optimum sonication time were required.

Water-based hybrid nanofluids

Many researchers used water for preparing the hybrid nanofluids due to better thermal properties. Moldoveanu et al. [139] considered two oxide nanoparticles dispersed in aqueous solution for the analysis: Al2O3 nanoparticle (45 nm) in 60% mass water and SiO2 (20 nm) in 40% mass aqueous solution. Initially, nanofluid was prepared at different φ of 1, 3 and 5% for Al2O3 and 1, 2 and 3% for SiO2–water-based nanofluids. The prepared suspension was sonicated for about 30 min to ensure uniform nanoparticle dispersion. Finally, the appropriate amount of distilled water was mixed into the initial solution and thoroughly mixed and sonicated to achieve the hybrid nanofluids. Suresh et al. [140, 141] produced Al2O3-Cu hybrid nanoparticles by hydrogen reduction from a mixture of Al2O3 and CuO in a 90:10 mass ratio. The prepared nanofluid was dried to get the precursor powder, and the powder was heated at 900 °C for 1 h to get a mixture of CuO and Al2O3. An appropriate amount of Al2O3–Cu nanoparticles was mixed in water, where SLS was used as a dispersant, using an ultrasonicator for 6 h to get a stable suspension. The Al2O3–Cu water-based hybrid nanofluid was prepared by a two-step method. Balla et al. [142] synthesized CuO–Cu (50 nm) water-based hybrid nanofluids by two-step method for different φ from 0.2 to 1%. Ultrasonic vibration was employed for better mixing of nanoparticles in the host fluid, and no additive was used in the synthesis process.

Nine et al. [143] prepared Al2O3–MWNT water-based hybrid nanofluids (mass ratios of 97.5:2.5 up to 90:10). They followed preparation methods like acid treatment and grinding of MWNTs by planetary ball mill to ensure better dispersion. Baghbanzadeh et al. [144] prepared hybrid silica nanosphere/MWCNT followed by wet chemical method. Sodium silicate was mixed with water, and a sonicator dispersed the solution in MWCNTs. Water was also added with CTAB and dimethylformamide by stirrer. Filtering and washing the compounds were done using EG and water until the grey products were demonstrated. Abbasi et al. [145] prepared MWCNT–TiO2 water nanofluid and TiO2 nanoparticles produced by hydrolysis of TiCl4. Hybrid nanoparticles of MWCNT–TiO2 were attained by dropwise adding TiCl4 to water by stirring process and mixing the oxidized MWCNTs in prepared suspension using a sonicator. Munkhbayar et al. [146] prepared Ag/MWCNT hybrid nanofluids for a constant φ of MWCNT and three different φ Ag nanoparticles. The wet grinding method (WGM) was used to mix MWCNTs–water nanofluid and Ag nanoparticles, synthesized by the pulsed wire evaporation (PWE) method. Baghbanzadeha et al. [147] considered distilled water as the host fluid, while MWCNTs, silica nanospheres and two types of their hybrids (ratio of silica and MWCNT are 80:20% and 50:50%) as the suspended particles and SDBS used as the dispersant for preparing nanofluids. First, a desired amount of SDBS was added with distilled water and placed under an ultrasonic disruptor for 2 min. Then, by adding nanomaterials to the prepared solution, the new mixture was sonicated by ultrasonicator for another 20 min. The most stable nanofluid obtained stable for about 1 month without any sedimentation. Jana et al. [148] prepared water-based CNT–Au and CNT–Cu hybrid nanofluid by mixing the water into Au and Cu with the CNT nanoparticle at different φ laurate salt used as a stabilizer for Cu nanoparticle. Megatif et al. [149] prepared Cu-TiO2hybrid nanoparticles by using the hydrolysis technique. The acid-treated CNTs were mixed with water and sonicated for 30 min. EG and 2-propanol were mixed into the suspension and stirred. Aravind and Ramaprabhu [150] synthesized graphene-MWCNTs through a two-step method. The composite of graphene-wrapped MWCNT was prepared by simple CVD technique and then purified. Madhesh et al. [149] developed a Cu–TiO2 hybrid nanocomposite by mixing TiO2 in aqueous solution and adding copper acetate during stirring. As reducing agents, sodium borohydride and ascorbic acid were added to the prepared solution. The prepared solutions remained stable for 2 h to get hybrid nanocomposite, and hybrid nanocomposite powder was again dispersed in host fluid (water) to obtain the hybrid nanofluid. Sundar et al. [151] synthesized the MWCNT-Fe3O4 nanocomposite by in situ method. The carboxylated CNT dispersed in water and mixed with ferric and ferrous chloride. Carboxylated-MWCNT was mixed in water by stirrer and washed with acetone to remove the impurities, using sodium hydroxide as a reducing agent. Baby and Ramaprabhu [152] used catalytic CVD to prepare MWNT, and hydrogen-exfoliated graphene (HEG) was synthesized by exfoliating graphite oxide. The hybrid nanostructure (f-MWNT + f-HEG) was dispersed in DI water-based nanofluids, and the solution was sonicated for 1 h and stirred for 24 h. The prepared solution was filtered, dried and used for making nanofluids. Shahsavar et al. [153] synthesized hybrid nanofluids of TMAH with Fe3O4 nanoparticles and gum arabic coated on CNT. FeCl2 and FeCl3 were mixed with NH3, and then, TMAH solution was added until the pure solution was prepared. The excess NH3 was evacuated from the solution with the help of a stirrer. Chen et al. [154] mentioned that a green method was applied to prepare a composite of MWNTs decorated with Ag nanoparticles. MWNTs were functionalized using a mechanical ball milling process in the presence of ammonium bicarbonate. The functionalized MWNTs were decorated with Ag–NPs by the traditional silver mirror reaction method. The resulting Ag–MWNT composites were used to prepare a water-based nanofluid. Sundar et al. [155] developed a nanocomposite of MWCNT and Fe3O4 by surface modification of MWCNT. Surface modification was involved in mixing MWCNTs in HCl and HNO3 acid solution by stirring. The in situ and chemical coprecipitation process involved mixing MWCNTs in water and adding FeCl2 to MWCNT.

Esfe et al. [156] prepared a hybrid nanofluid of Cu/TiO2–water/EG followed by a two-step method at different φ from 0.1 to 2% with the help of a stirrer, and various φ of nanoparticles were mixed with host fluid. Esfe et al. [157] prepared Ag–MgO water-based hybrid nanofluid at different φ followed by one-step and two-step methods. The solution was stabilized using different techniques such as surfactant (CTAB) addition, modifying the pH value, and ultra-sonication and the solution was found to be stable for several days. Madhesh et al. [158] prepared the hybrid nanocomposite of copper-titania. Titania aqueous and copper acetate solutions were stirred with sodium-reducing agent borohydride and ascorbic acid. Syam Sundar et al. [159] used two methods to prepare nanodiamond (ND)-Fe3O4 nanofluid. 1.5 g of acid-treated ND was mixed in 50 mL of water and stirred for 2 h. Yarmand et al. [160] synthesized graphene nanoplatelets-Ag nanocomposite and the graphene nanoplatelet was dispersed in a strong acid medium (1:3 ratios) for sonication, about 3 h. Water was used to clean graphene nanoplatelets and dried for a day, and finally, no sedimentation was noticed in the solution for two months. Batmunkh et al. [161] prepared the nanocomposite of Ag and TiO2 using a ball milling process, and water was used as dispersing media to prepare a hybrid nanofluid. From the above study, we can confirm that most hybrid nanoparticles can mix with water easily, whereas the optimum sonication time, surfactant ratio, and φ nanoparticle are key parameters for better stabilization of hybrid nanofluid.

Ethylene glycol (EG)-based single nanofluids

Robertis et al. [162] prepared EG based nanofluid by single-step technique. Copper nitrate hydrate, sodium hypophosphite monohydrate and polyvinyl pyrrolidone were used as a copper source, reducing agent and stabilizer, respectively. Nanofluid was prepared using the oven (frequency of 2.45 GHz and power of 400 W) and observed that the suspension was settled by about 28.5% in 50 days. They also mentioned that small particles can remain settling for long periods. Sharma et al. [163] mixed Ag particles in EG where silver nitrate was used as a precursor and poly acryl amide-co-acrylic acid (PAA-co-AA) was used as a dispersant. The nanoparticle size and stability were managed by the concentration of PAA-co-AA and the reaction condition. Tamjid and Guenther [164] prepared Ag-diethylene glycol-based nanofluid by VERL technique. The colloid was stirred for 5 minutes in an ultrasonicator to mix the nanoparticle in the host fluid properly. Philip [165] prepared the photoluminescent (PL) nanoparticles of Au with the particle size of 3, 4, 6, and 9 nm by chemical reduction of borohydride/citrate in the presence of PEG and tannic acid. An increase in PL intensity with the addition of KCl was observed for the colloids.

Mostafizur et al. [166] diluted Al2O3 with methanol to prepare nanofluid at different φ (particle size = 8 nm) by a two-step method. Ultrasonication homogenizer was used to reduce aggregation. A similar type of preparation was found by Esfe et al. [167]. Abdolbaqi et al. [168] prepared Al2O3 nanofluids by dispersing nanoparticles in two mixture ratios, glycol/water (40:60% and 60:40%), in a two-step method. Khdher et al. [169] used Al2O3 nanopowder (13 nm) with pure bio glycol as a base fluid to prepare nanofluid by two-step method. Raveshi et al. [170] mixed Al2O3 with SDBS, WEG 50 used as host fluid, suspension was sonicated in an ultrasonicator for 2 h and observed that no sedimentation was found even after 3 days. Chen et al. [171] used EG as the base fluid for preparing the TiO2 (25 nm) nanofluid by a two-step method. They investigated the stabilization and thermal properties with different sonication times (1–40 h). Yapic et al. [172] synthesized the TiO2 nanoparticle in PEG 200 over particle mass fraction up to 10% using a two-step method. The prepared nanofluid was agitated in an ultrasonicator for 6 h, and the suspension was stable for more than 4 days. Kulkarni et al. [173] prepared several nanofluids, including Al2O3 (45 nm), CuO (30 nm) and SiO2 (50 nm) nanoparticles with water and EG. The nanoparticle was mixed with EG-water solution (60:40), and to avoid accumulation during the experiment, the nanofluid sample was placed in an ultrasonicator bath for 2 h. Abdolbaqi et al. [174] prepared SiO2 water-based nanofluid, used in the experiment after appropriate dilution, and mixed it with glycol/water (BG/W) with the help of an ultra-sonicator for 2 h. The experiment used different φ of SiO2 nanofluids with two mixture ratios of BG/W at 20:80% and 30:70%.

Hu et al. [175] dispersed AlN nanoparticles (size of 20 nm) in ethanol where castor oil was used as a surfactant. The prepared solution was stirred in a magnetic stirrer and sonicated in an ultrasonicator for 10 min. They noticed that the nanofluid can be stable for over 2 weeks without agglomeration. A similar study was observed by Yu et al. [176]. Wozniak et al. [177, 178] mixed AlN in PPG 425 and PPG 2000, stirring the prepared solution for 3 h. They observed AlN-PPG 2000 solution was more stable than PPGs 425. Wozniak et al. [179] used AIN nanoparticles (< 100 nm), PPG and PPG 2000 to prepare nanofluid. PPG was used as the dispersive phase for nano-AlN. The liquid is commonly designated as PPG 2000 and AlN–PPG solution prepared by a two-step method. The powder was incrementally added to PPG continuously, and then, the suspension was homogenized. Subsequently, the dispersion was stirred for 40 min at 3000 rpm. It had been assumed that such an intensive stirring broke some AlN agglomerates. Various dispersants and physical treatments were involved in preparing AlN-nanofluids, and at least it can be stable for more than two weeks. Zyla and Fal [180] dispersed AIN (20 nm) nanoparticles in EG to prepare nanofluids. The prepared samples were subjected to mechanical stirring for 30 min, followed by ultrasonication for 200 min. Li et al. [181] prepared an EG-based SiC (30 nm) nanofluid with dispersants like polyvinyl pyrrolidone (PVP) and pH regulator sodium hydroxide (NaOH) by two-step method. SiC nanoparticle suspended in the weighed base fluid at different φ and magnetic stirring used for 1 h to mix the EG/SiC suspension. Simultaneously, a specific amount of PVP and NaOH (pH adjusted to 11) dispersant was added to ensure stable dispersion. An ultra-sonication homogenizer was continuously used for 12 h to obtain uniform nanofluid and was found stable after 30 days of preparation.

Moosavi et al. [182] mixed ZnO nanoparticles with different base fluids (EG and glycerol) in a two-step method. They used ammonium citrate as a dispersant and took the mass ratio of particle to dispersant as 1:1. They observed that the prepared suspension was stable for longer periods without any sedimentation and agglomeration. Yu et al. [183]dispersed ZnO nanoparticles (10–20 nm) in EG by stirring and sonicating 3 h for uniform dispersion. Based on the effect of sonication on particle size, it was mentioned that the size of the particle decreased rapidly in the first 3 h; after that, accumulation takes place, and particle size becomes larger. It can be concluded that there is an optimum time of sonication where maximum dispersion can be found. Suganthi et al. [184] prepared ZnO nanopowder using nitrate hexahydrate and ammonium carbonate by chemical precipitation method. Then, the ZnO nanoparticle was mixed in PG/water to prepare the nanofluid. ZnO–water nanofluid was prepared by sequential method in which 4% of ZnO–PG solution was prepared using probe ultrasonication. Water was added to the 4% ZnO–PG dispersion to prepare a 2% ZnO–PG–water nanofluid (PG:water = 50:50%). Other researchers have observed similar types of results [185, 186]. Li et al. [187] prepared EG-based nanofluid containing ZnO nanoparticles (30 nm) with different mass fractions between 1.75 and 10.5% by the two-step method. The nanofluid mixture was mixed with the help of magnetic stirring for 12 h and ultrasonicated for 4 h. Polyvinylpyrrolidone (PVP) was used as a dispersing agent in ZnO-EG nanofluid, and the prepared ZnO-EG nanofluid showed no sedimentation after 48 h. Lee et al. [188] suspended ZnO in EG to prepare nanofluid followed by one-step method (PWE). Saleh et al. [189] prepared ZnO nanoparticles using the chemical precipitation method and mixed those nanoparticles in EG using a magnetic stirrer and sonicator to prepare nanofluids. Kole and Dey [190] dispersed the appropriate amount of ZnO nanoparticles in the measured quantity of EG by ultrasonication (200 W). They observed the optimum time for sonication was 60 h so that the nanofluid could be stable for up to 1 month without any sedimentation. Moattar and Cegincara [191] dried ZnO nanoparticles with the help of an oven to remove moisture. The nanoparticle was mixed in PEG, and the colloid was stirred and agitated thoroughly to make a homogeneous nanofluid by an ultrasonic generator.

Meng et al. [192] dispersed 1 g of CNTs into 60 mL of HNO3. The particles were added to glycol, followed by a sonication process to obtain CNT glycol nanofluid and they noticed that the nanofluid remained stable for more than two months. Sheikhbahai et al. [193] prepared nanofluids by mixing Fe3O4 nanoparticles in EG and followed the sonication process for 1 h. The appropriate amount of water was added to the solution under agitation for 30 min before the experiment, and no sedimentation was found. Syam Sundar et al. [194] mentioned that nanoparticles prepared by the chemical precipitation method can be properly mixed using only an ultrasonication process for 2 h. Several other researchers have prepared EG-based stable nanofluid containing magnetic nanoparticles by different methods [195,196,197]. Heris [198] prepared CuO (40 nm)–EG/water (60/40)-based nanofluids at different φ of 0.1%, 0.2% and 0.5%. They used an agitator for 6 h to mix the nanoparticle with host fluid, then the solution was kept in a sonicator for 2 h and found that the nanofluid could be stable for up to 1 day. Liu et al. [199] prepared CuO (30 nm)/EG-water-based nanofluid with the help of a super-sonic water bath, surged for ~ 12 h and noticed that the prepared solution was stable for several days. Namburu et al. [200] used CuO nanoparticles (29 nm) with EG and water mixture (60:40). The prepared nanofluid was stirred and agitated for 30 min using an ultrasonicator. Kim et al. [201] synthesized TiO2 nanoparticles with water and EG, using SDS as a surfactant. The suspension was sonicated in an ultrasonicator for 1 h and stirred for 10 h to get a stabilized nanofluid. Lu et al. [202] dispersed Cu (20 nm) nanoparticles with water and ethanol (host fluid) using an ultrasonic box (25–40 kHz) for 10 h without any surfactant. Mostafizur et al. [203] suspended the spherical SiO2 nanoparticle (5–15 nm) in methanol at different φ by using a two-step method. First, the suspension was shaken in an incubator. Then, the suspension was stirred using an ultrasonication homogenizer to overcome strong cohesion between the particles and encourage even distribution of particles. Pang et al. [204] prepared SiO2 and Al2O3 nanofluids by mixing the nanoparticle in methanol, and a sonicator was used for 2 h. The value of ZP of Al2O3 nanofluid was more than 60 mV, and for SiO2 nanofluids, more than 30 mV, indicating good stability for both the nanofluids. From the above literature survey, we can assure that most of the nanoparticles can easily mix with EG, whereas optimum sonication time, particle and surfactant ratio are considered the key parameters for better stabilization of nanofluid.

Ethylene glycol (EG) based on hybrid nanofluids

Paul et al. [205] followed two-step method to prepare EG-based hybrid nanofluid. Initially, Al–Zn nanoparticles were prepared by mechanical alloying and diluted in EG using sonication and stirring. Toghraie et al. [206] prepared a nanofluid by mixing the same amount of ZnO and TiO2 nanoparticles into EG using a two-step method. Proper mechanisms like sonication, stirring process and pH value control are employed for a better mixture of nanoparticles in the host fluid. With a stirring of 0.5 h, the nanofluid at different φ was exposed to an ultrasonic processor for 6–7 hrs and no sedimentation was identified for a long time. Sundar et al. [207] prepared hybrid nanodiamond (ND)-nickel (Ni) nanoparticles by in situ method. Initially, ND nanoparticles were mixed in EG, and nickel chloride was added to the solution. Nanosperse was used as a surfactant to secure a homogenous mixture, and the suspension was sonicated for 1 h. ND–Ni-based nanofluids were prepared with 5 different base fluids of EG–water mixtures. Asokan et al. [208] prepared three different types of fluid (water + EG, mono and hybrid nanofluids) for application in compact heat exchangers. Al2O3and CuO nanoparticles were selected to prepare the hybrid (particle ratio 50:50) and mononanofluids mixed in water–ethylene glycol (60:40)-based mixture fluid. Three different φ (0.02%, 0.04% and 0.06%) of nanofluids were prepared, and the volume of each sample prepared was 8 L. Two-step method was selected to prepare the hybrid nanofluid, and PVP surfactant (ratio of surfactant and particle 1:10) was added in the sample to prevent accumulation. The hybrid nanofluid mixture was stirred for two hours and then kept in an ice bath to prevent the solution from overheating. Finally, an ultrasonicator probe was dipped into the solution.

Aravind and Ramaprabhu [209] prepared a stable nanofluid by mixing the graphene and graphene-MWCNT composite materials in water and EG with a sonication of 30 min. Abbasi et al. [210] prepared γ-Al2O3–MWNT hybrid nanofluids using a solvothermal process with ethanol. Aluminium acetate powder was mixed in absolute ethanol under stirring at a normal temperature for 30 min until the aluminium acetate powder dispersed completely. The pristine MWCNTs and the functionalized MWCNTs were added to the solution and synthesized using a sonicator at room temperature until no black agglomerates could be observed in the suspension. Farbod and Ahangarpour [211] prepared hybrid nanofluids using Ag and MWCNTS nanoparticles. Functionalization of MWCNTs executed by mixing the MWCNTs in sulphuric and nitric acid solution. The MWCNTs and pristine decorated with Ag nanoparticles with several φ. AgNO3 was kept in a storage vessel, and ethanol was mixed as a solvent and stirred for 15 min. MWCNTs were added with Ag nanoparticles and ethanol by sonication process and dried to get hybrid nanomaterials. Nearly 42 mg of Ag/MWCNTs nanomaterial was kept in a vessel, and 20 mL of water was mixed to prepare hybrid nanofluids. Afrand et al. [212] obtained a two-step method for preparing SiO2–MWCNT/SAE 40 and Fe3O4–Ag/EG hybrid nanofluids at different φ. The above survey confirmed that most researchers followed the two-step method, sonicator and surfactant, to prepare an EG-based hybrid nanofluid.

Engine oil (EO)-based single nanofluids

Saeedinia et al. [213] mixed CuO nanoparticles with a particle size of 50 nm in oil using an ultrasonicator and noticed that the suspension stable for 24 h and completely sedimented after a week. Colangelo et al. [214] prepared Al2O3 (45 nm)-Therminol 66-based nanofluid, used as a surfactant, with an ultrasonic vibrator. Sonawane et al. [215] prepared stable Al2O3- aviation turbine fuel (ATF) nanofluid where OA and Tween 20 LR were used as surfactants, and the suspension was sonicated for different hours. They found that the prepared solution was independent of sonication time and stable for more than 24 h. Choi et al. [216] mixed Al2O3 (13 nm) and AlN (50 nm) nanoparticles with transformer oil (TO) at φ = up to 4%. Nanoparticles were added with n-hexane and surfactant (OA), and the mixture was subjected to bead-milling with ZrO2 beads. The solution was added with TO and dried off the n-hexane using a vacuum evaporator. Xuan and Li [47] prepared Cu–TO and Cu–water-based nanofluids, where the particle size was 100 nm. The TO–Cu suspension stabilized for one week without sedimentation, using 22% of OA. Different percentages of OA were used, and the solution was vibrated for 10 h in an ultrasonicator. Meanwhile, the water–Cu solution stabilized using 9% laurate salts and sonicated in an ultrasonicator. Then, it was noticed that the solution could stable for more than 1 day.

Li et al. [217] followed a two-step method to prepare diathermic oil based SiC nanofluid. SiC nanoparticles were suspended in the weighted base fluid at different φ and stirred at 500 rpm for 1 h. Then, an ultra-sonication homogenizer was used to sonicate the solution for 6 h to obtain a uniform nanofluid. They found no sedimentation was observed in the nanofluid after 3 days of preparation. Wei et al. [218] prepared Fe3O4 nanoparticles by coprecipitation, mixing the OA to change the nanoparticles. Kerosene was mixed into the mixture after 1 h, and the phase-transfer process occurred spontaneously. After removing the aqueous phase, Fe3O4kerosene-based nanofluid was found at φ = 1%. Goshayeshi et al. [219] considered OA as surfactant, kerosene as host fluid, as well as α- and γ-Fe2O3 nanoparticles prepared from neutrino nanovation to prepare nanofluids. They prepared a 2% nanofluid by dispersing the Fe2O3 nanoparticles with host fluid and stirring constantly, and a bath sonicator sonicated the fluid up to 5 h for better stabilization.

Timofeeva et al. [220] added SiO2 nanopowder (15 nm) in therminol 66 (TH66) as host fluid, where BAC, BZC, and CTAB were used as a surfactant. Surfactant dispersed into the host fluid, and the mixture was stabilized by a magnetic stirrer and sonicator 10 times, 5 min each. The prepared solution can be stable for at least 7 days without any visual phase separation and noticed that BAC was the best surfactant for SiO2/TH66. Lee et al. [221] used AlN nanoparticles and two dispersing solvents: coolant oil and n-Hexane. They prepared nanofluids by adding 0.5% of AlN nanoparticle into coolant oil or n-Hexane, then premixed by a homogenizer for 30 min. A surfactant (polyoxyethylene alkyl acid ester, 30 mass% of the particles) was added to the slurry to enhance the dispersion stability. The bead mill operated using different bead sizes and rotation speeds. Katiyar et al. [222] dispersed the required proportion (0–7%) of nanoparticles such as Fe, Ni, and Co in the fluids by utilizing a mechanical stirrer for 60 min at 800–1200 rpm. Subsequently, sonication was carried out for 2–3 h duration at 60% amplitude by utilizing a probe-type ultra-sonicator to break and disperse any accumulation or cluster of nanoparticles. OA was used in minute proportion as a surfactant in the oil-based nanofluids and CTAB for the EG-based magnetic nanofluids to enhance the stability of the suspension and synthesized fluid observed to have a stable of 7–10 days. Agrawal et al.[223] synthesized the CuO nanoparticle using the wet chemical method for two different precursors. Nanofluid from the synthesized nanoparticles was prepared by using a two-step method. Copper acetate, copper sulphate and sodium hydroxide (pellets) were used to synthesize analytic reagent grade, where water, EG, and EO were used as base fluids. Initially, the suspension was stirred for 1 h to increase the stability and remove the accumulation. The nanofluid suspension was sonicated for 30 min using a probe ultrasonic processor (at 220 V), and they observed that the nanofluid was stable for up to 10 days without any sedimentation. Sateesh et al. [224] prepared different nanofluids by dispersing Al2O3 nanoparticles in diary scum oil methyl ester for dual fuel engine application with the help of a homogenizer and an ultrasonicator, used to disperse nanoparticles and reduce agglomeration for 1 h. From the above study, it can be concluded that EO-based nanofluid can also be synthesized and stabilized for long periods, mostly with the help of surfactant and sonication.

Engine oil (EO) based on hybrid nanofluids

Han et al. [225] synthesized hybrid sphere/CNT particles by producing the nanoparticle through spray pyrolysis. Al(NO3)3 and Fe (NO3)3 nanoparticle composites prepared by thermal decomposition and hybrid sphere/CNT particles were compiled on a membrane filter and dispelled in oil. Kato et al. [226] prepared a polymer hybrid by adding PbTiO3 in silicone oil. PbTiO3 precursor was synthesized from titanium isopropoxide, lead acetate, and 2-(methacryloyloxy) ethyl acetoacetate (MEAA). DI water was used to hydrolyse solid lead acetate and EG monomethyl ether solution. The sonication process was also done for proper dilution of suspension, followed by a stirring process up to 24 h. Botha et al. [227] prepared hybrid nanofluids by one-step method containing silica and silver nanoparticles. Kumar et al. [228] followed the in situ method for preparing Cu–Zn (50:50) hybrid nanofluids. The calculated amount of nanopowder was mixed in vegetable oil and sonicated for 2 h to obtain a stable nanofluid. Esfe et al. [229] used Fe3O4–Ag, MWCNTs, SiO2 nanoparticles and EO (base fluid) to prepare hybrid nanofluids. The measured amount of each component was mixed, stirred and sonicated to obtain a stable condition of the suspension, and the prepared suspensions showed great stability with negligible deposition. Hisham et al. [230] mixed the cellulose nanocrystal and CuO nanoparticle with SAE 40 to prepare hybrid nanofluids followed by a two-step method and noticed that slight sedimentation was found in the 4th week of the preparation. Soltani et al. [231] followed a two-step method to prepare tungsten oxide–MWCNTsEO-based hybrid nanofluids. Once the samples were ready, they were placed on a magnetic stirrer and ultrasonic device for 30 min to have more stability dispersion. Liu et al. [232] dispersed MWCNT–TiO2 nanoparticles in SAE 20 by a two-step method with the help of a homogenizer. To reduce the nanoparticle clustering in the prepared sample and to improve the stability, an ultrasonic processor was used for 3 h. They observed no sedimentation in any nanofluid sample even after 72 h. Mousavi et al. [233] considered ZnO and molybdenum disulfide (MoS2) as the precursor, SAE-40 engine oil as the base fluid and Triton X-100 as the surfactant. Initially, the ZnO and MoS2 particles were added to the oil at different φ. Suspension was mixed using a stirrer for about 3 h and agitated with an ultrasonic agitator for about 45 min. More number of kinds of literature was not available for EO-based hybrid nanofluids. Based on the previous survey, preparing and stabilizing the EO-based hybrid nanofluid is possible.

Characterization of single and hybrid nanofluids:

A long-term and homogeneous stable nanofluid is a big challenge for scientists and researchers to prepare and apply it for real life because the properties of unstable nanofluids varying with time. Researchers used different techniques and suggested various tests to confirm the homogeneity and stability of the nanofluid. Stability means the nanoparticle is mixed uniformly in the host fluid without agglomeration. Aggregation in the nanoparticle is calculated by the frequency of collisions caused by Brownian motion. The stability of the nanofluid is also related to its electro-kinetic properties, so pH control is required to keep the fluid away from the isoelectric point, which increases stability due to strong repulsive forces. The stability of nanofluid depends on different parameters such as pH, preparation method, characteristics of nanoparticle, sonication time, type of shape and size of the nanoparticles with different base fluids φ and surfactants, etc. The important factor that makes nanofluid unstable is the nanoparticle’s aptitude for aggregation due to its high surface charge. The stability of the nanofluid is also calculated by adding van der Waals attractive forces and electrostatic repulsive forces [234]. When electrostatic repulsive forces are more than attractive, stability is achieved. Agglomeration in the nanofluids not only settles and clogs microchannels but also varies thermal properties like thermal conductivity, viscosity, density, etc. So, the aim of the researcher should be to prepare stable nanofluids by using different parameters. The nanoparticle’s characterization technique is to identify the nature of the chemical, particle size distribution, morphological behaviour, stabilization, size of the aggregation and many more. This technique is important for verifying particles’ interaction or reaction with the base fluid in each nanofluid preparation step. Few techniques have been used in previous studies, such as:

  1. 1.

    Fourier transforms infrared spectroscopy (FTIR): This is required to study the surface chemistry of solid or liquid particles.

  2. 2.

    Scanning electron microscopy (SEM) is done to identify the microstructure and morphological study of nanoparticles or nanostructure materials. Field electron scanning electron microscopy (FESEM) is also a similar technique to SEM.

  3. 3.

    Transmission electron microscopy (TEM) using the wet-TEM technique to analyse the dispersion state is similar to SEM but much higher resolution than SEM.

  4. 4.

    X-ray diffraction (XRD) images are done to identify the particles’ crystal structure, and the vibration sample magnetometer (VSM) measures the magnetic properties.

  5. 5.

    Energy-dispersive X-ray spectroscopy (EDX) is required for a sample’s elemental analysis or chemical characterization. Thermal gravimetric analysis (TGA) is done to know about the influence of heating and melting on the thermal stability of nanoparticles.

  6. 6.

    UV–Vis spectroscopy is required in analytical chemistry to quantitatively calculate different analytes, such as transition metal ions, highly conjugated organic compounds, and biological macromolecules. Infrared absorption spectroscopy is used to identify chemical substances or functional groups in solid, liquid, or gaseous forms.

  7. 7.

    DLS analysis is conducted to identify the particle size distribution of nanoparticles in the host fluid.

  8. 8.

    Inductively coupled plasma–optical emission spectroscopy (ICP-OES) is used to detect chemical elements. Particle size analyser (PSA) and ZP is tested to identify the stability of the nanofluids.

SEM and TEM are two usual methods to identify the presence of particles in nanofluids. Still, these are not enough to calculate statistical results because only from 10 to 100 particles are identified among many numbers of particles. The particle size was measured using the DLS process to confirm particle size over a larger sample size. The particle undergoes random thermal motion by Brownian movement, while the size of the particle is within 10 µm. The speed of particle movement differs according to the Stokes–Einstein equation depending on the particle size. Lee et al. [235] mentioned that particles small in size move faster than the larger particles. A summary of a few research works on the stability and characterization of single and hybrid nanofluids are mentioned in Tables 4 and 5, respectively.

Table 4 Summary of characterization and stability of single nanofluids
Full size table
Table 5 Summary of characterization and stability of hybrid nanofluids
Full size table

Application of single and hybrid nanofluids

Nanofluids have improved thermal properties and energy efficiency in various industries and solar systems. Nanofluid can be applied in several areas, such as HT intensification, different kinds of cooling such as electronic, industrial, &nuclear systems, transportation, space and defence, mass transfer enhancement, intensified microreactors, sensing and imaging, energy, mechanical, biomedical and few other applications such as microreactors, brake fluids, microbial fuel cell and unique optical filter properties. Convective HT is one of the most widely studied thermal phenomena in nanofluids, related to several applications. High heat flow processes have conceived important concern for new technologies to increase HT, and microprocessors have become smaller and more energetic, so the heat flux has developed over time, leading to new challenges in thermal systems. In an automotive system, enhanced HT may lead to a smaller heat exchanger for cooling; the result showed that the vehicle’s mass was reduced. Managing high thermal loads with conventional coolants possessing inferior HT characteristics is very difficult. Nanofluid plays an important role in several application areas, leading to a major impact in developing future generations of equipment. The application of single and hybrid nanofluids in different areas is mentioned in Tables 6–10. In addition, Figs. 10–13 indicate the experimental setup for the use of single and hybrid nanofluids in various regions.

Table 6 Application of single nanofluid in the area of electronics appliances
Full size table
Table 7 Application of single nanofluids in the area of automobile
Full size table
Table 8 Application of single nanofluids in the area of solar energy
Full size table
Table 9 Application of single nanofluids in the area of healthcare/biomedical
Full size table
Table 10 Application of hybrid nanofluids in other areas
Full size table
Fig. 10
figure 10

Shows the experimental setup of a flat plate solar collector [286], b direct absorption solar collector [271]

Full size image
Fig. 11
figure 11

Shows the experimental setup of cylindrical microchannel heat sink [251]

Full size image
Fig. 12
figure 12

Shows the experimental setup of car radiator [260]

Full size image
Fig. 13
figure 13

Shows the experimental setup of plate heat exchanger [323]

Full size image

Challenges of single and hybrid nanofluid

Industries need better thermal management systems for their miniaturized and efficient equipment. Since all the conventional methods have reached their maximum development of new fluids that can transfer heat more efficiently and effectively, it is necessary. Nanofluids have several advantages that can reduce cooling system size, improve reliability, and increase energy and fuel efficiency. It can reduce pumping power compared to a micro-sized particle system. Several challenges and problems of nanofluids are addressed and overcome before their commercial application in the industries. The main issues are as follows:

  1. (a)

    Stability of nanoparticle dispersion: The nanofluid requires long-term stability. Systematic preparation methods for reliable and stable nanofluid have not been established. Preparation of the homogeneous solution is a big challenge since nanoparticles always form aggregates due to the strong van der Waals interaction. The toxicity of some nanofluids is high.

  2. (b)

    Degradation of nanofluids: The particle in the static nanofluid subjected to Brownian motion may lead to diffusion, limited agglomeration and gravitational settlement. So, it will affect the properties of nanofluid and the feasibility of real applications.

  3. (c)

    Cost of nanofluids and Production process:Another concern for nanofluids is high cost. The size of the nanoparticle causes high manufacturing costs. For production of the nanofluid, it requires a complex and advanced instrument. Due to the difficulties in its production, the cost of nanofluid is high, and there may be a chance of environmental damage when it is drained out after its usage.

  4. (d)

    Higher viscosity increased pumping power and pressure drops: The flow of nanofluid showed a high-pressure drop because of the increase in density and viscosity compared to the conventional fluid, whereas the high pumping power requirement may be the obstacle to enhancing efficiency.

  5. (e)

    Lower specific heat and boiling characteristic: Compared with the host fluid, the specific heat of the nanofluid is low. So, to exchange more heat, researchers have mentioned that the working fluid should have high specific heat. When φ of the nanoparticle increases, the surface T of the nanofluid increases and causes overheating.

  6. (f)

    Corrosion and erosion: The nanoparticle presence in nanofluid may lead to erosion and sometimes corrosion, which may damage the equipment.

  7. (g)

    Thermal performance in turbulent flow: Apart from nanofluid’s thermal properties, researchers also focused on the HT performance of nanofluid. Most of the research papers reported that this property was increased with applying nanofluid. The thermal performance of nanofluid in turbulent flow must be addressed.

  8. (h)

    Preparation of hybrid nanofluids: The major technical challenge is preparing and stabilizing the hybrid nanofluids; it can be obtained with proper dispersion. Mixing two different nanoparticles may pose a surface charge problem, so the selection process for the preparation should consider the following points: an appropriate combination of particles and proper particle bonding. The synthesis method may suppress these in-stabilities to some extent and apply this hybrid nanofluid for solar devices and other HT applications. Hybrid nanofluids are to be prepared to be used for industrial applications. More research needs to be done to understand the complex mechanism behind the augmentation of HT with hybrid nanofluids.

  9. (i)

    The selection process of the particles for hybrid nanofluid: The selection process of hybrid nanoparticles is crucial to obtain the maximum effect. The problems are technical perception, preparation cost and experimentation. More knowledge required about the concept of thermal network and rheological behaviour of hybrid nanofluid. However, many challenges need to be analysed to overcome for various applications.

Future direction of Nanofluids

In the previous section, we have discussed about the application and challenges of normal and hybrid nanofluids. The future research probably focuses on developing efficient energy transfer methods by using hybrid nanofluids. Nanofluid particles can be used to develop microchips that lead to better computers. Additionally, nanofluids can be used as different industrial cooling (microchips and automobile), nanofluids can implement on biomedical (nanocryosurgery, drug delivery, cancer therapeutics, cryopreservation). Hybrid nanoparticles can be used to develop new and improved materials with different properties. Improvement in TC of hybrid nanofluids also helps to make better engines. Nanoparticles can be utilized in various areas, including removing pollutants from water and air, cleaning up contaminated soil, improving the performance of batteries and fuel cells, enhancing sunscreens and personal care products, and serving as food additives to advance preservation and packaging of food items.

Conclusions

Nanofluid is necessary due to its magical properties such as thermal conductivity, viscosity, HT coefficient, and optical properties. Compared with single nanofluid, hybrid nanofluid shows improvement in thermophysical properties. The main aim of this present study is to give detailed knowledge about the preparation of different types of single and hybrid nanofluid with various base fluids, preparation techniques, stabilization processes, characterization, application and challenges.

  • The difference between single and hybrid nanofluid is explained in the study’s first part. Then, several techniques for the single-step and two-step methods of single and hybrid nanofluid preparation were demonstrated with different base fluids (water, EG, and EO).

  • Various surfactants are described that help stabilize the prepared solution, and several characterization methods are suggested for the prepared solution to find stability.

  • The author has identified the application of single and hybrid nanofluids in different areas such as electronics, automobiles, solar energy, and healthcare. A few challenges have been identified about nanofluids in this literature, which must be checked carefully before being implemented for industrial application.

  • From the above literature study, we have found that all kinds of nanoparticles and hybrid nanoparticles can mix with different base fluids by sonicator and stirrer for better stabilization of the prepared solution; low φ, particle-to-surfactant ratio, optimum sonication time, particle size and chemical composition of nanoparticle are the key parameters. This review article based on single and hybrid nanofluid will help the present and future researchers working in the same field.