Thermal Conductivity Enhancement of Metal Oxide Nanofluids: A Critical Review

Advancements in technology related to energy systems, such as heat exchangers, electronics, and batteries, are associated with the generation of high heat fluxes which requires appropriate thermal management. Presently, conventional thermal fluids have found limited application owing to low thermal conductivity (TC). The need for more efficient fluids has become apparent leading to the development of nanofluids as advanced thermal fluids. Nanofluid synthesis by suspending nano-size materials into conventional thermal fluids to improve thermal properties has been extensively studied. TC is a pivotal property to the utilization of nanofluids in various applications as it is strongly related to improved efficiency and thermal performance. Numerous studies have been conducted on the TC of nanofluids using diverse nanoparticles and base fluids. Different values of TC enhancement have been recorded which depend on various factors, such as nanoparticles size, shape and type, base fluid and surfactant type, temperature, etc. This paper attempts to conduct a state-of-the-art review of the TC enhancement of metal oxide nanofluids owing to the wide attention, chemical stability, low density, and oxidation resistance associated with this type of nanofluid. TC and TC enhancements of metal oxide nanofluids are presented and discussed herein. The influence of several parameters (temperature, volume/weight concentration, nano-size, sonication, shape, surfactants, base fluids, alignment, TC measurement techniques, and mixing ratio (for hybrid nanofluid)) on the TC of metal oil nanofluids have been reviewed. This paper serves as a frontier in the review of the effect of alignment, electric field, and green nanofluid on TC. In addition, the mechanisms/physics behind TC enhancement and techniques for TC measurement have been discussed. Results show that the TC enhancement of metal oxide nanofluids is affected by the aforementioned parameters with temperature and nanoparticle concentration contributing the most. TC of these nanofluids is observed to be actively enhanced using electric and magnetic fields with the former requiring more intense studies. The formulation of green nanofluids and base fluids as sustainable and future thermal fluids is recommended.


Introduction
To enhance heat dissipation and efficiency of thermal systems, an improvement in thermal fluids is necessary, especially when different heat-enhancing methods have been exhausted and presently reaching their practicable limits [1,2]. This challenge ignited the engineering of superior thermal fluids with higher TC compared with the conventional thermal fluids (engine oil (EO), water, and ethylene glycol (EG)) as pioneered by Maxwell, Ahuja, and Masuda [1,3,4]. The first two researchers intended to enhance the thermal conductivity (TC) of conventional thermal fluids by the suspension of micro-scaled particles of metals and non-metals known to have higher TC than the conventional base fluids. range of 0.615-0.712 W/m K whereas CuO + Cu nanofluids exhibited a TC range of 0.629-0.779 W/m K, all at the temperature range of 25-50 °C. Maximum TC was recorded for 2.5 g CuO + 1.5 g Cu nanofluid.
Progress in nanofluid research demonstrated the use of either green nanoparticles or green base fluids for the formulation of green nanofluids [48][49][50][51][52]. TC improvement of 16.1% was determined for EG-DIW (50:50)-based palm kernel nanofluid (at 0.5 vol% and 60 °C) [48]. With an EG-based hybrid nanofluid formulated using fruit bunch and GO nanoparticles, TC was recorded to be improved [50]. Bioglycol-based Al2O3 nanofluids were reported to yield higher TC compared with the use of conventional base fluids [51]. The deployment of a magnetic field to promote the TC of nanofluids has also been published [53][54][55].
The huge number of literature available in the public domain concerning studies on the measurement and enhancement of TC via the deployment of nanofluids (mono-particle and hybrid) has necessitated an updated review of this subject. Owing to the importance of TC to nanofluid applications, such as heat transfer and efficiency in thermal systems and devices, this present paper has focused on the TC of metal oxide nanofluid as they have attracted more attention, subject to their chemical stability, low density, and oxidation resistance. The effect of temperature, volume/weight concentration, nano-size, sonication, shape, surfactants, base fluids, alignment, TC measurement techniques, and mixing ratio (for hybrid nanofluid) on the TC of metal oxide nanofluids has been compiled and discussed. The measurement techniques and mechanisms related to the TC of nanofluids have also been presented. A special feature of this work is the deployment of the magnetic and electric fields, and green metal oxide nanofluid as active and passive techniques, respectively, to improve the TC of metal oxide nanofluids, which is lacking in previous review studies. In addition, the influence of alignment on TC of metal oxide nanofluid is discussed as an under-reported parameter affecting nanofluid TC. This work aims to present a holistic document on the TC of metal oxide nanofluids which will immensely contribute to nanofluid research and benefit the research community. The trend of nanofluid TC from 1999 to 2022 is provided in Figure 1 and in Table 1.   This paper is divided into eight sections. The first section is the introduction of this paper which entails the motivation and objectives of this work. Techniques deployed in the measurement of TC of nanofluids are highlighted and discussed in Section 2. The TC enhancement mechanisms and contributing factors are presented and deliberated in Sections 3 and 4, respectively. TC enhancement techniques and green nanofluid development studies have been compiled and discussed in Sections 5 and 6, respectively. The challenge and future research, and conclusion are presented in Sections seven and eight, respectively.

TC Measurement Methods
The need to measure the properties of materials, especially thermal fluids, is very important and strongly related to the application of the same. The development of an appropriate measuring technique is seriously connected to the state, type, physical, and chemical composition of the material, the physics and mechanisms behind the measured property, etc. Additionally, reliability and correctness of the measured properties is important. TC is an indicator of the heat transfer ability of a material. In the quest to measure the TC of nanofluids via an experimental approach by researchers, various techniques have been developed and reported in the literature. These techniques are generally classified as steady-state, transient, and thermal comparator techniques [31]. The steady-state method is sub-divided into parallel-plate and cylindrical cell methods while the transient technique is further classified as 3ω, temperature oscillation, transient hot-wire, laser flash, and thermal constant analyzer techniques. Figure 2 illustrates the classification of nanofluid TC measurement techniques. Additionally, the transient hot-wire method is further categorized as transient short hot-wire and liquid metal transient hot-wire methods [35]. TC is mathematically expressed based on the Fourier law, as given in Equation (1) [29]: where κ = TC W/(m K); Q = quantity of heat passing through a cross-sectional area A (m 2 ) which leads to a temperature difference; dT/dx = temperature gradient over a distance of dx (K/m).
Nanomaterials 2023, 12, x FOR PEER REVIEW 6 of 2 nanofluid was found to yield higher enhancement (16.5%) than that conducted using th laser flash approach (4.95%) [74]. A comparison study conducted on the use of differen TC techniques of transient hot wire, laser flash, and thermal constant analyzer showe that the transient hot wire produced the best results in terms of repeatability and precisio [75]. Figure 3 presents a comparison of measurement techniques for the TC of Al2O nanofluids. It is worth mentioning that all the TC techniques are well-developed as the have undergone improvement over time. For further studies on the design, developmen and evolution, operation, and uncertainty analysis of all the techniques, please see th literature [29,71].  Of the above-mentioned TC measuring techniques, the transient hot wire and the thermal constants analyzer are the most used techniques for measuring the TC of nanofluids [29]. These techniques fall under the broad category of transient technique that is characterized by a local temperature difference varying as dependent on time. The design and construction of a high-precision and accurate thermal constants analyzer and transient hot-wire measurement device is challenging. However, the use of the transient hot wire and the thermal constants analyzer techniques for nanofluid TC measurement are found to be associated with systematic errors due to natural convection currents and the capacitance effect within the measured nanofluids [71]. At higher temperatures and using the transient hot wire technique, the initiation of natural convection is reported to lead to higher TC than obtained when the steady-state method is used [31]. Other sources of systematic errors in nanofluid TC measurement include dependence and high sensitivity to samples' initial conditions, nanofluid stability difficulty, nanofluid concentration, and specific heat of nanofluid components [72].
Although the devices developed via the mentioned techniques have different degrees of sophistication, they have their merits and demerits. The deployment of different techniques for measuring nanofluid TC is reported to be marked by inconsistency in measured values [71]. The use of the transient hot wire approach was observed to measure a higher TC for water-based Ag and Al 2 O 3 nanofluids compared to the use of the laser flash technique for the same purpose [73]. This was due to the demonstration of more collision flux with the wall by the nanoparticles using the transient hot wall approach. Additionally, the engagement of the transient hot wire technique to measure the TC of Al 2 O 3 /water nanofluid was found to yield higher enhancement (16.5%) than that conducted using the laser flash approach (4.95%) [74]. A comparison study conducted on the use of different TC techniques of transient hot wire, laser flash, and thermal constant analyzer showed that the transient hot wire produced the best results in terms of repeatability and precision [75]. Figure 3 presents a comparison of measurement techniques for the TC of Al 2 O 3 nanofluids. It is worth mentioning that all the TC techniques are well-developed as they have undergone improvement over time. For further studies on the design, development and evolution, operation, and uncertainty analysis of all the techniques, please see the literature [29,71]. Nanomaterials 2023, 12, x FOR PEER REVIEW 6 of 28 nanofluid was found to yield higher enhancement (16.5%) than that conducted using the laser flash approach (4.95%) [74]. A comparison study conducted on the use of different TC techniques of transient hot wire, laser flash, and thermal constant analyzer showed that the transient hot wire produced the best results in terms of repeatability and precision [75]. Figure 3 presents a comparison of measurement techniques for the TC of Al2O3 nanofluids. It is worth mentioning that all the TC techniques are well-developed as they have undergone improvement over time. For further studies on the design, development and evolution, operation, and uncertainty analysis of all the techniques, please see the literature [29,71].

TC Enhancement Mechanisms
Early studies on nanofluid heat conduction show enhancement of up to 40% in TC with a nanoparticle concentration of less than 5% [5,76,77]. Different mechanisms also have been proposed for this anomalous enhancement [75,78] which include static mechanisms-nanolayering, aggregation and percolation, interface thermal resistance, fractal geometry, and dynamic mechanisms-Brownian motion, ballistic nature of nanoparticles, and nanoscale convection.

Brownian Motion of the Nanoparticles
Generally, there are three types of motion regarding the movement of nanoparticles in nanofluids. These are Brownian, thermophoretic, and osmophoretic motions which are

TC Enhancement Mechanisms
Early studies on nanofluid heat conduction show enhancement of up to 40% in TC with a nanoparticle concentration of less than 5% [5,76,77]. Different mechanisms also have been proposed for this anomalous enhancement [75,78] which include static mechanisms-nanolayering, aggregation and percolation, interface thermal resistance, fractal geometry, and dynamic mechanisms-Brownian motion, ballistic nature of nanoparticles, and nanoscale convection.

Brownian Motion of the Nanoparticles
Generally, there are three types of motion regarding the movement of nanoparticles in nanofluids. These are Brownian, thermophoretic, and osmophoretic motions which are due to force, temperature difference, and concentration gradient, respectively. The anomalous improvement in nanofluid TC was linked to the effect of Brownian motion which involves the random motion of nanoparticles in the base fluid owing to continuous bombardment of the particles and base fluid molecules. During Brownian motion, first there is thermal transport due to particle-particle interaction leading to improved TC as nanoparticles have a high volume-to-area ratio. The second is heat transfer via micro-convection due to particle-fluid interaction [30]. Brownian motion has the most effect on nanofluid TC than thermophoretic and osmophoretic motions [79]. However, the insignificance of Brownian motion to TC improvement of nanofluid has been reported [71].

Nanolayer Effect
The nanolayer is the ordered solid-fluid interface formed owing to the strong particlefluid force of interaction. The TC of the nanolayer is reported to be more than that of the bulk base fluid and lower than that of the nanoparticle [80]. It is said to function as a thermal bridge between the base fluid and the nanoparticle as the thickness of the nanolayer increases the concentration of nanoparticles in the base fluid causing TC enhancement of nanofluids [30,80]. The presence of a particle-fluid interface introduces an interfacial thermal resistance called "Kapitza resistance", which serves as an obstacle to heat transfer and therefore reduces the overall TC within the system. Although, nanolayer thickness is of the order of a nanometer, but due to the high specific surface area of nanoparticles, the nanolayer effect becomes critical and plays a key role in heat transfer across the particlefluid interface [71].

Nanoparticle Clustering
Nanoparticle constituent of nanofluid clusters as the distance between nanoparticles becomes smaller during the collision as the weak force of attraction (van der Waals) increases [71]. At high nanoparticle concentration, nanoparticle clustering possibility increased. The clustering of nanoparticles in nanofluids has been reported to improve nanofluid TC [81]. This is due to localized rich-particle portion development with lower thermal resistance to heat transfer compared to the less-particle portion. The formation of larger particle-free portions due to the settling of heavier aggregates lowers the TC. The cluster (particle-rich portion) of nanoparticles in the nanofluid contains more particles than the less-particle portion leading to a quicker transfer of heat [30].

Ballistic Nature of Nanoparticles
In solid and micro-scale, heat is transferred as phonons that are formed and propagated at random and dispersed by one another [71]. Heat is conducted in a solid material via the vibration of atoms jointly held together. The vibrating arrangement of atoms releases or losses energy in quantized form as a phonon. Thus, a phonon performs a key role in the TC of a material. In a hot region, a higher phonon density exists compared to that of a cold region, thus implying heat transfer is largely due to phonon diffusion subject to temperature gradient [30]. This is easily related to the Ballistic behaviour of nanoparticles as the size of nanoparticles is smaller than the atomic scale phenomenon of phonon heat transfer mechanisms. Higher ballistic phonon transport mechanisms are experienced in a nanofluid as the nanoparticle size reduces [82].

Concentration
Factors contributing to the TC of nanofluids are provided in Figure 4. Nanofluid is formulated by the suspension of nanoparticles in a base fluid. Increasing the quantity of nanoparticles suspended in the base fluid will directly increase the concentration of the nanoparticles in the base fluid. The presence of nanoparticles is expected to enhance the TC of the formulated nanofluid as the existence of Brown motion and other mechanisms aid TC enhancement. Two-fold studies have been published concerning the effect of con- centration on the TC of nanofluids. Classical studies for both mono-particle and hybrid nanofluids were conducted at room temperature to measure their TC while subsequent works measured TC under varying temperatures. The work of [37] revealed the TC enhancement of water-based TiO 2 and Al 2 O 3 nanofluids as the concentration increased, while no improvement in TC was observed for SiO 2 /water nanofluid. An increase in volume concentration (1-5 vol%) was observed to augment the TC of water and EG-based CuO and Al 2 O 3 nanofluids with a maximum enhancement of 20% for CuO/EG nanofluids [76]. The TC of Al 2 O 3 and CuO nanoparticles suspended in vacuum pump oil, EG, engine oil, and DIW at room temperature demonstrated augmentation with concentration with a maximum enhancement for Al 2 O 3 /EG (40% with 8 vol%) [69]. With a maximum enhancement of 1.44%, the TC of Al 2 O 3 /water nanofluids showed an improvement as the volume fraction increased from 0.01% to 0.3% [63]. An enhancement of the TC of 0.2 vol% TiO 2 /water nanofluid measured at room temperature was reported [61]. aid TC enhancement. Two-fold studies have been published concerning the effect of concentration on the TC of nanofluids. Classical studies for both mono-particle and hybrid nanofluids were conducted at room temperature to measure their TC while subsequent works measured TC under varying temperatures. The work of [37] revealed the TC enhancement of water-based TiO2 and Al2O3 nanofluids as the concentration increased, while no improvement in TC was observed for SiO2/water nanofluid. An increase in volume concentration (1-5 vol%) was observed to augment the TC of water and EG-based CuO and Al2O3 nanofluids with a maximum enhancement of 20% for CuO/EG nanofluids [76]. The TC of Al2O3 and CuO nanoparticles suspended in vacuum pump oil, EG, engine oil, and DIW at room temperature demonstrated augmentation with concentration with a maximum enhancement for Al2O3/EG (40% with 8 vol%) [69]. With a maximum enhancement of 1.44%, the TC of Al2O3/water nanofluids showed an improvement as the volume fraction increased from 0.01% to 0.3% [63]. An enhancement of the TC of 0.2 vol% TiO2/water nanofluid measured at room temperature was reported [61]. Under increasing volume concentration of TiO2/EG, the TC was observed to increase with an improvement of 18% [83]. The TC of water-based CuO and Al2O3 nanofluids was found to enhance with an increase in concentration from 1 vol% to 4 vol% at room temperature [62]. Enhancements of 2-9.4% and 6.5-14% at room temperature were recorded for Al2O3 and CuO nanofluids at 1 vol% and 4 vol%, respectively. The TC of Fe3O4/DIW nanofluids showed an increase as the volume concentration increased from 0.2-2 vol% with a maximum enhancement of 48% [64]. An increase in volume concentration of Fe3O4/kerosene nanofluids (0-1 vol%) was observed to directly enhance the TC [65]. The TC of MgO/glycerol nanofluid was enhanced by 19% as the volume fraction increased from 0.5% to 4% [14]. At 30 °C, the TC of ZnO/EG nanofluid was enhanced by 40% when the nanofluid concentration was increased from 0.5 vol% to 3.75 vol% [84]. The TC of EGbased αFe2O3 and Fe3O4 nanofluids at a volume fraction of 0.69% was observed to be augmented by 15% and 11%, respectively [85]. The TC of Al2O3/DIW nanofluids was enhanced by 15% for a weight fraction of 0.8% at room temperature [86]. Under increasing volume concentration of TiO 2 /EG, the TC was observed to increase with an improvement of 18% [83]. The TC of water-based CuO and Al 2 O 3 nanofluids was found to enhance with an increase in concentration from 1 vol% to 4 vol% at room temperature [62]. Enhancements of 2-9.4% and 6.5-14% at room temperature were recorded for Al 2 O 3 and CuO nanofluids at 1 vol% and 4 vol%, respectively. The TC of Fe 3 O 4 /DIW nanofluids showed an increase as the volume concentration increased from 0.2-2 vol% with a maximum enhancement of 48% [64]. An increase in volume concentration of Fe 3 O 4 /kerosene nanofluids (0-1 vol%) was observed to directly enhance the TC [65]. The TC of MgO/glycerol nanofluid was enhanced by 19% as the volume fraction increased from 0.5% to 4% [14]. At 30 • C, the TC of ZnO/EG nanofluid was enhanced by 40% when the nanofluid concentration was increased from 0.5 vol% to 3.75 vol% [84]. The TC of EG-based αFe 2 O 3 and Fe 3 O 4 nanofluids at a volume fraction of 0.69% was observed to be augmented by 15% and 11%, respectively [85]. The TC of Al 2 O 3 /DIW nanofluids was enhanced by 15% for a weight fraction of 0.8% at room temperature [86].
With the use of Al 2 O 3 /bioglycol nanofluid, an increase in volume concentration was noticed to yield a maximum TC enhancement of 24% using a concentration of 2 vol% for 40 bioglycol-60 water-based nanofluid [67]. The formulation of bioglycol-based Al 2 O 3 nanofluids was observed to afford improved TC (17%) for the concentration of 1 vol% and at 30 • C [51]. Under increasing volume fraction, the TC ratio of Ag-MgO (50:50)/DW nanofluid was observed to enhance [87]. The TC of Cu-Al 2 O 3 (10:90)/DIW nanofluid was found to be augmented by 1.47-12.11 as the concentration rose from 0.1 vol% to 2 vol% [60]. A maximum TC of 27.8% was recorded for MWCNT-Fe 2 O 3 /water nanofluid with 0.05 wt%:0.02 wt% concentration [45]. Generally, increasing the concentration of nanoparticles has been found to augment the TC of nanofluids. The TC of MWCNT-CuO/therminol55 nanofluids was observed to enhance as the concentration rose from 0.005 wt% to 0.08 wt% with the highest augmentation of 128.4% [46].

Temperature
Temperature is an important property that influences the TC of nanofluids. As the temperature of nanofluids rises, an increase in the kinetic energy of base fluid molecules and nanoparticles occurs. This leads to intensified micro-convention, Brownian motion, and bombardment between particle-molecule and particle-particle resulting in increased TC of nanofluids. The TC ratio of TiO 2 /EG and Al 2 O 3 /DIW increased as the temperature rose from 20-60 • C with TC enhancement of 18% (5 vol% and 60 • C) and 12% (1 vol% and 60 • C), respectively [61]. The TC of water-based CuO and Al 2 O 3 nanofluids was found to enhance with an increase in concentration from 1 vol% to 4 vol% at room temperature [62]. At 4 vol% and 51 • C, TC enhancements of 9.4-24.3% and 14-36% were observed for Al 2 O 3 and CuO nanofluids, respectively. As the temperature of Fe 3 O 4 /DIW nanofluids increased from 20 • C to 60 • C, the TC was enhanced from 8.4-17% and 25% to 48% of Fe 3 O 4 /DIW nanofluids for the concentration of 0.2 vol% and 2 vol%, respectively [64].
An increase in temperature from 10-60 • C was observed to enhance the TC of Fe 3 O 4 /kerosene nanofluids by a maximum value of 34% [65]. At a concentration of 2 vol%, temperature increase from 10 • C to 70 • C for EG and DW-based Al 2 O 3 nanofluids was found to enhance their TC by 31% and 30%, respectively [39]. At 0.3 wt%, the TC of SiO 2 nanofluid was reported to diminish with an increase in EG content of the EG-water base fluid and enhance with a temperature rise from 25-45 • C [66]. Recently, the TC of WO 3 /EG nanofluids was found to be improved by 32.4% as the temperature increased from 5-65 • C for a mass fraction of 1.5 wt% [88]. Under increasing temperature (10-50 • C) and concentration (1-7 vol%), the TC of TiO 2 /EG was enhanced by 2.7-19.52% [68].
Contrary to the above results, an increase in temperature from 20 • C to 45 • C for MgO/glycerol nanofluids with volume fractions of 0.5-4% was observed not to affect the TC despite the improvement of the TC of glycerol through the suspension of MgO nanoparticles in it [14]. Additionally, the TC of EG-based Fe 2 O 3 and Fe 3 O 4 nanofluids was found to be independent of temperature rise from 10 • C to 50 • C with the enhancement of 15% and 11%, respectively, at a volume fraction of 0.69% [85]. Owing to the effect of nanoparticle clustering as a result of nanofluid stability which can negate Brownian motion, the temperature change of nanofluids may not always favor the enhancement of TC of nanofluids. A different result was demonstrated when the TC of water and EG-based Fe 3 O 4 nanofluids were measured [91]. It was reported that the TC diminished with an increase in concentration and temperature, which was related to the combined effect of interfacial thermal resistance and surfactant layer charge. The effect of temperature and concentration on the TC of nanofluid is illustrated in Figure 5.
Nanomaterials 2023, 12, x FOR PEER REVIEW 10 found to be independent of temperature rise from 10 °C to 50 °C with the enhanceme 15% and 11%, respectively, at a volume fraction of 0.69% [85]. Owing to the effect o noparticle clustering as a result of nanofluid stability which can negate Brownian mo the temperature change of nanofluids may not always favor the enhancement of T nanofluids. A different result was demonstrated when the TC of water and EG-b Fe3O4 nanofluids were measured [91]. It was reported that the TC diminished wit increase in concentration and temperature, which was related to the combined effe interfacial thermal resistance and surfactant layer charge. The effect of temperature concentration on the TC of nanofluid is illustrated in Figure 5.

Nanoparticle Size
The size of a particle is a unique characteristic for classification and identifica attributable to some important properties. In the case of nanoparticles, the particle si traceable to the name and the distinct properties (thermal and convective) connecte its use for nanofluid formulation. The use of different sizes of nanoparticles to formu nanofluids is critical to the TC and stability of the resulting nanofluids. The nanof scientific community remains divided on the influence of nanoparticle size on the T nanofluids. This is marked by different scientific opinions behind their perceived res An increment in nanoparticle size has been observed to either enhance or reduce th of nanofluids [31]. A school of thought based on experimental works reported a d relationship between nanoparticle size and TC. The increase in nanolayer thickness, n clustering, surface area, nano-convection, and Brownian motion due to a reduction in noparticle size has been linked to the enhancement of nanofluid TC [31]. The influen nanoparticle size on nanofluid TC is presented in Figure 6.

Nanoparticle Size
The size of a particle is a unique characteristic for classification and identification attributable to some important properties. In the case of nanoparticles, the particle size is traceable to the name and the distinct properties (thermal and convective) connected to its use for nanofluid formulation. The use of different sizes of nanoparticles to formulate nanofluids is critical to the TC and stability of the resulting nanofluids. The nanofluid scientific community remains divided on the influence of nanoparticle size on the TC of nanofluids. This is marked by different scientific opinions behind their perceived results. An increment in nanoparticle size has been observed to either enhance or reduce the TC of nanofluids [31]. A school of thought based on experimental works reported a direct relationship between nanoparticle size and TC. The increase in nanolayer thickness, nanoclustering, surface area, nano-convection, and Brownian motion due to a reduction in nanoparticle size has been linked to the enhancement of nanofluid TC [31]. The influence of nanoparticle size on nanofluid TC is presented in Figure 6. The measurement of the TC of water-based Al2O3 with nanoparticle sizes of 28 nm [69], 38 nm [76], and 13 nm [37] resulted in enhancements of 12%, 8%, and 20%, respectively. This showed an increase in the TC as the nanoparticle size reduced. The TC of water and EG-based ZnO (10-60 nm) and TiO2 (10-70 nm) nanofluids showed an improvement in this property as the concentration increased (1-3%), and the sizes of the nanoparticles for both nanofluids diminished [92]. The influence of micro-convection on the TC of 5.5 vol% Fe2O3/water nanofluids with nanoparticle sizes of 2.8 nm and 9.5 nm was studied [93]. Enhancement of TC by 5% and 25% was recorded with nanoparticle sizes of 2.8 nm and 9.5 nm, respectively, which is strongly related to micro-convection as a result of Brownian motion. Additionally, a trend of improvement in TC as nanoparticle size decreased was observed when EG and water-based Al2O3 nanofluids were examined for their thermal conductivities under changing temperature (20-50 °C), volume fraction (0.5-3%) and nanoparticle size (11-150 nm) [94]. Peak enhancement of 11-32% and 9.5-11% were reported with nanoparticle size of 11 nm, temperature of 50 °C, and volume fraction of 3% for Al2O3/water and Al2O3/EG nanofluids, respectively.
The impact of nanoparticle size (MgO-20 and 100 nm), temperature, and mixing ratio on the TC of DIW-based MgO and MgO-ZnO nanofluids (at 0.1 vol%) was conducted [43]. A decrease in the nanoparticle size of MgO was observed to intensity TC of MgO and MgO-ZnO nanofluids. At 25 °C and under changing nanoparticle size and volume concentration, the TC of water-based SiO2, TiO2, Al2O3, and ZrO2 nanofluids was measured [95]. It was observed that increasing nanoparticle size enhanced the TC ratio for all nanofluids. The study showed that subject to varying volume fraction and nanoparticle size (at ambient temperature), the TC of EG and water-based Al2O3 nanofluids enhanced as the nanoparticle size increased in the range of 2 nm to 50 nm [74]. This finding was linked to the phonon scattering at the particle-fluid interface. The influence of varying nanoparticle size (Al2O3-5 nm and 30 nm), temperature, mixing ratio (10:90-90:10), and volume concentration on the TC was examined [96]. A direct relationship was observed between the nanoparticle size and TC with a maximum augmentation of 45.1%. The measurement of the TC of water-based Al 2 O 3 with nanoparticle sizes of 28 nm [69], 38 nm [76], and 13 nm [37] resulted in enhancements of 12%, 8%, and 20%, respectively. This showed an increase in the TC as the nanoparticle size reduced. The TC of water and EG-based ZnO (10-60 nm) and TiO 2 (10-70 nm) nanofluids showed an improvement in this property as the concentration increased (1-3%), and the sizes of the nanoparticles for both nanofluids diminished [92]. The influence of micro-convection on the TC of 5.5 vol% Fe 2 O 3 /water nanofluids with nanoparticle sizes of 2.8 nm and 9.5 nm was studied [93]. Enhancement of TC by 5% and 25% was recorded with nanoparticle sizes of 2.8 nm and 9.5 nm, respectively, which is strongly related to micro-convection as a result of Brownian motion. Additionally, a trend of improvement in TC as nanoparticle size decreased was observed when EG and water-based Al 2 O 3 nanofluids were examined for their thermal conductivities under changing temperature (20-50 • C), volume fraction (0.5-3%) and nanoparticle size (11-150 nm) [94]. Peak enhancement of 11-32% and 9.5-11% were reported with nanoparticle size of 11 nm, temperature of 50 • C, and volume fraction of 3% for Al 2 O 3 /water and Al 2 O 3 /EG nanofluids, respectively.
The impact of nanoparticle size (MgO-20 and 100 nm), temperature, and mixing ratio on the TC of DIW-based MgO and MgO-ZnO nanofluids (at 0.1 vol%) was conducted [43]. A decrease in the nanoparticle size of MgO was observed to intensity TC of MgO and MgO-ZnO nanofluids. At 25 • C and under changing nanoparticle size and volume concentration, the TC of water-based SiO 2 , TiO 2 , Al 2 O 3 , and ZrO 2 nanofluids was measured [95]. It was observed that increasing nanoparticle size enhanced the TC ratio for all nanofluids. The study showed that subject to varying volume fraction and nanoparticle size (at ambient temperature), the TC of EG and water-based Al 2 O 3 nanofluids enhanced as the nanoparticle size increased in the range of 2 nm to 50 nm [74]. This finding was linked to the phonon scattering at the particle-fluid interface. The influence of varying nanoparticle size (Al 2 O 3 -5 nm and 30 nm), temperature, mixing ratio (10:90-90:10), and volume concentration on the TC was examined [96]. A direct relation-ship was observed between the nanoparticle size and TC with a maximum augmentation of 45.1%.
The impact of volume fraction (0-5%) and nanoparticle size (spherical (15 nm) and rodshaped (40 nm) on the TC of Ti 2 O/DIW nanofluids showed enhancement (33%-40 nm and 30%-15 nm) with an increase in nanoparticle size [97]. A study on the influence of different base fluids, volume fractions, specific surface areas, and nanoparticle sizes (12.2-302 nm) on the TC of Al 2 O 3 nanofluids revealed that for all the nanofluids the TC augmented with a rise in nanoparticle size from 12.2 to 60.5 nm while the reverse was reported when the nanoparticle size increased from 60.5-302 nm [98]. Additionally, the specific surface area was noticed to increase as the nanoparticle size increased. The obtained results were connected to the phonon mean free path. When the nanoparticle size is larger than the phonon means free path, TC enhancement occurs as the specific surface area increases for improved particle-fluid interaction. With a smaller or equal nanoparticle size to that of the phonon means free path, a reduction in TC is observed as a result of phonon scattering at the particle interface. In addition, excessive particle clustering especially for small-size nanoparticles has been reported to lead to a reduction in nanofluid TC as nanoparticle size was reduced [99].

Base Fluid Characteristics and Alignment
The characteristics (such as polarity, viscosity, and hydrogen bonding) of base fluids utilized in the formulation and application of nanofluids are very important [34]. Suspending different nanoparticles with their peculiar chemical and physical properties in various base fluids with dissimilar characteristics is a complex exercise and a good understanding of these two basic materials and their peculiarities is key to the choice of these materials, experimental results, and nanofluid applications. The thermal properties of nanofluids (for example TC) are strongly connected to the existence and the degree of thermal interfacial resistance of the base fluid molecules and the suspended nanoparticles. Metal oxide-based nanoparticles have been reported to be well-dispersed in highly polarized base fluids [34]. The influence of EG and EG-W as base fluids on the TC of 4 vol% SiC nanofluids with difference nano-sizes was studied [100]. Using EG-W-based SiC nanofluid was noticed to exhibit higher TC enhancement than W-based SiC nanofluid. The outcome was connected to the reduction in the interfacial thermal resistance value of the EG-W compared to W.
A deeper understanding of the effect of base fluid characteristics (polarity, hydrogen bonding, and viscosity) on the TC of Fe 2 O 3 nanofluids with and without a magnetic field was provided by the work of Christensen et al. [34]. Fe 2 O 3 nanoparticles were suspended in twelve different solvents with diverse characteristics. In the presence and absence of magnetic effect, the suspension and alignment of Fe 2 O 3 nanoparticles were enhanced as base fluids with a single OH group exhibiting inter-molecule hydrogen bonding caused lower viscosity and higher polarity which improved the TC of the corresponding nanofluids. Exposure of the nanofluids to a magnetic field increased nanoparticle alignment leading to an increased TC enhancement. In addition, Hong et al. [101] demonstrated that the alignment of SWNT-Fe 2 O 3 nanofluid (using NaDSSB as a surfactant) caused TC improvement, especially under the influence of a magnetic field. TC was enhanced to the maximum (1.36 W/m K) when the nanofluid was exposed to the magnetic field for 30 s. A change in nanoparticles (from Fe 2 O 3 to NiO) and surfactant (from NaDSSB to CTAB) was reported to corroborate the effect of alignment on TC.
A study on the influence of alignment and base fluids (water, EG, and water-NaDDBS) on the TC of Fe 2 O 3 and CuO nanofluids (at 0.4 vol%) was conducted [102]. The Fe 2 O 3 nanofluids exhibited higher TC values compared to CuO nanofluids as the particles of the Fe 2 O 3 nanofluids were observed to align without a magnetic field. Water-based nanofluids have the highest TC value, followed by water-NaDDBS, then EG. In the presence of a magnetic field, the TC of Fe 2 O 3 /water and Fe 2 O 3 /water-NaDDBS nanofluids was enhanced while that of EG-based Fe 2 O 3 nanofluid diminished. The influence of alignment of 0.017 wt% MgO-SWCT + 0.17 wt% NaDDBS nanofluid on TC was investigated [103].
Highest TC (0.92 W/m K) was recorded when SWCT nanoparticles were aligned on MgO under the influence of a magnetic field. This was around a 35% improvement over a case of no magnetic field effect. Sundar et al. [104] studied the effect of DIW and EG on the TC of GO-Co 3 O 4 nanofluids. TC improvement of 11.85% and 19.14% was obtained using EG and DIW, respectively at 0.2 vol% and 60 • C.

pH
Different base fluids have different pH values. The suspension of different nanoparticles in base fluids alters the base fluids' surface charge, and thus the pH, which is a function of the stability of the corresponding nanofluids. Modification of nanofluid surface charge via the pH to improve nanofluid stability affects the TC of the nanofluid [105]. Hydroxyl group formation is experienced when metal oxide nanoparticles are suspended in water. The nanoparticle surface charge polarity is linked to the isoelectric point of the solid phase and the base fluid pH. The pH of a cylinder boehmite Al 2 O 3 /EG nanofluid with 5 vol% concentration was modified from 2.54 to 4.10 to study its effect on thermal conductivity [106]. A slight improvement in thermal conductivity was observed as the pH increased.
A study on the influence of pH on the TC of water, EG, and water-NaDDBS-based Fe 2 O 3 and CuO nanofluids (at 0.4 vol%) was conducted [102]. Although the pH of CuO nanofluids was higher than Fe 2 O 3 nanofluids with EG demonstrating the highest pH of all the base fluids, changes in pH of Fe 2 O 3 nanofluids were observed to appreciably improve the TC away from the iso-electric point. With DIW-based ZrO 2 and TiO 2 nanofluids, the impact of pH on the TC was examined [107]. Near the iso-electric point (pH = 6.2), the TC was significantly enhanced when altering the pH from 4 to 10. The effect of pH of 0.017 wt% MgO-SWCT + 0.17 wt% NaDDBS nanofluid on TC was examined [103]. Increasing the pH of the nanofluid from 7 to 11.5 was found to reduce the TC. In addition, the effect of altering the pH of αAl 2 O 3 /water nanofluids (with a volume fraction of 1.8 to 5) on TC was investigated [98]. An increase in the pH of the nanofluids was noticed to improve the TC. The farther the pH from the isoelectric point (9.2), the higher the TC. Stability and pH influence on the TC of 0.5 wt% DIW-based Al 2 O 3 and CuO nanofluids were investigated [108]. Stable nanofluids were achieved at a pH of 8 (Al 2 O 3 ) and 9.5 (CuO). Below these values, the TC enhancement (15% for Al 2 O 3 and 18% for CuO) of these nanofluids was observed, and above these pH values, depreciation in TC was reported. In another study, the effect of pH on the TC of CeO 2 -MWCNT (80:20)/water nanofluids (0.25-1.5 vol%) formulated using six different surfactants under increasing sonication duration and surfactant-particle ratio was conducted [109]. At a surfactant-particle ratio of 3:2, sonication time of 90 min, and using CTAB, the best stability was exhibited at a pH of 9.5. A linear increment in TC was observed as the pH rose from 8 to 9.5 with a reduction in TC noticed as the pH increased beyond 9.5. This trend was observed for all the studied surfactants. Increasing the pH from 8 to 9.5 caused TC improvement from 7.2% to 13.1%.

Surfactants
The deployment of surfactants in the nanofluid formulation is aimed at improving the stability of nanofluids and preventing their segregation and settlement. Nanofluid TC is affected by nanofluid stability status. At low surfactant concentration, nanofluid TC is increased while at high concentration, TC is reduced [105]. Stability, surfactant (SDBS) weight fraction, and pH influence on the TC of 0.05 wt% DIW-based Al 2 O 3 and CuO nanofluids were investigated. At pH of 8 (Al 2 O 3 ) and 9.5 (CuO), stable nanofluids were obtained. At optimal SDBS weight fractions, optimal stability in terms of zeta potential and particle size was achieved. The influence of different surfactants (CTAB, SDS, and SDBS) on the TC of GnP and GnP-TiO 2 nanofluids under varying sonication duration was investigated [110]. SDS and CTAB (at 30 min sonication) were the best surfactants for mono and hybrid nanofluid formulation with maximum TC improvement of 23.7% and 21.6%, respectively, at 60 • C and concentration of 0.1 wt%. Arasu et al. [111] studied the effect of different surfactants (SDS and SDBS) on the stability and TC of TiO 2 -Ag/water nanofluids (0.1-0.7 wt%). Results revealed the change in TC improvement with the use of different surfactants. SDS produced more stable nanofluids that exhibited a higher TC enhancement of 29.6% compared to SDBS with 2.1% TC improvement.
The effect of four different surfactants (acetic acid, SDS, CTAB, and SDBS) on the TC of TiO 2 /water nanofluids was studied [112]. Most stable nanofluids were formulated using SDS and CTAB. Highest improvement (5.8% at 60 • C and volume fraction of 1%) in TC was recorded with the use of SDS for the nanofluid formulation. Under varying sonication duration and surfactant-particle ratio, the influence of six different surfactants on the TC and stability of CeO 2 -MWCNT (80:20)/water nanofluids was examined [109]. Peak stability as measured by the zeta potential was attained at the optimum surfactant-nanoparticle ratio, and sonication time of 3:2 and 90 min, respectively. Maximum TC ratio was achieved using a CTAB-nanoparticle ratio of 3:2, volume concentration of 0.75%, 90 min sonication, and at 30 • C.
The above studies demonstrated that since surfactants have different characteristics, a specific surfactant does not apply to all types of nanoparticles and base fluids. The compactivity of base fluids and nanoparticles is crucial to the selection of surfactants for nanofluid formulation as the stability of nanofluids impacts their thermal properties and performance.

Mixing Ratio (Hybrid Nanofluids)
The influence of varying mixing ratio (20:80-80:20) and temperature (30-80 • C) on the TC of 1 vol% TiO 2 -SiO 2 /water-EG (60:40) nanofluid was found to improve this thermal property by 16% (peak) using mixing ratio of 20:80 at 80 • C [113]. The TC of 0.1 vol% MgO-ZnO/DIW nanofluids under the effect of changing mixing ratio and temperature was observed to be augmented by 15-22% using a mixing ratio of 40:60 (MgO-ZnO) at 50 • C [43]. The effect of the mixing ratio on the TC was observed to be higher than the temperature. The impact of different mixing ratios (30:70-70:30) on the TC of Al 2 O 3 -Ag/DW nanofluid under increasing volume fraction and temperature was conducted [114]. The TC was enhanced as the temperature and concentration increased with a peak enhancement of 23.6% using 0.1 vol% Al 2 O 3 -Ag/DW nanofluid having a mixing ratio of 50:50 and at 52 • C. The TC of water-based Fe 3 O 4 + CNT nanofluids with varying mixing ratios (1:2, 1:1, and 2:1) was studied [115]. Maximum TC enhancement of 45.4% was recorded using 0.9 vol%  [116]. Volume concentration and temperature rise were observed to augment TC. Maximum TC was achieved using 0.5 vol% CuO-MgO-TiO 2 /water nanofluid with a mixing ratio of 60:30:10 and at 60 • C. The TC of MgO-TiO2/DW nanofluids with mixing ratios (50:50, 80:20, 20:80, 60:40, and 40:60) was examined by varying the temperature and volume concentration [117]. As the temperature and concentration increase enhanced the TC, a peak enhancement of 21.8% was observed for 0.  Figure 7 shows the impact of the mixing ratio on the TC of hybrid nanofluids.

Sonication Characteristics
The stability of nanofluid is very important to its application. Whether a surfactant is used or not to improve the stability of nanofluid, the deployment of the ultrasonication process is crucial to break up nanoparticles suspended in base fluids and aid even dispersion of nanoparticles into them. Sonication is to provide sufficient energy to overcome the interparticle attraction forces holding the nanoparticles together. The ultrasonication process is a complex exercise as it involves several variables such as sonication duration, amplitude, frequency, pulse time, probe depth, etc. Stability is known to affect the thermal and convection properties of nanofluids and this is strongly related to the sonication variables [32,119]. The influence of sonication characteristics on the TC of different nanofluids has been studied and contradictory results have been published in this regard in the scientific community [70,84,110,120]. Figure 8 illustrates the effect of sonication time on nanofluid TC.

Sonication Characteristics
The stability of nanofluid is very important to its application. Whether a surfactant is used or not to improve the stability of nanofluid, the deployment of the ultrasonication process is crucial to break up nanoparticles suspended in base fluids and aid even dispersion of nanoparticles into them. Sonication is to provide sufficient energy to overcome the interparticle attraction forces holding the nanoparticles together. The ultrasonication process is a complex exercise as it involves several variables such as sonication duration, amplitude, frequency, pulse time, probe depth, etc. Stability is known to affect the thermal and convection properties of nanofluids and this is strongly related to the sonication variables [32,119]. The influence of sonication characteristics on the TC of different nanofluids has been studied and contradictory results have been published in this regard in the scientific community [70,84,110,120]. Figure 8 illustrates the effect of sonication time on nanofluid TC.

Mono Nanofluids
The influence of sonication time (2-8 h) on the stability and TC of DIW-based ZnO and CuO nanofluid (0.1 wt%) [121] was examined. Increasing sonication time was observed to affect TC and the need to optimize the TC relative to stability and sonication time was proposed. The effect of sonication time (20-60 min) on the stability and TC of WO 3 /EG nanofluids (0.005-5 wt%) was assessed [88]. For all the studied samples, the TC was augmented as the sonication time increased. The influence of MWCNT (0.01-1 w/v) and  [120]. Results showed that the TC improved as sonication time increased along with concentration to a certain point after which it reduced as sonication time increased further due to reagglomeration. The highest TC enhancement of 4.6% and 16.1% for 1.5 vol% and 2 vol% concentration at sonication time of 90 min and 120 min, respectively, was recorded.  The impact of sonication time (0.5-5 h) and energy (8.46-42.30 kJ) on the TC of 0.5 vol% Al2O3/DW nanofluid was investigated [124]. Increment in the sonication time was noticed to augment TC. By changing surfactant mass fraction (0.25-4 wt%), surfactants (SDS and PVP), concentration (0.1-2.5 vol%), nanoparticle (13 nm and 20 nm), and sonication time (0.25-2 h), the stability and TC of αAl2O3/DIW nanofluids were examined [70]. It was demonstrated that PVP provided better stability, but with lower TC. The TC remained constant when the mass fraction was >1.0 wt% but enhanced with sonication time (up to 1 h). Nanofluid with 13 nm particle yielded higher TC for both surfactants. The TC of γAl2O3/DIW nanofluids with varying volume concentration (1-3 vol%) and sonication duration (15-180 min) was examined [125]. It was demonstrated that the TC enhanced with sonication time.
Sonication time (4-100 h) influence on the TC and stability of ZnO/EG nanofluids (0.5-3.75 vol%) was conducted [84]. Increasing the sonication time from 4 h to 60 h was observed to cause TC improvement by 21-40% with the opposite noticed after a further increase in the sonication time. At 60 h sonication, TC remained constant with no sedimentation for 30 days for 1 vol% ZnO nanofluid. By varying the sonication time (1-5 h) to formulate 0.5 vol% Al2O3/W nanofluid, the TC was monitored [126]. The sonication time increment was found to enhance TC as nanoparticles sedimentation reduced.
Sonication time (4-100 h) influence on the TC and stability of ZnO/EG nanofluids (0.5-3.75 vol%) was conducted [84]. Increasing the sonication time from 4 h to 60 h was observed to cause TC improvement by 21-40% with the opposite noticed after a further increase in the sonication time. At 60 h sonication, TC remained constant with no sedimentation for 30 days for 1 vol% ZnO nanofluid. By varying the sonication time (1-5 h) to formulate 0.5 vol% Al 2 O 3 /W nanofluid, the TC was monitored [126]. The sonication time increment was found to enhance TC as nanoparticles sedimentation reduced.

Magnetic Field
Some nanoparticles used in the formulation of nanofluids have magnetic properties that distinct them from other nanoparticles. Magnetic nanofluids are formed by the suspension of ferromagnetic nanoparticles in different base fluids. Depending on the nature of magnetic nanoparticles, magnetic field strength and direction, and type of magnetic field source, particles of magnetic nanofluid form diverse arrangements of chain clusters which tend to affect the thermal properties. The alignment of the particles to form chains on exposure to the magnetic field has been reported to be a veritable tool for the manipulation of the TC of nanofluids [128]. Tuning of the magnetic field has found potential applications in sealing, heat transfer, sensors, nuclear and solar systems, ink jet printers, biomedical, loud speakers, dampers, etc. [128]. The influence of magnetic field strength on nanofluid TC is presented in Figure 9. which reduction is noticed. The influence of varying SDS mass percent, sonication tim (20-150 min), mixing ratio, and concentration on the TC of DW-based nanofluids (CuO MgO-TiO2) was investigated [116]. Maximum TC was reached with increasing concentra tion when the nanofluid with a mixing ratio of 60:30:10 was sonicated for 140 min. Varia tion in the SDS weight concentration, mixing ratio, sonication time (20-80 min), and vo ume concentration of MgO-TiO2/DW nanofluids was examined. TC was augmented wit increasing concentration using the nanofluid with a mixing ratio of 80:20 and 0.35 wt% o SDS, and sonicating the same for 75 min.

Magnetic Field
Some nanoparticles used in the formulation of nanofluids have magnetic propertie that distinct them from other nanoparticles. Magnetic nanofluids are formed by the su pension of ferromagnetic nanoparticles in different base fluids. Depending on the natur of magnetic nanoparticles, magnetic field strength and direction, and type of magnet field source, particles of magnetic nanofluid form diverse arrangements of chain cluster which tend to affect the thermal properties. The alignment of the particles to form chain on exposure to the magnetic field has been reported to be a veritable tool for the manipu lation of the TC of nanofluids [128]. Tuning of the magnetic field has found potential ap plications in sealing, heat transfer, sensors, nuclear and solar systems, ink jet printers, b omedical, loud speakers, dampers, etc. [128]. The influence of magnetic field strength o nanofluid TC is presented in Figure 9. . Effect of increasing magnetic field intensity and CuO + 3 wt% Fe3O4 nanofluids on relativ thermal conductivity under increasing exposure duration [129].
Studies on the deployment of the magnetic field to augment the TC of magnet nanofluids were pioneered using DIW-based Fe and Fe3O4 nanofluids with varying vo ume fractions and under increasing and different orientations of magnetic field [130]. Th TC of water and n-decane based γ-Fe2O3 and CoFe2O4 nanofluids at 25 °C and exposed t an external magnetic field was examined. A reduction in the TC of the nanofluids (40% b CoFe2O4 and 50% by γ-Fe2O3) as the magnetic field intensity increased up to 30 mT wa observed [131]. At ambient temperature, the TC of Fe3O4/kerosene nanofluids with vary ing volume fractions (0.031-7.8%) and under different (parallel and perpendicular) an Studies on the deployment of the magnetic field to augment the TC of magnetic nanofluids were pioneered using DIW-based Fe and Fe 3 O 4 nanofluids with varying volume fractions and under increasing and different orientations of magnetic field [130]. The TC of water and n-decane based γ-Fe 2 O 3 and CoFe 2 O 4 nanofluids at 25 • C and exposed to an external magnetic field was examined. A reduction in the TC of the nanofluids (40% by CoFe 2 O 4 and 50% by γ-Fe 2 O 3 ) as the magnetic field intensity increased up to 30 mT was observed [131]. At ambient temperature, the TC of Fe 3 O 4 /kerosene nanofluids with varying volume fractions (0.031-7.8%) and under different (parallel and perpendicular) and increasing magnetic field (0-500 G) was accessed [132]. Peak enhancement of 300% for 6.3% concentration when the magnetic field of 82 G was positioned parallel to the temperature gradient was recorded. Exposure of the nanofluid to higher magnetic field strength resulted in TC depreciation. However, no appreciable enhancement was observed when the magnetic field was positioned perpendicular to the temperature gradient. The TC improvement and trend agreed with the results published using a similar nanofluid exposed to the same strength and orientation of magnetic field except that the peak enhancement occurred using volume fraction of 0.078% [93].
The claim that nanofluid TC was enhanced by exposing a magnetic field parallel to the direction of the temperature gradient is also supported by [53], and the existence of peak TC enhancement at a certain magnetic field strength is corroborated by [54]. The parallel arrangement of the magnetic field to the direction of the temperature gradient aids energy transport in the nanofluid as the formed chain structure aligns with the magnetic field direction to quicken energy transportation process [133].
On exposing Fe 3 O 4 /kerosene nanofluids with varying volume fraction (1.12-4.7%) and temperature (25-65 • C) to increasing magnetic field strengths (0-1200 G) and different magnetic field directions, the TC was investigated [133]. Increasing concentration and magnetic field strength (up to 885 G) were observed to enhance the TC with a peak enhancement of 30% when the magnetic field was applied parallel to the temperature gradient direction. The enhancement recorded was attributed to the formation of a zipper-like structure which was reversible. However, temperature increase was noticed to reduce the TC on exposure to increasing magnetic field magnitude. This was in agreement with a later study on the impact of temperature (20-60 • C), volume fraction (0.25-4.8%), and magnetic field (0.021-0.145 T) on the TC of water-based Fe 2 O 3 and Fe 3 O 4 nanofluids [134]. Increasing magnetic field strength and volume fraction enhanced Fe 2 O 3 and Fe 3 O 4 nanofluids by 15-38.5% and 13-176%, respectively, but temperature rise detracted it. However, an improvement in the TC of Fe 3 O 4 /glycerol nanofluids with temperature (20-40 • C), volume fraction (0.5-3%), and magnetic field strength (120-600 G) rise was reported [55] which contradicted the findings of reported in previous studies [115,133,134]. A maximum enhancement of 16.9% was observed.
An investigation of the effect of utilizing a constant and oscillating magnetic field on the TC of water-based 1 vol% Fe 3 O 4 and 2 vol% CNT-Fe 3 O 4 nanofluids was conducted [135]. The use of a constant magnetic field revealed an up-and-down trend with an increase in magnetic field strength over time which was due to the alignment of chainlike structure leading to increased TC, and the thickening of chains and settling causing TC reduction. However, engaging an oscillating magnet field showed an increment in TC as the magnetic field intensified with time. At magnetic field strength of 700 G, average TC was improved by 24.3% and 22.6% for CNT-Fe 3 O 4 and Fe 3 O 4 nanofluids, respectively, when the influence of an oscillating magnetic field was compared to that of a constant magnetic field. Similarly, an alternating magnetic field was found to be better than a constant magnetic field for the augmentation of the TC of NiO/DIW nanofluids under increasing volume fraction, temperature, and magnetic field strength [136]. This was because imposing an alternating magnetic field intensifies the velocity and randomness of nanoparticles leading to TC improvement while the use of a constant magnetic field causes nanoparticle chain formation leading to the magnification of TC. The impact of volume fraction and magnetic field strength was observed to be significant.
The above studies revealed that there exist contradictory results concerning the influence of temperature rise on the TC of nanofluids when exposed to increasing magnetic field intensity, which needs to be further investigated to have a better view of this observation and to have a well-informed understanding of the physics and mechanism behind this. Additionally, maximum TC attained at a certain magnetic field strength is observed to be a function of nanoparticle type, base fluid type, nanoparticle concentration, temperature, magnetic field strength, and magnetic field type.

Electric Field
Very limited studies have been performed concerning the influence of electric fields on the TC of nanofluids. In a pioneering work, the impact of temperature (26.6-90 • C) and electric field (0-1000 V/mm) intensity on the TC of 30 vol% Al 2 O 3 /silicone oil nanofluid was investigated [137]. Exposure of the nanofluid at 26.6 • C to an increasing electric field from 0 V/mm to 700 V/mm showed a slight increase in the TC from 0.2454 W/m K to 0.2916 W/m K, which surged by 48% on increasing the electric field to 800 V/mm. The TC remained unchanged with a further rise in the electric field. An increment in the temperature of the nanofluid under exposure to an increasing electric field revealed a reduction in the TC. The effect of nanoparticle size (20 nm and 50 nm), temperature (15-55 • C), concentration (0.1-1.5 wt%), and electric field (0-1.2 MV/m) on the TC of αAl 2 O 3 /transformer oil nanofluids [138]. Increasing temperature, concentration, and electric field were observed to augment the nanofluid TC while increment in nanoparticle size has an insignificant effect on the TC. The TC recorded is strongly related to the Brownian motion phenomenon. A contradiction regarding the effect of temperature on the TC of nanofluids exposed to an electric field is observed which calls for further studies on this concern in addition to the scarcity of literature in the public domain. The effect of electric field intensity on nanofluid TC is illustrated in Figure 10. The above studies revealed that there exist contradictory results concerning the influence of temperature rise on the TC of nanofluids when exposed to increasing magnetic field intensity, which needs to be further investigated to have a better view of this observation and to have a well-informed understanding of the physics and mechanism behind this. Additionally, maximum TC attained at a certain magnetic field strength is observed to be a function of nanoparticle type, base fluid type, nanoparticle concentration, temperature, magnetic field strength, and magnetic field type.

Electric Field
Very limited studies have been performed concerning the influence of electric fields on the TC of nanofluids. In a pioneering work, the impact of temperature (26.6-90 °C) and electric field (0-1000 V/mm) intensity on the TC of 30 vol% Al2O3/silicone oil nanofluid was investigated [137]. Exposure of the nanofluid at 26.6 °C to an increasing electric field from 0 V/mm to 700 V/mm showed a slight increase in the TC from 0.2454 W/m K to 0.2916 W/m K, which surged by 48% on increasing the electric field to 800 V/mm. The TC remained unchanged with a further rise in the electric field. An increment in the temperature of the nanofluid under exposure to an increasing electric field revealed a reduction in the TC. The effect of nanoparticle size (20 nm and 50 nm), temperature (15-55 °C), concentration (0.1-1.5 wt%), and electric field (0-1.2 MV/m) on the TC of αAl2O3/transformer oil nanofluids [138]. Increasing temperature, concentration, and electric field were observed to augment the nanofluid TC while increment in nanoparticle size has an insignificant effect on the TC. The TC recorded is strongly related to the Brownian motion phenomenon. A contradiction regarding the effect of temperature on the TC of nanofluids exposed to an electric field is observed which calls for further studies on this concern in addition to the scarcity of literature in the public domain. The effect of electric field intensity on nanofluid TC is illustrated in Figure 10.

Green Nanofluids
Though with very limited studies, the TC of green base fluid and nanofluid has also been investigated. At 2 vol% and 80 • C, the TC of Al 2 O 3 /40 biogylcol-60 water and Al 2 O 3 /60 biogylcol-40 water nanofluids was found to be enhanced by 24% and 13%, respectively [67]. The higher TC of water was suggested to have influenced the obtained results. By investigating the TC of bioglycol-based Al 2 O 3 nanofluids, a maximum enhancement of 17% was observed at 30 • C while peak TC was recorded at 70 • C, all at a concentration of 1 vol%, despite subjecting the nanofluids to increasing temperature from 30 • C to 80 • C [51]. Additionally, the use of the green base fluid was observed to result in a higher enhancement of the nanofluid TC compared to those of EG (9%) and PG (3.6%). A green nanofluid formulated by the suspension of TiO 2 -SiO 2 (20:80) nanoparticles into bio-glycol-water (60:40) was examined for TC under changing temperature (30-70 • C) and concentration (0.5-3 vol%) [52]. Both the concentration and temperature increase were observed to augment nanofluid TC with peak improvement of 12.5% for 3 vol% at 70 • C.

Green Nanofluids
Though with very limited studies, the TC of green base fluid and nanofluid has als been investigated. At 2 vol% and 80 °C, the TC of Al2O3/40 biogylcol-60 water and Al2O3/6 biogylcol-40 water nanofluids was found to be enhanced by 24% and 13%, respectivel [67]. The higher TC of water was suggested to have influenced the obtained results. B investigating the TC of bioglycol-based Al2O3 nanofluids, a maximum enhancement o 17% was observed at 30 °C while peak TC was recorded at 70 °C, all at a concentration o 1 vol%, despite subjecting the nanofluids to increasing temperature from 30 °C to 80 °C [51]. Additionally, the use of the green base fluid was observed to result in a higher en hancement of the nanofluid TC compared to those of EG (9%) and PG (3.6%). A gree nanofluid formulated by the suspension of TiO2-SiO2 (20:80) nanoparticles into bio-gly col-water (60:40) was examined for TC under changing temperature (30-70 °C) and con centration (0.5-3 vol%) [52]. Both the concentration and temperature increase were ob served to augment nanofluid TC with peak improvement of 12.5% for 3 vol% at 70 °C.

Future Perspective and Challenges
Different classifications of nanoparticles, base fluids, and surfactants have been de ployed as materials for the formulation of nanofluids. The advantages of metal oxid

Future Perspective and Challenges
Different classifications of nanoparticles, base fluids, and surfactants have been deployed as materials for the formulation of nanofluids. The advantages of metal oxide nanofluids over other classes of nanofluids have attracted the attention of the nanofluid research community. TC is considered to be the foremost thermal property of nanofluids as this relates to different applications of nanofluids. Therefore, the TC of metal oxide nanofluids is crucial to the future of nanofluid research, which has prompted this present review work. The reported inconsistency in the TC measurement of metal oxide nanofluids needs to be addressed through the development of TC devices using various TC measurement techniques. Improved development of transient hot wire and thermal constants analyzer techniques is very important to correct the disparity in TC results for different nanofluids. Sensitivity Stability remains a critical factor in nanofluid research. Nanofluid stability is directly related to the TC of metal oxide nanofluids. Stable nanofluids are to be formulated by optimizing the various preparation characteristics [32] to improve their TC values and nanofluid applications. In addition, the functionalization of metal oxide nanoparticles in the formulation of nanofluids can be considered as an option in the future to enhance their stability and augment TC values.
Moderate studies have been conducted on the impact of magnetic field on the TC of metal oxide. However, very limited ones have experimented the effect of alignment on metal oxide nanofluid TC. More works need to be conducted to investigate the alignment of different nanofluids (mono, hybrid, magnetic, and non-magnetic) under the influence of magnetic field with different orientation and intensity. The effect of electric field on nanoparticle alignment in nanofluid and TC of nanofluids is very scare in the open literature and further studies in this respect are expected in the future. In addition, studies are to be intensified concerning the effect of electric field intensity and orientation on the TC of nanofluids. In terms of sustainability and eco-friendly environment, investigation on less toxic green base fluids, nanoparticles, nanofluids, and synthetic routes marked with increased TC is expected to increase shortly as studies are presently very limited [28,47,[140][141][142].

Conclusions
A review of the TC measurement and enhancement of metal oxide nanofluids has been conducted. Generally, the suspension of nanoparticles of different metal oxides in diverse base fluids has been observed to enhance the TC of the base fluids, even regarding the use of hybrid nanoparticles (with one metal oxide or both metal oxides). The TC and the resultant enhancement were noticed to be dependent on several contributing factors, such as temperature, nanoparticle and base fluid characteristics, measurement technique, alignment, concentration, sonication characteristics, surfactant presence, type, and concentration with key ones reviewed and discussed in this present work. The transient hot wire is the most used TC measuring technique with the issue of nanofluid stability being critical to its usage and accuracy. The concentration of nanofluids has a direct influence on the TC while nanoparticle size, temperature, mixing ratio, and sonication characteristics have conflicting effects on it. The deployment of electric and magnetic fields with increasing intensity and concentration was found to augment the TC of metal oxide nanofluids, but the influence of increasing temperature was marked by controversial results. Undoubtedly, electric and magnetic fields can be utilized to control nanofluid TC with the former requiring intense future studies. Brownian motion, nanoparticle clustering, nanolayer, and Ballistic nature remained the most important mechanisms responsible for the uncharacteristic TC enhancement of nanofluids. The development of green synthetic processes, base fluid, nanoparticles, nanofluid, and hybrid nanofluid is envisioned to be critical to the future of nanofluid research and enhancement of TC for thermal transport applications.