RETRACTED ARTICLE: Nanofluids: properties and applications

Abstract

Nanofluids are liquid suspensions of hard nanometer-sized particles suspended in a base fluid. The suspension of small solid particles in energy transmission fluids enhances their thermal conductivity and provides an inexpensive and creative way to greatly boost their heat transfer (HT) properties. It is possible to add nanofluids to various industrial and technical issues, such as heat exchangers, electrical equipment cooling, and chemical processes. In comparison to traditional fluids utilized for HT, which include water, oil, ethylene glycol, and single nanoparticles (NPs) involving nanofluids, hybrid nanofluids are new forms of fluids that display strong HT efficiency. In terms of cooling, hybrid nanofluids function well where temperature scales are high and have a wide variety of thermal applications. In general, hybrid nanofluids are developed by diffusing two distinct forms of NPs in base fluids, which has emerged as a novel nanotechnology.

Highlights

• An overview of nano fluids and their application to cooling and heating.

• An overview of nano fluids and characteristics of cooling and heating.

• Review recent progress in nano fluids in pharmaceutical and medical.

• Review recent progress in nano fluids in mechanical engineering and civil engineering and medical physics.

• Review recent progress in nano fluids in chemical processes and electrical equipment and thermal conductivity and thermophysical properties of nanofluids and thermal conductivity coefficient and nanoparticle viscosity concentration effect and effect of temperature on viscosity.

1 Introduction

Various fluids are commonly utilized as heat porters in heat transfer systems. Heat transfer fluids (HTF) are used in the systems of power plants to exchange heat [1,2,3,4,5], temperature-changing systems of flats [6,7,8,9], vehicle dispensing systems in transportation systems [10,11,12], and dispenser mechanisms in many manufacturing plants [13,14,15]. Across all of the foregoing cases, the thermal conductivity of the HTF has a significant impact on the performance of the HT process and, as a result, the overall productivity of the device [16,17,18,19,20]. In this regard, researchers have worked on creating improved HTFs with a significantly better thermal conductivity than currently-utilized fluids [21,22,23,24,25]. So far, the major efforts to boost heat transmission through geometric modification have been exercised [26,27,28,29,30], but they have been hampered by the weak thermal conductivity of HT fluids. Choi, on the other hand, invented a new revolutionary group of HT fluids in 1995 based on the suspension of nanoscale metallic particles whose average size was less than 100 nm in classic HT fluids and called them "nanofluids" [31,32,33,34].

Considerable efforts were made on heat transfer enhancement through geometrical modification up to now but were all constrained by the low thermal conductivity of the heat transfer fluids used [35,36,37,38,39]. However, in 1995, Choi developed a newly innovative class of heat transfer fluids that depends on suspending nanoscale particles of metallic origin with an average particle size of less than 100 nm into conventional heat transfer fluids and gave such type of fluids the term “nanofluids” [40,41,42]. In other words, the term nanofluid is used to describe a mixture containing nanoscale particles of average size less than 100 nm with any basefluid that does not dissolve the particles hosted by it. Maxwell proposed the notion of dispersing particles in fluids in his study in the 19th century [42,43,44,45,46]. Their research focused on the greater metals’ thermal conductivity as in comparison to fluids at ambient temperature [47,48,49,50]. At normal temperature, the thermal conductivity of copper is 3000 and 700 times higher than motor oil and water, respectively. A similar disparity exists in terms of thermal conductivity among liquids, with metallic liquids having significantly higher thermal conductivity than non-metallic liquids [51,52,53,54,55].

Figure 1 depicts the thermal conductivity of various biological materials, HT fluids, metals, and metal oxides. In this sense, the fluid’s suspending metallic particles is thought to increase its heat conductivity [56,57,58,59,60]. One of the issues emerging from the use of fluids involving m-sized particles is blocking narrow channels produced by huge agglomerations of solid particles, which makes it difficult to use HT devices with tiny channels [61,62,63]. Nanofluids, on the other hand, are able to overcome this barrier as their particles are tiny enough to move through those pipes (i.e., they do not thwart the passage of flow) [64,65,66]. Another benefit of employing nanoparticles is their very big surface area on which heat transmission between the environment and particles occurs [67,68,69]. Due to this issue, reducing particle size from mm and m to nm greatly improves surface area and, as a result, heat transmission [70].

Fig. 1

Comparison of popular polymers, liquids, and solids with thermal conductivity [64]

Nanofluids were described by Xuan and Li in 2000 as any metallic, non-metallic, or polymeric nanoscaled particles mixed with a non-carcinogenic base fluid [12, 71,72,73,74]. They also stated that by adding nanoparticle concentrations as low as 1–5 vol% to the base fluid, efficient thermal conductivity can increase up to 20% [75,76,77,78,79]. Moreover, they argued that the enhancement is strongly influenced by particle structure, particle lengths, increased volume fraction of the ns the in base fluid, particle thermophysical characteristics, and other related factors [13, 80,81,82,83].

Figure 2 depicts the important factors that generally affect the effectual thermal conductivity of nanofluids. When selecting nanomaterials to manufacture nanofluids for HT uses, consider (I) chemical stability, (ii) thermophysical characteristics, (iii) toxicity, (iv) availability, (v) consistency in the base fluid, and (vi) cost [84,85,86,87,88]. The oft-employed nanoparticles for the creation of nanofluids include (Al), (Cu), (Ag), (Fe), (Ti), (Si), (Zn), (Mg), (CNTs), graphene oxide, and diamonds [89,90,91,92,93]. For nanofluid formulation, air, (EG), EG-H₂O combinations, and oils are often utilized base fluids [63]. Using nanofluids in high-temperature applications, such as within the heat exchanger annulus, several studies have reported the formation of scales, commonly recognized as the fouling effect, on surfaces [94,95,96,97,98,99]. The fouling effect operates like surface nanocoating and could be beneficial in lowering the losses of pressure produced by nanofluids’ large viscosity relative to the base fluid due to their form, which is based on nanoparticles [100,101,102,103,104]. This occurs because, as shown in Fig. 3a, the layer created seeks to smooth the surface (b).

Fig. 2

Parameters that control efficient thermal conductivity for nanofluids

Fig. 3

a Rough surface area and (b) nano-coated surface [65]

Kang et al. demonstrated in their work how coating a riser surface with nanoparticles reduced the pumping power and improved the system efficiency by 25% [66]. This is because coating the riser surface has affected the contact angle between the fluid and the surface, making it more hydrophobic to the liquid in contact to it [105,106,107,108]. Figure 4 demonstrates the relation between the surface contact angle and fluid Fig. 5.

Fig. 4

Relation between surface contact angle and fluids [66, 67]

Fig. 5

The number of publications having nanofluids in their title

Ali et al. [65] also confirmed the changes in surface wettability behaviour caused from nanocoating, where they deposited Al particles on the surface of an Al substrate and then examined the film thickness, fluid pH value, and fluid temperature effects on the fluid-surface contact angle [109,110,111,112,113]. Their findings showed that water of pH values above and below 7 tends to develop higher contact angles as the deposited layer thickness and fluid temperature increased, in contrast to water of neutral pH which showed the opposite behaviour [114,115,116,117,118,119].

Nanofluids fouling effect can also increase or decrease the nucleation boiling heat transfer depending on the surface/liquid contact angle as demonstrated by Phan et al., where they showed in their work that the highest heat transfer coefficient was obtained at a contact angle close to either 90° or 0° [89,90,91,92,93,94,95,96, 120,121,122,123].

Except for 2018, which is highly likely to alter with the planned details on the website [97], data taken from the database of Scopus in the period of 1995–2018 indicates an immense growth of published papers using the keyword “nanofluids” in the description. As Fig. 6 indicates, the bulk of published publications are from scholarly journals [124,125,126,127,128,129]. Hybrid nanofluids are made by mixing two types of nanoparticles in the same basefluid to improve thermophysical, optical, rheological, and morphological qualities. Relatedly, Shah and Ali [2] provided numerous strategies in order to achieve industrial reality of hybrid nanofluids, as shown in Fig. 7 Nanofluids can be created to behave as effective lubricants. They can also be utilized to reduce frictional losses in turbines in hydro, tidal, and wind power facilities [130,131,132,133]. Table 1 summarizes various nanofluids.

Fig. 6

% of usable paper forms [2]

Fig. 7

Phases for hybrid Nan fluids industrial realization [2]

Table 1 Various Forms of Nanofluid

2 Forms of nanofluid

Nanofluid, a term that is employed to represent fluids involving nanoscale scattered particles, can be shaped by single-element nanoparticles (copper, iron, and silver), single-element oxides (Copper(II) oxide, Aluminium oxide, and Titanium dioxide), (Cu-Zn, Fe-Ni, and Ag-Cu), and multi-component oxides) Cu-Zn, Fe-Ni, and Ag-Cu) (CuZnFe₄O₄, NiFe₂O₄, and ZnFe₂O₄), or Metal carbides (SiC, B4C, and ZrC), metal nitrides (SiN, TiN, and AlN) and suspended carbon compounds (graphite, carbon nanotubes, and diamonds) in water, gasoline, EG, gasoline, and coolants [134,135,136,137,138]. They could be divided based on two major types: nanofluids from a single substance and nanofluids from hybrids [139,140,141].

2.1 Single material of nanofluids

It was initially suggested in 1995 by Choi, and is known as the traditional shape of nanofluid utilized in manufacturing suspensions using a single type of nanoparticles through various preparation methods [5, 142,143,144,145]. Many scholars have claimed that nanofluids in this group are superior in efficiency since they have far more desirable thermophysical characteristics than the base fluids [51, 146,147,148,149,150].

2.2 Hybrid nanofluids

Hybrid nanofluids are considered as an innovative group of nanofluids that are suspended in a base fluid from a mixture of more than one form of NPS [151, 152]. To increase fluid thermal conductivity more than a standard single material kind of nanofluid [104, 153,154,155], this kind of fluid was first tested in 2007 by Jana et al. [104]. Copper NPS, (CNTs) and Gold NPS dispersed in water were investigated in analyzing them, in tandem with their hybrids (Corbon nano Tube-Cu/H₂O and Corbon nano tube -Au/H₂O) [156,157,158]. The findings indicated that Cu/H₂O nanofluid thermal conductivity was the greatest in all the examined cases and linearly rose with the increase in particle concentrating [159, 160]. The nanofluid’s stability ac Corbon nano tube -Cu/H₂O Despite this finding, the stability of the Corbon nano tube -Cu/H₂O nanofluid was greater than that of the other forms of nanofluid. This aids the fluid’s thermal conductivity retention until it becomes further deteriorated [161,162,163].

2.3 Nanofluid preparation methods

The uniformity of particle dispersion is mostly determined by the utilized preparation process, and it is likely to have a major impact on the nanofluid’s thermophysical characteristics [164, 165]. This issue indicates that if two comparable nanofluids are generated using different procedures, their thermophysical characteristics and agglomeration propensity most likely change [166,167,168]. This happens due to the point that nanofluids cannot be made simply from a solid-liquid combination; they require certain suspension characteristics, like uni-dimensionality, chemical and physical stability, sustainability, and dispersibility [169,170,171]. Two major methods are utilized to manufacture nanofluids, the one-step method (the bottom-up approach) and the two-step method (the top-down approach) [105, 172,173,174,175]. The description of the various methods involved in preparing TiO₂ nanofluids is shown in Fig. 8.

Fig. 8

Nanofluid preparation methods [280]

2.4 The single-step process

In this method, the approach is based on a single step of combining nanoparticles’ processes of producing and dispersing in the basefluid [176,177,178]. This procedure has some characteristics. An oft-employed method for synthesizing nanofluids (the one-step approach to direct evaporation) relies on solidifying nanoparticles within the basefluid itself, which are primarily in the gaseous phase [179,180,181,182,183,184,185].

Akoh et al. [106] developed the method and on a Running Oil Substrate (VEROS) method was called the Vacuum Evaporation. The original concept of this technique was to manufacture nanoparticles, but it was found that it was incredibly arduous to extract a dry shape of NPS from the generated mixture of fluid [186,187,188,189]. An updated VEROS method was proposed by Wagener et al. [107], wherein sputtering magnetron with high pressure was used to synthesize the compounds comprising iron and Silver NPS. An updated VEROS process was also developed by Eastman et al. [108], wherein Cu vapor was directly condensed to generate their Cu/EG nanofluid with a flowing low-vapor-pressure EG. A one-step approach to obtaining Cu nanofluid was used by Zhu et al. [109] through chemical reaction. NaH₂PO₂·H₂O was irradiated with CuSO₄·5H₂O in EG in their work to chemically respond to nanofluid output. Additionally, Tran and Soong [110] employed a one-step process of laser ablation to synthesize nanofluid Al₂O3. There is also another one-step approach [111, 112, 190,191,192,193], both of which are favorable for minimizing the agglomeration of basefluid nanoparticles. The drawback of utilizing the one-step method, though, is the existence of chemicals which are hard to get rid of [105, 194,195,196,197]. An example of the one-step technique utilized to ready nanofluids via vapour sedimentation is shown in Figs. 9, 10. The resulting SEM images for the Single-step processes can be seen in Fig. 11.

Fig. 9

Single step method [105]

Fig. 10

Nanofluid preparation using the one-step vapor deposition process [105]

Fig. 11

Reduction of particle adhesion in the Single-step process [105]

2.5 The two-step process

Nanoparticles are generated or acquired in the shape of a dry powder and are then distributed in the base fluid in this way [125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215]. To disperse basefluid nanoparticles, magnetic stirrers, ultrasonic baths, homogenizers, high-shear blenders, and bead stirrers are commonly utilized [216,217,218,219]. Contrary to the one-step process, this method is the most extensively utilized to generate nanofluids due to cheaper manufacturing costs and a big supply of widely provided nanoparticles by various companies [220,221,222]. An instance of the method of the two-step technique utilized for nanofluid synthesis is shown in Figs. 12, 13. In order to structure their Al₂O₃ nanofluids, Eastman et al. [108], Wang and Xu [113], and Lee et al. [114] followed this method. TiO₂/H₂O nanofluid was synthesized by Murshed et al. [115] along the same path. In order to generate transformer oil-based and water-based nanofluids, Xuan and Li [12] utilized the as-existing Cu nanoparticles. It was also stated that uni-walled and multiple-walled carbon nanotubes were utilized with or without incorporating surfactants in using the two-step process [56, 116,117,118,119, 223,224,225,226] to prepare nanofluids. A number of researchers contend that the two-step procedure is advantageous for the processing of oxide-containing nanoparticulate nanofluids, whereas it is little efficient for metallic nanoparticles [120, 227,228,229]. The major drawback of the two-step methodology, as compared to the one-step approach, is the high buildup of particles that occurs as a result of the process [230,231,232,233,234]. Despite these shortcomings, this approach is still the most frequent way to make nanofluids in big or small quantities, and it can be used to practically make any type of nanofluid [23, 235,236,237,238,239]. Figure 14 shows the resulting SEM and TEM images for Two-step methods.

Fig. 12

Two step method [120]

Fig. 13

The graphical framework for preparing two-step nanofluids [120]

Fig. 14

TEM and SEM micrograph of nanoparticles in the two-step process [120]

3 Preparing nanoparticles

3.1 Sol-gel of method

The Polyol method is a chemical method for the synthesis of nanoparticles. This method uses nonaqueous liquid (polyol) as a solvent and reducing agent. The nonaqueous solvents that are used in this method have an advantage of minimizing surface oxidation and agglomeration. This method allows flexibility on controlling of size, texture, and shape of nanoparticles. Polyol method can also be used in producing nanoparticles in large scale [7, 240,241,242].

The polyol process can be taken as a sol-gel method in the synthesis of oxide, if the synthesis is conducted at moderately increased temperature with accurate particle growth control [8, 243,244,245]. There are several reports that have studied the synthesis of oxide sub-micrometer particles and these include Y₂O₃, V_xO_y, Mn₃O₄, ZnO, CoTiO₃, SnO₂, PbO [9,10,11,12,13,14,15,16, 246,247,248,249]. The solvent that is mostly used in polyol method in metal oxide nanoparticles synthesis is ethylene glycol because of its strong reducing capability, high dielectric constant, and high boiling point. Ethylene glycol is also used as a crosslinking reagent to link with metal ion to form metal glycolate leading to oligomerization [17, 250,251,252]. It has been reported that as-synthesized glycolate precursors can be converted to their more common metal oxide derivatives when calcined in air, while maintaining the original precursor morphology [8, 253,254,255,256,257].

Due to its flexibility in creating particles with a high surface region, the Sol-gel process is often utilized for synthesizing nanoparticles [258,259,260]. The Sol-gel process was explained very clearly by Behnajady et al. [261] and this is seen in Fig. 15. In four separate stages, they completed the whole process; the precursor titanium was distilled into the solvent in the first step and then the blend is sonicated with the aid of an ultrasonic washing. The acquired substance, which appears like a cream, is dried and calcinated in the final phase to produce a crystalline powder [262,263,264,265,266,267].

Fig. 15

Steps in preparing nanoparticles of TiO₂ via sol-gel [261]

3.2 Hydrothermal of synthesis

The production of various metal oxide particles such as TiO₂ [4, 27], K₂Ti₆O₁₃ [5, 17, 18, 268,269,270], K₄Nb₆O₁₇ [6, 271, 272], KNbO₃ [7, 273,274,275], KTiNbO₃ [8,9,10, 276,277,278], KTaO₃ [11], Zn₂SiO₄:Mn [12,13,14,15,16, 279, 280], ZrO₂ [19, 27, 67, 261, 281, 282], AlOOH [20, 283, 284], Al₂O₃ [21, 69, 285,286,287,288], Ba(Sr)Ti(Zr)O₃ [22,23,24, 51,52,53,54, 72, 73, 289, 290], Ca0.8Sr0.2Ti1-xFeO₃- [25, 291, 292], YSZ [26, 293,294,295,296], (Fe,In)₂O₃(ITO) [32, 57, 297, 298], LiFePO₄ [33, 71, 299, 300], (Ce,Zr)O₂ [34, 39, 77, 80, 81, 300,301,302], YVO₄ [35, 303,304,305], (Co,Cu,Ni)(Fe,Co)₂O₄ [36, 45, 74, 78, 306,307,308,309], Fe₂O₃ [37, 70, 310,311,312], YAG [38, 46, 58, 59, 313, 314], ErOOH [40, 315,316,317], Mg₃.5H₂(PO4)₃ [41, 318,319,320], CuAlO₂ [42, 321,322,323,324], ZnO [47,48,49, 79, 325,326,327], LiMn₂O₄ [55, 328, 329], LaxNiyO₃ [60, 76, 330, 331], SnO₂ [68, 332,333,334], (Ca,Mg)(PO4)₃ [75, 335,336,337] has been demonstrated by hydrothermal batch and flow reaction systems.

In an instrument involving a slice-walled steel vacuum tube, which is known as an autoclave, at greater pressure and temperature, the hydrothermal synthesis process was used to mix uni-crystals of an aqueous solution [338, 339]. Figure 16 shows the resulting SEM images TiO₂ nanoparticles for hydrothermal synthesis [281].

Fig. 16

a FESEM template image (anodic aluminum membrane) [282], (b) FESEM synthesized TiO₂ nanorod array image [282], (c) FESEM synthesized TiO₂ nanorod array image [283], and (d) FESEM synthesized TiO₂ nanorod film image [284]

4 Thermophysical properties of nanofluids

Nanofluids are superior to their base fluid as they represent a novel class of fluid with fundamentally distinct thermophysical characteristics like density, particular heat power, thermal conductivity, available HT, thermal diffusivity, and viscosity [13, 340,341,342]. The term “effective” is widely utilized to represent the thermophysical characteristics of nanofluids (efficient viscosity and efficient density). The reason is to distinguish between the basefluid’s thermophysical characteristics and the generated nanofluid [342,343,344,345]. The thermophysical characteristics of nanofluids, as explained in greater depth later, are seen in Fig. 17. There are four thermophysical characteristics of a fluid that alter by adding nanoparticles to the base fluid. These characteristics involve density, viscosity, thermal conductivity coefficient and specific heat [17,18,19, 346, 347]. Multiple researchers have explained differential views on the impact of the inclusion of nanoparticles on the values of these characteristics, but the addition of nanoparticles usually improves the properties, with the exception of real heat, which reduces by adding nanoparticles [20,21,22,23,24, 348,349,350,351]. The degree of the increase is contingent upon various factors such as the volume percentage of nanoparticles, nanoparticle properties, base fluid properties and temperature. Nanofluids have found many applications because of their properties, which makes the study of these properties of particular importance [352,353,354,355,356]. Additionally, because these properties depend on the nanoparticles’ concentration in the base fluid, the characteristics of the nanofluid can be adjusted by altering the concentration of nanoparticles [25,26,27,28, 357,358,359].

Fig. 17

Thermophysical Properties of Nanofluids [17]

4.1 Thermal conductivity coefficient

A primary motivating element underlying nanofluids is the increase in thermal conductivity in comparison to ordinary fluids, which bears a positive influence on the transmission of heat in the fluid convective [360,361,362,363]. If the inserted nanoparticles have higher thermal conductivity than their base fluid, adjoining nanoparticles to a traditional fluid increases its thermal conductivity. Several most typical thermal conductivity nanoparticles and basefluids have been indicated in Tables 2 and 3, respectively. THW Transient hot-wire strategy, steady-state parallel-plate technique, cylindrical cell method, temperature oscillation technique and 3-omega technique are the most common methods for calculating thermal conductivity. In Table 4, a description of experiments and suggested hypotheses is presented [364,365,366]. This rise in efficient thermal conductivity could be attributed to multiple causes, like the Brownian movement (Fig. 18a), which is central to regulating the thermal dispersion activity of fluid nanoparticles. Another explanation for shaping layered constructions, which is recognized as the nanolayer, is the liquid molecules covering the nanoparticles (Fig. 18b). Because a thermal loop in the nanoparticles and the huge liquid, these layered structures are known to improve nanofluid’s thermal conductivity [56, 367,368,369]. Furthermore, crystalline solids’ heat is borne by spontaneously generated phonons, propagated in a random direction, spread by deficiencies or colliding [225,226,227, 370,371,372]. In addition, particle clustering was considered as affecting efficient thermal conductivity [227]. This happens because of the settling of particle agglomerations with lower thermal resistance to heat flow, which results in the creation of concentrated regions rich in particles. Thermophoresis (also known as thermodiffusion, thermomigration, Ludwig-Soret effect), a phenomenon found in a particle mixture that appears to react differentially to the power of a temperature gradient, however, has been reported to influence nanofluids’s efficient thermal conductivity of at elevated temperatures; but, none of the published literature to date has ever proved such a theory [228, 229, 373,374,375].

Table 2 Thermal conductivity nanoparticles are widely used [51]

Table 3 Thermal conductivity basefluids are widely used [51]

Table 4 Summary of different tests that conduct to a theory

Fig. 18

Nanoparticles (a) Brownian movement and (b) liquid/solid interface nanofluid configuration involving huge fluid, nanoparticles, and nanolayers [82]

Various studies have been done to explore improvements in the thermal conductivity of nanofluids. The Maxwell model (1), proposed in 1881 and used the thermal conductivity of both nanoparticles (k_np) base fluids (k_bf) to anticipate the effective thermal conductivity (k_nf) of solid-liquid dispersion [6, 230, 295], was the initial correlation utilized to predict the efficient thermal conductivity (k_nf) of solid-liquid dispersion.

$$K_{nf} = K_{bf}.\frac{{K_{np} + 2.K_{bf} + 2.\left( {K_{np} - K_{bf}} \right).F_V}}{{K_{np} + 2.K_{bf} - \left( {K_{np} - K_{bf}} \right).F_V}}$$

(1)

$$K_{nf} = K_f\left[ {1 + A\emptyset + B\emptyset ^2} \right]\qquad(1)$$

This model takes into account the nanofluid’s two phases (solid and liquid) and accurately predicts the nanofluid’s efficient thermal conductivity when the additional particles are spherically shaped, modest in volume, and suspended in air circumstances. Subsequently, in 1935, Bruggeman [6, 231, 295] proposed an implicit model (2) of effective thermal conductivity that could study the nexus between particles diffused at random.

$$\left[ {\left( {\frac{{K_{np} - K_{nf}}}{{K_{np} + 2K_{nf}}}} \right).F_v + \left( {1 - F_v} \right)\left( {\frac{{K_{bf - 2K_{nf}}}}{{K_{bf} + 2K_{nf}}}} \right)} \right] = 0$$

(2)

$$K_{nf} = K_f\left[ {1 + \left( { - 49.796 + 0.178T} \right)\emptyset + \left( {535.576 - 1.840T} \right)\emptyset ^2} \right]\qquad(2)$$

The Bruggeman model can be extended to suspensions formed at any concentration from particles of spherical form, where (2) produced exactly the same results as low volume percentages (1). Equation (1) was revised multiple times to account for numerous phenomena such as Brownian motion, surface charge, liquid-particle interface layer, particle clustering, and ballistic phonon transmission in order to improve the precision of the predicted findings. Electrophoresis-induced convection, particle-driven natural convection, thermophoresis, and other factors, on the other hand, are still not taken into account and must be discovered in order to aid in estimating efficient thermal conductivity. Table 5 contains examples of some of the known relationships with their observations, while the below studies [229, 232,233,234,235,236,237,238,239,240,241,242,243,244,245,246,247,248,249,250, 376,377,378,379] involve more models. Several researchers conducted experimental measurements nanofluids’ effective thermal conductivity utilizing transient hot-wire methods (economical and simple to employ, wherein the measurement rests on Fourier’s law and 5 percent uncertainty is stated to be effective thermal conductivity) [251,252,253]; 3ω Method (using temperature oscillation frequency dependency to calculate thermal conductivity) [76, 254, 255]; method of temperature fluctuation (resting on the fluctuation method and including calculation of the sample’s temperature answer) [59, 256]; analyzer of thermal constants (less difficult to do, very swift and capable of measuring thermal conductivity in the 0.02–200 W/m·K range) [257]; parallel-plate steady-state strategy (which utilizes the one-aspect equation of heat occurrence in calculating); micro-hot strip technique (substantially less measurement time and far more precise than the hot-wire technique); and the technique of optical beam deflection (self-constructed system that needs high measurement time and could only reliably anticipate thermal conductivity at ~100 vol percent) [113, 258, 259, 380, 381]. The thermal constant analyzer is the oft-adopted methodology utilized by a great number of researchers among all the above-mentioned techniques.

Table 5 Examples of multiple effective associations in thermal conductivity are available in the literature

A transient hot wire is in interaction with the liquid being tested, and Fig. 19 indicates the effects of temperature on thermal conductivity. The nanofluids’ thermal conductivity was experimentally developed and the thermal conductivity data for metal and metal oxides, such as Al₂O₃, Fe₃O₄, TiO₂, ZnO, ZrO₂, and CuO nanofluids, which are accessible in the previous research, were utilized in developing nanofluids. In developing regression, researchers used The Eq. (3).

$$\begin{array}{ll}k_r &= \frac{{knf}}{{kf}}\\& = \left[ {0.8938} \right]\left( {1 + \frac{\emptyset }{{100}}^{1.37}} \right)\left( {1 + \frac{{Tnf}}{{70}}^{0.2777}} \right)\\&\quad\,\,\times \left( {1 + \frac{{dp}}{{150}}^{ - 0.0336}} \right)\left( {\frac{{\alpha p^{0.01737}}}{{\alpha f}}} \right)\end{array}$$

(3)

Fig. 19

Temperature effect on thermal conductivity

4.2 Viscosity

In evaluating the convective HT coefficient, researchers have considered viscosity as a crucial factor. However, because of the low knowledge of viscosity processes and the absence of a mathematical model that represents viscosity activity in nanofluids, this property is problematic. There have been many attempts to develop a model that accounts for nanofluids’ viscosity. The initial model is the model of effective viscosity developed by Einstein [71] as a function of volume for liquids’ suspended rigid layered solids. In 1906, the model was created and it was developed from linear hydrodynamic equations. However, only the viscosity behaviour for layered stiff components and for a less concentrating of particle of 1.0-degree percentage could be predicted by Einstein’s model. To boost the viscosity relationships, several changes were made to Einstein’s model. Another model resting on Einstein’s equation was developed by Brinkman [290] to include higher concentrations of particles, while Bachelor [291] applied Brownian motion to the model he developed. Nevertheless, studies have demonstrated inconsistent outcomes from the described models. Analysis on alumina and titanium nanofluids, according to Einstein-Batchelor correlations, shows higher viscosity levels [293]. In addition, the aforementioned models are all a result of the nanoparticles’ volume fraction; nonetheless, they lack providing the influence of temperature. Other scholars have further focused on viscosity calculations for various nanofluids and have established their related associations. The relationships mentioned in Table 6 are just a volume fraction function, ψ. Research has begun utilizing methods called viscometers to calculate nanofluids’ viscosity in recent years.

Table 6 . Theoretical models for viscosity forecasts

Rheological behaviour of nanofluids affects pressure drop of nanofluids. Additionally, it gives an idea of nanoparticle structuring, which can be helpful in predicting the thermal conductivity of nanofluids. The rheological behaviour can be measured by rheometers [64,65,66,67,68, 70, 72]. Some researchers [62, 63, 69, 71] have used viscometers to measure viscosity. Nowadays viscometers are considered inadequate as they are not capable to read the feature of shear dependence, especially for low viscosity liquid-based nanofluids containing non-spherical particles. Water-based nanofluid containing microsized Al₂O₃ particle exhibits shear thinning behaviour [382,383,384]. The rheological behaviour of various nanofluids enriched with nanoparticles, such as, CuO, BaTiO₃, Ni, Al, Ag, graphite, grapheme, CaCO₃, TNT, Gold, Carbon black powder and Yttrium oxide has been systematically summarized and analysed in Table 7.

Table 7 Summary of rheological behaviour of different nanofluids [408]

4.2.1 Nanoparticle viscosity concentration effect

Several research studies have reported that the aggregation of nanoparticle degree in nanofluids augments the coefficient of HT in tandem with increased viscosity. Changing Al₂O₃’s concentration in water with degrees of 0.3, 0.5, 0.7, 1, and 2 percent was shown to contribute to a rise in viscosity, contributing to a rise in the friction component in turn [385,386,387,388,389]. In both water and ethylene glycol-based Al₂O₃ and water-based SiC nanofluids, a related behaviour was found. This effect also works for non-metallic nanofluids, wherein a number of experiments on the rheology of nanofluid carbon nanotubes have reported that augmenting the loading of carbon nanotubes makes the nanofluid’s viscosity rise. It is important to note that there are certain contradictions surrounding viscosity behaviours in the literature [390,391,392]. Pak and Cho [74] analyzed water-dependent nanofluids based on Al₂O₃ and TiO₂ and found that the HT decreased dramatically at a degree concentrating of 3% and was lesser than the HT of pure water. Variables other than volume concentration influence the viscosity of the nanofluid, like the form, scale, and surface chemistry of the nanoparticle [293]. Similarly, a water dependent Al₂O₃ and TiO₂ analysis found that the caliber and form of the nanoparticle and the degree fraction and temperature were all essential factors for viscosity determination. Nevertheless, in the literature, the parameters listed are weakly researched and more studies are needed.

4.2.2 Effect of temperature on viscosity

As described before, temperature is not considered by the theoretical viscosity models. Previous simulations can also only be valid at low concentrating and considering the condition of room temperature, but not at greater temperatures [289]. A number of researchers agree that by increasing the temperature, viscosity decreases [290]. Previous experiments have included nanofluids from CuO, Al₂O₃, SiC, and CNT, with an emphasis on nanofluids from Al₂O₃. In comparison, with temperature increases in CuO, Al₂O₃, and SiO₂ distributed in both water and ethylene glycol, viscosity was observed to decrease exponentially [292]. Analysis has also shown that if viscosity’s increase is four times greater than the nanofluids’ thermal conductivity, then increase in friction factor [293] makes it pointless. Viscosity experimental data that were obtained at 4% volume fraction, consisting of many data spots, were analyzed via regression and the following value was obtained (Fig. 20).

Fig. 20

Temperature effect on viscosity

4.3 Specific heat capacity (C _ρ)

Relevant heat potential tests a material’s ability to retain and exchange energy in the shape of heat when there is a temperature differential [110, 111]. Because specific heat is utilized to measure significant properties, including thermal conductivity, thermal diffusivity, and spatial temperature of flow, it is significant to obtain precise values with regard to the specific heat. For the calculation of nanofluids, researchers often use the deferential scanning calorimeter and double hot wire. The effect of size and concentration of the nanoparticle on nanofluids Many researchers have found that as the fraction degree of the nanoparticle rises, the relative heat reduces in nanofluids, because of the reduced heat power of the nanoparticles relative to their base fluid. The basic heat of five distinct nanofluids, which are Al₂O₃, ZnO, TiO₂, CuO, and SiO₂, with 60:40 propylene glycol and water ratios respectively, was investigated in a recent paper. The paper stated that the particle size had no major effect on the real heat after changing the nanoparticles’ concentration degrees from 0.5 to 6 percent and the particle calibers from 15 to 76 nm. On the other hand, the accumulation of volume had a major part in modifying the action of the heat power. The decrease in real heat was tolerable at low concentrations, mainly because it contributed to improving thermal conductivity, which increased the efficiency of HT. However, the heat potential decreases more as the fraction volume of the nanoparticle increases [202]. Likewise, the particular heat of the mixture of MgO, ZnO, and ZrO2 nanofluids based on water and ethylene glycol was examined, and the result was that while the nanofluids displayed a 30 percent rise in specific heat relative to the base fluids, the volume fraction of nanoparticles is still decreasing [203].

A number of researchers performed comparable researches and all recorded the same activity across a spectrum of nanofluids [204,205,206,207]. It is stated for carbon nanotubes nanofluids that as the multi-walled CNT concentration increased in 30:70 EG-water, the specific heat decreased [208, 209]. In comparison, however, a rise in specific heat was recorded with augmenting concentrations of single-walled CNT in water [210]. Carbon nanotubes (CNTs) are known to have high specific heat power. This is why augmented loading contributes to the rise in the real heat, although this needs to be proved. Impact on temperature on CP, most research studies have stated that with increasing the temperature, the real heat rose. Multi-nanofluid tests have reported that increasing temperatures would result in improved specific heat capacity [111, 202, 211]. Several reports, however, have observed the opposite impact and have stated that with elevated temperatures, real heat capability decreases [134,135,136]. With regard to volume concentration, the previous activity of particular heat does not apply for all CNT nanofluids where the temperature is varied. Relevant multi-walled CNT heat was found to increase with rising temperatures [137, 209], whereas it grew with rising temperatures [137, 209]. In the single-walled CNT nanofluid [210], it was the opposite. From Fig. 21, it can be inferred that the model could estimate nanofluids’ efficient specific heat value well when considered in the margin of ±5 percent.

Fig. 21

Comparison of efficient real heat technically estimated with measured evidence [294]. The dotted line corresponds to the error margin (±5 percent error margin)

5 Nanofluid applications

Nanofluids have many applications. Such applications are divided into two parts: heat transfer and mass transfer. Most of the industrial applications of nanofluids are related to cooling and heating, which is a subset of heat transfer, and the application of nanofluids in the field of mass transfer is more related to pharmaceutical and medical topics; for example, nanofluids can be used to send drugs to specific places in the body, without damaging the tissue. In fact, it is the size of these materials that makes such a difference in their properties and they have completely different properties from their base fluid [65,66,67,68,69,70]. Exceptional properties of nanofluids include mass transfer, thermal conductivity, and higher viscosity than conventional suspensions. Exceptional properties, along with stability, relatively easy preparation method, and acceptable viscosity have made nanofluids as one of the most suitable and strongest choices in the field of mass transfer and HT phenomena. Although nanofluids are man-made, the interesting point is that nanofluids exist in nature and the most important nanofluid found in nature is blood, as a complex of biological nanofluids [71,72,73,74].

An increasing number of applications related to energy conversion and storage rely on graphene because of its extraordinary combination of properties [1, 2]. Graphene is a solid material and it has been used as such in all these applications, however, fluids are strategic materials used in a wide range of industrial applications, which span from thermal to biomedical or to electrochemical systems. In particular, nanofluids, which integrate solid nanoparticles dispersed in a base liquid and constitute a new type of materials with ground-breaking new properties, provide new opportunities to advance in many fields. Heat transfer is currently the most intensively explored application. However, magnetic ferrofluids, health applications, and energy storage appear as other promising fields of study and potential application [3, 4]. The nature of the solid phases used in the preparation of nanofluids is extremely varied. In the case of heat transfer fluids (HTFs), all types of solids, from metals to oxides to carbons have been widely studied given the superior thermal conductivity of solids as compared to liquids [5], however, magnetic or electrochemical nanofluids are much more restricted to phases with the necessary magnetic or electroactive nature. In the latter type, electroactivity can be redox [6, 7] or capacitive [8], although hybrid materials and devices combining both of those are also possible [9]. In electroactive nanofluids, nanoparticles are dispersed into a base fluid that must be an ionic-conducting electrolyte. This represents an additional challenge in order to avoid coagulation processes which are frequently associated with the presence of ionic salts in the medium. Graphene nanofluids are prepared by dispersing graphene (or RGO) nanosheets in an adequate base fluid. They can be stabilized in organic or aqueous solvents [6, 8, 10, 11] in the form of pure, non-oxidized graphene [11] or rGO [8], but also in the form of hybrids [6, 10, 393, 394].

5.1 Heat transfer in nanofluids

This paper focuses on nanofluids, highlighting their uses and various mechanisms involved in their work. Modern nanotechnology has enabled the production of metallic or nonmetallic nanoparticles with average crystallite sizes below 100 nm. The mechanical, optical, electrical, magnetic, and thermal properties of nanoparticles are superior to those of conventional bulk materials with coarse grain structures. Nanofluids are a new class of nanotechnology-based heat transfer fluids engineered by dispersing nanometer-sized particles with typical length scales on the order of 1–100 nm in traditional heat transfer fluids. Due to their large surface area, less particle momentum, and high mobility, nanoparticles emerged as suitable candidates for suspending in fluids. Nanofluids are used in cooling and related technology overcoming the usual problems with common slurries such as sedimentation, clogging, increased pressure drop, erosion, and applicability to micro-channels [395,396,397,398,399]. Nanofluids of ceramic and pure metallic particles have been produced by the conventional two-step method where the particles are first produced by methods such as IGC or chemical vapor deposition and then the particles are dispersed in the fluid using various methods such as physical dispersion and chemical dispersion methods where various techniques such as ultrasonic vibration, use of surfactants, or control of pH can be used. For measuring thermal conductivity of nanofluids, the very first need is to standardize the measurement techniques. The observed enhancement of effective thermal conductivity over that of the base fluid is often few times for nanofluid compared to what would have been given by usual micrometer-sized suspensions.

A prime need in many industries and projects is to have high efficiency HT environments. In many industries, including heat sources, manufacturing processes, transportation and electronics, fluid cooling and heating play a significant role, and many ways have been documented to improve the rate of HT in such processes. Most of such ways are based on changes in equipment structure, like increased thermal surfaces (blades), thermal surface vibration, fluid injection or suction, and the application of electric or magnetic current. Such ways can scarcely keep up with the growing demand for HT and equipment compression including electronic chips, laser systems, and high-energy processes. Among the issues that have received less attention is the effect of fluid HT coefficient on the development of high-efficiency HT equipment. HT media are often composed of fluids such as water, ethylene glycol, or oil. These fluids have a very low HT coefficient compared to metals and even metal oxides. Thus, fluids containing very fine particles of these compounds are likely to show better thermal properties than pure fluids. Due to technological problems, studies in this field are mostly focused on odor suspensions that contain solid particles suspended in millimeters or up to micrometers. Particles on this scale cause serious problems in HT equipment. For these particles to fix swiftly in the system and if the channel involves a tinier diameter, the problem exacerbates. For example, when passing through microchannels, they become clogged and cause the path to become clogged, which leads to a huge drop in pressure. Also, the strike between these particles and the walls of the system and equipment causes wear [75,76,77,78,79,80]. Recent advances in nanoparticle production can be considered a step forward in methods of increasing HT because of the small particle size and low volume fraction used to solve problems such as agglomeration and pressure drop. In addition, the large relative surface area of nanoparticle increasesase particle stabiliy, reduces the problem of sedimentatio, and reduces the cost of fluid storage and transport. Table 8 lists some of the applications of nanofluids in heat transfer.

Table 8 Applications of nanofluids in various fields [12]

5.2 Advanced nanofluids and trucks

Due to the need for more powerful engines, truck manufacturers constantly look for ways to extend aerodynamic designs to their vehicles. One of the efforts in this field is to reduce the amount of energy required to deal with high resistances. In a typical heavy truck, at a speed of 110 km/h, about 65% of the total engine efficiency is spent on overcoming aerodynamic traction; one of the major reasons for this is air resistance. In cooling systems, different radiators are required depending on the type of fluid used. In order to transfer heat from the engine to the radiator and finally release this heat to the surrounding environment, it is necessary to use fluids with high heat capacities. These fluids are able to absorb heat without increasing their own temperature, and then transfer it very slowly to the environment without the need for more fluid, which slows down the heat of conventional vehicle radiators. If the HT rate by the fluids is increased in a way, the design of the radiators becomes easier and more efficient and they can be made smaller. Also, the size of vehicle cooling pumps can be reduced. Truck engines can also generate more power due to operating at higher temperatures. Increasing the thermal conductivity of coolers can also be a good idea for the production of advanced fuel cells and dual-fuel/electric vehicles [81,82,83,84,85, 400].

5.3 Metal nanofluids and cooling engines

The characteristics of diesel engines are rapidly changing in terms of limitations in reactions and efficiency. Cooling systems must be able to operate at higher temperatures and transfer more heat to the environment. The size of the radiators should also be reduced to remove extra car equipment and make it easier to get around. Realistically, confining more cooling power to less space will only be possible with the use of new technologies such as nanofluids [86,87,88,89].

5.4 Application of nanofluids in medicine and drug transfer

Nanotechnology using nanoparticles has made it possible to transfer drugs to specific cells. By placing the active agent only in the disease area and not at a higher dose than required, it is possible to significantly reduce the overall use of the drug and its side effects. The aim of purposeful delivery of drug is to decrease the side effects of drugs along with reducing ensuing use and treatment costs. With the use of nano mechanized devices and molecular targeting, it is possible to achieve the potential of the target. One advantage of utilizing nanoscale for medical technology is that the tinier the device, the less aggressive it is and the more likely it is to be placed in the natural environment. In addition, the biochemical reaction time is much shorter. These devices are swifter and more sensitive than conventional medications. The effect of drug delivery through nano medicine is profoundly based on these factors: (a) effective encapsulation of drugs, (b) successful drug delivery to the target area in the body, and (c) successful drug release [90,91,92,93,94].

This paper is focused on the application of nanofluid in drug delivery systems and disease treatment. Nanofluids can increase the mass and heat transfer through the different media. Repairing or regenerating damaged cells, human organs, and tissues are based on different technologies, for example, drug delivery, tissue engineering, etc. Biological function components like nanostructure materials are one of the main essential parts of human-related technologies. In this case, many functional nanomaterials and nanofluids have been investigated for drug delivery systems, gene therapy, tissue engineering, and cancer therapy [401,402,403].

5.5 Tissue engineering

Nano medicine may use bodies based on suitable nano materials and growth factors as part of tissue engineering to aid in the reproduction or repair of damaged tissue. Nanoparticles such as grapheme, carbon nanotubes, molybdenum disulfide, and tungsten disulfide are utilized as reinforcing factors to make powerful biodegradable mechanical nano composites for bone tissue engineering uses. For example, a meat cooker has been shown to use a gold-plated nano shell suspension activated by an infrared laser to combine two pieces of meat into one piece. It can be used to weld arteries during surgery [95,96,97].

5.6 Heat transfer in medicine

Heat transfer plays a crucial role in many biomedical applications in cryobiology (biopreservation and cryosurgery) and hyperthermic biology (thermal therapies). In these applications, thermal excursions are used to selectively preserve or destroy cells and tissues. Biopreservation is an enabling technology for many biomedical fields including cell and tissue banking, cell therapeutics, tissue engineering, organ transplantation, and assisted reproductive technologies. Thermal therapies including cryosurgery are increasingly important in all surgical sub-specialties for minimally invasive thermal destruction of tissues for cancer and cardiovascular disease treatment. In this talk work predominantly from our lab will be reviewed focusing on cellular and molecular phenomena that are important in defining outcomes of both cryobiological and hyperthermic biomedical applications. During these applications, microscale cellular phenomena linked to viability are mechanistically shown to depend on the heat transfer process in vitro. These events include cellular dehydration, intracellular ice formation, and membrane hyperpermeability, and blebbing [404,405,406,407]. In addition, new approaches to assess molecular targets of heating and cooling using calorimetric and spectroscopic methods (i.e. lipid hydration, protein denaturation, and solute segregation) will be discussed. In vivo, new approaches will be reviewed to define gene-regulated events (inflammation and apoptosis) and control them with targeted adjuvants such as TNF-a for cancer treatments. Finally, recent work will be reviewed with nanoparticles showing their dramatic potential to both enhance and control thermal therapy outcomes through adjuvant (drug) delivery, and laser and inductive (RF) heating within the body [408].

The use of nanofluids as effective coolants in the surgery of a particular organ reduces the risk of organ damage and safer surgery, and increases the patient’s chances of survival. Nanofluids can also kill cancer cells by creating high temperatures around the tumor without affecting healthy surrounding cells [98,99,100,101].

6 Nanofluid stability

The agglomeration of nanoparticles results in not only the settlement and clogging of microchannels but also the decreasing of thermal conductivity of nanofluids. So, the investigation on stability is also a key issue that influences the properties of nanofluids for application, and it is necessary to study and analyze influencing factors to the dispersion stability of nanofluids. This section will contain (a) the stability evaluation methods for nanofluids, (b) the ways to enhance the stability of nanofluids, and (c) the stability mechanisms of nanofluids.

Many methods have been developed to evaluate the stability of nanofluids. The simplest method is sedimentation method [20, 21]. The sediment weight or the sediment volume of nanoparticles in a nanofluid under an external force field is an indication of the stability of the characterized nanofluid. The variation of concentration or particle size of supernatant particle with sediment time can be obtained by special apparatus [5]. The nanofluids are considered to be stable when the concentration or particle size of supernatant particles keeps constant. Sedimentation photograph of nanofluids in test tubes taken by a camera is also a usual method for observing the stability of nanofluids [5].

Surfactants used in nanofluids are also called dispersants. Adding dispersants in the two-phase systems is an easy and economic method to enhance the stability of nanofluids. Dispersants can markedly affect the surface characteristics of a system in small quantity. Dispersants consists of a hydrophobic tail portion, usually a long-chain hydrocarbon, and a hydrophilic polar head group. Dispersants are employed to increase the contact of two materials, sometimes known as wettability. In a two-phase system, a dispersant tends to locate at the interface of the two phases, where it introduces a degree of continuity between the nanoparticles and fluids. According to the composition of the head, surfactants are divided into four classes: nonionic surfactants without charge groups in its head (include polyethylene oxide, alcohols, and other polar groups), anionic surfactants with negatively charged head groups (anionic head groups include long-chain fatty acids, sulfosuccinates, alkyl sulfates, phosphates, and sulfonates), cationic surfactants with positively charged head groups (cationic surfactants may be protonated long-chain amines and long-chain quaternary ammonium compounds), and amphoteric surfactants with zwitterionic head groups (charge depends on pH).

Nanofluids are not merely a blend of liquid and solid particles, but nanoparticles are likely to agglomerate due to their high surface activity, and this agglomeration causes microchannels to settle and capture, reducing the physical properties of the nanofluid. Therefore, the stability of nanofluid should be seriously considered. The most significant factors influencing nanofluids’ stability are: nanoparticle concentration, dispersants, fluid viscosity, pH value, nanoparticle type, nanoparticle diameter and ultrasonic time [102,103,104,105].

6.1 Nanofluid stability mechanisms

Sustainable nanofluid preparation is a prerequisite for leveraging nanofluid characteristics. The aggregation and agglomeration of nanoparticles increases the likelihood of precipitation and as a result decreases the stability. The deposition degree of layered particles in a stationary fluid could be calculated by Stokes’ law:

$$V = \frac{{2R^2}}{{9\mu }}\left( {\rho _P - \rho _1} \right)g$$

This equation is obtained by striking a balance in the forces of gravity, buoyancy, and drag acting on the particles. R is the particle radius, the fluid viscosity, ⍴p the particle density, and ⍴l the fluid density. According to this law, as the particle size decreases, the velocity of the particles settles. As the caliber of the particle gets to a critical radius (Rc), no precipitation occurs because of the particles’ brown motion. Although radius-bearing particles less than the Rc do not settle, tinier particles involve greater levels of power and are more probable to deposit. Therefore, to provide a stable nanofluid, small particles must be used to prevent them from accumulating. Nanofluid stability means the non-accumulation of nanoparticles and significant precipitation, and therefore the concentration of floating nanoparticles becomes constant [106,107,108,109]. Based on the Derjaguin-Landau-Verwey-Overbee (DLVO) theory, nanoparticles’ stability in a fluid is measured as a result of the forces of gravity and stabilization. Generally, there exist four intermolecular powers between particles. The forces of absorption between particles are: (a) van der Waals forces and (b) magnetic dipole forces if the particles are magnetic. Particles’ stabilization power is because of the electrostatic stabilization forces on the surface of nanoparticles mixed with an electric charge, and the steric stabilization forces on the nanoparticles surface as mixed with polymers or surfactants. If the stabilization force of the particles overcomes the gravitational force, the nanofluid reaches a steady state; otherwise, the two particles strike and stick together. Thus, for stable nanofluids, stabilization powers must be predominant among particles [110,111,112,113,114]. The fundamental mechanisms that affect the stability of colloids are grouped into two categories on the basis of the types of stabilization: steric stabilization and electrostatic repulsion. Figure 22 shows a schematic illustration of these two types of stabilization.

Fig. 22

Steric stabilization and electrostatic stabilization [7]

6.2 Surfactant

An easy and economical alternative to augmenting the stability of the nanofluid is to add a surfactant to the nanofluid. Surfactants substantially affect the surface characteristics of the system. Such materials involve a hydrophilic polar end and a hydrophobic end (often a hydrocarbon chain). Surface active ingredients are grouped into four categories on the basis of the composition of the hydrophilic head:

(A) There is no non-pregnant group in the hydrophilic head.

(B) Anion with negatively charged group

(C) Cation with positive pregnant group

(D) Amphoteric that the charge on the hydrophilic head can be positive or negative.

To opt for the suitable surfactant, it must be borne in mind that if the base fluid is polar, surfactants with a hydrophilic head are used; otherwise, surfactants that are soluble in oil are used [115,116,117,118,119]. Care should also be taken in the use of these materials because excessive being of such contents in the nanofluid changes the nanofluid’ properties and affects mass transfer and HT. The major surfactants utilized by the researchers are Sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfate (SDBS), acetyl trimethylammonium bromide (CTAB), oleic acid, dodecyl trimethylammonium bromide (DTAB), polyacrylonitrile (DTAB). (PVP) [120].

Although using surfactants is a common way to enhance the stability of the nanofluid, adding such materials to the nanofluid may bring about problems such as foaming and decreasing the nanofluid’s thermal conductivity. Also, as a result of the collapse of the bond between the surfactant and the nanoparticle at temperatures above 60 °C, the stability of the nanofluid is lost [121,122,123].

6.3 Nanofluid pH control

A nanofluid’s stability is strongly related to its electro-kinetic properties. Thus, if the charge density is high on the surface of the nanoparticles, the nanoparticles will be stable in the fluid due to the electrostatic repulsion force. Therefore, the desired stability can be achieved by adjusting the pH of the nanofluid [124,125,126,127]. The Hamaker equation: EA = A₁₃₂r/ (12x). The Hamaker constant A₁₃₂ of metal oxide is usually on the order of 10⁻²⁰ J. Using Hamaker equation and the estimated Wd, E_tot is measured as a function of x at different pHs as Fig. 23 indicates. The pH for the point of zero charge also alters by the variation of temperature as indicated in Table 9 [50].

Fig. 23

Potentials for contact at different pHs as a result of interparticle distance [53]

Table 9 Values of pH_pzc of the TiO₂ between 5 and 55 C [50]

6.4 Ultrasonic vibration

Ultrasonic vibration can be utilized to increase the stability of the nanofluid. The previous two methods assist with improving the stability of nanofluids by changing the surface of nanoparticles, but in this method, ultrasound waves cause weak surface connections between nanoparticles and thus break down agglomerates and augment the stability of nanofluids [128,129,130,131,132,133, 198]. A summary of researchers reaching diverse duration of stability using ultrasonic ways is presented in Table 10.

Table 10 Summary of different ultrasonic processes

6.5 External field application and settling

In this method, the amount of weight or volume of nanoparticles deposited in the nanofluid, under the force of external gravitational field or centrifuge, is a measure of the stability of the nanofluid. Thus, the more the nanoparticles precipitate, the less stable the nanofluid is [199,200,201,202].

6.6 Ultraviolet-visible absorption spectroscopy (UV-Vis Spectrophotometry)

This method is one of the easy methods to study the stability of nanofluids. Changes in concentrating floating particles in the nanofluid are obtained over time by calculating the attraction of nanofluids, because there is generally a linear correlation between the absorption intensity and concentrating nanoparticles in the fluid [203, 204, 408]. The disadvantage of this method is that it is not suitable for high concentration nanofluids. In addition, there is a snippet of different absorptions of nanofluid peaks by Ultraviolet-visible absorption spectroscopy (UV-Vis Spectrophotometry) in Table 11. The sizes of Au nanoparticles from different preparation methods calculated by TEM and peak wavelength are shown in Table 12.

Table 11 Description of the peak absorption of various nanofluids measured by the UV-Vis spectrophotometer

Table 12 Volumes of gold nanofluid in different synthesis conditions [71]

6.7 Zeta potential analysis

The amount of zeta potential is related to the colloidal solution’s stability. Colloidal solutions with high zeta potential (positive or negative) have better stability. In general, it is said that nanofluids with a zeta potential of 40 mV to 60 mV have acceptable stability and nanofluids with zeta potential above 60 mV have very good stability. Table 13 shows the relationship between nanofluid stability and the amount of zeta potential. The problem with this method is the limitation of the viscosity of the base fluid [205].

Table 13 Shows the relationship between nanofluid stability

7 Conclusion

• 1. For nanofluids, the problems created by degradation, impurities and pressure drops are dramatically decreased due to the limited size of the particles, and the stability of fluids against sedimentation is substantially enhanced. Since nanoparticles have a strong conductivity, as they are dispersed in a base fluid, they improve the fluid’s thermal conductivity, which is a significant factor in HT. Nanoparticles also improve mass transfer, but the exact mechanism of this phenomenon has not yet been determined and more research is needed. Due to the unique properties of nanofluids, they have many applications and the most important of them is the use of heat transfer and medicine.

• 2. Nanofluids are a novel generation of fluids with great potential in industrial cartridges. In nanofluids, because of the small caliber of the particles, corrosion, impurities, and pressure drop problems were immensely decreased and the stability of fluids against deposition was significantly improved. In general, two main methods for making nanofluids were described. In the two-step method, after preparing the nanoparticles, they are added to the fluid, at which point the particles may stick to each other. In the one-step method, nanoparticles are synthesized in the target carrier fluid. The agglomeration of nanoparticles in the nanofluid causes sedimentation and capture of micro channels, and reduces the physical properties of the nanofluid; so, it is very important that the nanofluid has a good stability. According to DLVO theory, nanoparticles’ stability within a fluid is measured as a result of the powers of gravity and repulsion. The main methods to increase the stability of nanofluids are the addition of surfactants, pH adjustment and the use of ultrasonic devices. There are several methods to study the stability of nanofluids, the most important of which is zeta potential analysis and ultraviolet-visible absorption spectroscopy.