Chapter 15 Thermophysical Characteristics of Nanofluids: A Review Chou-Yi Hsu, Gargibala Satpathy, Fatma Issa Al Kamzari, E. Manikandan, Yathrib Ajaj, and Aithar Salim Al Kindi Abstract Rapid advancement in nanotechnology over the past 10 years has given scientists and technologists plenty to consider. One of the amazing results of such innovation is nanofluid, in which metallic and non-metallic nanoparticles are suspended colloidally in commonly used base fluids. The peculiar thermal behaviour of nanoscale fluids was only recently discovered by leading researchers, and since then, nanofluids have been the subject of intense investigation all around the world. This is because they have the potential to be an improved thermophysical heat transmission fluid and because they are crucial for uses like oil recovery and medicine delivery. Recent research on nanofluids has revealed that these fluids have superior heat transmission and wetting properties. In addition, water-based nanofluids are more environmentally friendly than mineral oil quench medium. Due to these potential benefits, quench media based on nanofluids have been developed for use in heat treatment procedures. It was unexpected and outside the realm of thencurrent theories that nanofluids with comparable low particle concentrations would have a larger thermal capacity (conductivity). According to the experimental findings, the thermal conductivity of nanofluids increases with decreasing particle size. C.-Y. Hsu Department of Pharmacy, Chia Nan University of Pharmacy and Science, Tainan, Taiwan G. Satpathy Central Research Laboratory, Sree Balaji Medical College & Hospital (SBMCH), Bharath Institute for Higher Education & Research (BIHER), Bharath University, Chennai, Tamil Nadu, India F. I. Al Kamzari · Y. Ajaj (*) · A. S. Al Kindi Engineering Department, Faculty of Engineering and Computer Science, German University of Technology in Oman, Halban, Oman E. Manikandan Central Research Laboratory, Sree Balaji Medical College & Hospital (SBMCH), Bharath Institute for Higher Education & Research (BIHER), Bharath University, Chennai, Tamil Nadu, India Solid-State Nanoscale Laboratory, Department of Physics, Thiruvalluvar University, Vellore, Tamil Nadu, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 J. A. Malik, M. J. S. Mohamed (eds.), Modern Nanotechnology, https://doi.org/10.1007/978-3-031-31104-8_15 337 338 C.-Y. Hsu et al. Micrometre-sized particle-based fluid suspensions reveal no such significant augmentation. Another critical characteristic that demands the same focus is viscosity, which has a significant influence on heat transmission. Nanofluid stability is essential to their successful operation in thermal systems. In addition to this, a number of other thermophysical characteristics, for instance, thermal conduction, heat flux, density, viscosity, and specific heat, are crucial for materials to keep their thermophysical characteristics after manufacture for an extended period of time. But before nanofluids can be used commercially, there are a few problems that need to be fixed. Stability and functioning performance are the two key concerns of nanofluids. In light of the fact, the intention of this chapter is to provide thorough analysis on thermophysical features of nanofluids and some potential opportunities for upcoming research that could be important as the literature in this field is lengthened across an extensive range of fields, including heat transfer, physics, chemical engineering, material science, and synthetic chemistry. Keywords Nanotechnology · Nanofluid · Thermophysical · Heat transmission · Wetting properties · Thermal conductivity · Nanofluid stability · Operational performance · Synthetic chemistry 15.1 Introduction Nanoparticle-containing base fluids (nanofluids) have evolved as a novel category of fluids with extraordinary features of improved heat transfer. A new category of fluids called nanofluids has been developed with increased heat transfer qualities and were synthesized in 1965 by Choi (1995) by means of dispersing nanoparticles (nanorods, nanotubes, nanofibers, nanosheet, nanowires, or droplets) in base fluids (Fig. 15.1). Compared to colloidal solutions, the diluted suspensions of nanofluids have a higher heat transmission surface between the fluid and the particles. These benefits make nanofluids useful in various divisions such as electronic applications, power generation, air conditioning, chemical synthesis, nuclear system cooling, heating and cooling procedures, space and defence, solar absorption, transportation and microelectronics energy storage, magnetic sealing, friction reduction, biomedical application, and nano drug delivery (Das et al. 2006; Yu and Xie 2012). It is therefore possible to construct lighter and more compact heat exchangers as significantly less fluid is needed than the base fluid. Further, nanofluids might be crucial in the removal of extra energy produced by the burning of fuel in the automotive sectors. Nanofluids can lose heat through the radiator’s walls when flowing through the tubes. If there are big and small particles floating in the base fluid, heat transfer is improved. Larger particles sediment and corrode the channels through which they flow, causing overall wear and tear of the pipes and channels. Both of these issues have been resolved by nanofluids for heat transmission. The suspensions are significantly steadier and can produce lesser attritions along with efficient transfer of heat features than the base fluids (Keblinski et al. 2002; Das 2015). 15 Thermophysical Characteristics of Nanofluids: A Review 339 Fig. 15.1 Nanoparticles suspended in base fluid Rapid advancement in nanotechnology over the past 10 years has facilitated the development of new materials that outperform and possess superior qualities than prior materials. This development of nanomaterials has led to the creation of versatile nanofluids. Compared to solids, conventional fluids have inferior thermal–physical characteristics. The use of extended-surface materials such as fins and microchannels, surface vibration, fluid suction/injection, and the use of electrical and magnetic fields have all been abandoned to boost heat transfer. Consequently, much research has been done on novel technologies with the aim of improving the thermophysical characteristics of conventional fluids (Duangthongsuk and Wongwises 2010; Gupta et al. 2017). In light of the fact that the literature in this field is expanded across an extensive range of fields, comprising material science, heat transfer, chemical engineering physics, and synthetic chemistry, the goal of this article is to exhaustively review and elucidate the thermal conductivity, viscosity, and specific heat of nanofluids and some potential opportunities for upcoming research that could be useful. 15.2 Various Types of Nanofluids Nanoparticles were added to thermofluids in 1995 to augment their thermal characteristics (Choi and Eastman 1995). Since then, extensive work has been done on creating nano-based fluids with distinctive thermophysical characteristics, for example, thermal conductivity, thermal diffusivity, and viscosity (Prasher et al. 2006; Yoo et al. 2007; Chamsa-Ard et al. 2017). Studies have revealed that adding small quantity of nanoparticles can increase the thermophysical features of a variety of fluids. Various types of nanoparticles that have been studied comprise metal oxides (Fe3O4, Al2O3, CuO, SiO2, TiO2, and ZnO), carbides (SiC, TiC), pure metals (Au, Ag, Cu, Al, and Fe), and a variety of carbon materials (graphite, diamond, and 340 C.-Y. Hsu et al. single- and multiwall carbon nanotubes). Many of these different kinds of nanoparticles have been used in liquids including water, water/ethylene glycol, ethylene glycol, and oils. Consequently, they fall into two basic categories: single-material as well as hybrid nanofluids (Chamsa-Ard et al. 2017; Prasad et al. 2017; Sawant et al. 2021). 15.2.1 Single-Material Nanofluids Choi and Eastman (1995) suggested that in single-material nanofluids a specific kind of nanoparticle is employed to produce the suspension using several procedures (Fig. 15.2). Different studies have claimed that this type of nanofluids performs better because they have base fluids that are much beneficial thermophysically (Tawfik 2017; Modak et al. 2018; Sawant et al. 2021). 15.2.2 Hybrid Nanofluids In the most sophisticated subcategory of nanofluids, hybrid nanofluids which are suspended in a base fluid contain a variety of nanoparticles of different types (Fig. 15.3). Jana et al. (2007) conducted the first experimental research on this kind of fluid with the goal of improving the fluid’s thermal conductivity exceeding that of a typical single-material type nanofluid. They looked at Cu nanoparticles, CNTs Fig. 15.2 Single type nanofluid 15 Thermophysical Characteristics of Nanofluids: A Review 341 (carbon nanotubes), Au nanoparticles, and their subsequent hybrids such as CNT-Cu/ H2O as well as CNT-Au/H2O that are all dispersed in water. In accordance with the test results, Cu/H2O nanofluids had the highest thermal conductivity of all the samples examined, and this conductivity increased linearly as the particle concentration increases. Although the CNT-Cu/H2O nanofluid was less stable than other nanofluid types, it did achieve a longer settling time compared to other varieties of nanofluids. Because of this, the fluid may maintain its thermal conductivity for a much longer before it degrades (Otanicar et al. 2010; Ali et al. 2018). 15.3 Methods of Preparation of Nanofluids It takes more than merely combining nanoparticles with the base fluid to produce nanofluids. Stabilization and correct mixing are necessary under specific environmental circumstances to produce nanofluids with homogeneously distributed nanoparticles and may have a substantial influence on the thermophysical features of the nanofluid. The thermophysical characteristics and tendency to agglomerate two nanofluids that are comparable to one another but were created using different techniques (Fig. 15.4) are most likely to differ from one another. This is due to the Fig. 15.3 Hybrid nanofluid 342 C.-Y. Hsu et al. fact that a solid–liquid combination alone cannot form nanofluids; rather, a suspension with a number of qualities including homogeneity, chemical and physical stability, dispersibility, and durability are required. The size range of the nanoparticles employed in nanofluids is 1–100 nm, and they come in a variety of geometries, including nanoreefs, nanospheres, nanoboxes nanotubes, and nanoclusters. According to a few reports (Lee et al. 2016; Harikrishnan et al. 2017; Kaggwa and Carson 2019), the synthesis process determines the nanoparticles’ morphology; additionally, there is a great relation between the nanoparticle size and the thermal conductivity, which is a major factor in improving heat transfer. There are several ways to prepare nanofluids, and they may be further divided into two categories: the one-step method, also referred to as the bottom-up technique, and the top-down strategy, also referred to as the two-step method (Chamsa-ard et al. 2017; Ali et al. 2018). 15.3.1 Single-Step Method In this approach, the synthesis as well as distribution of nanoparticles into the base fluid is combined in single step as presented in Fig. 15.5. One of the most popular methods for making nanofluids, the single-step direct evaporation approach, depends on solidifying nanomaterials that were gaseous in the base fluid before. This technique identified as vacuum evaporation onto a running oil substrate (VEROS) method was devised by Akoh et al. (1978). The main objective of this procedure was to create nanoparticles; nonetheless, it ended up being quite challenging to separate the created fluid mixture of nanoparticles into their dry form. In this situation, condensation and phase change of a material from the vapour stage Fig. 15.4 Physical and chemical methods of synthesizing nanofluids 15 Thermophysical Characteristics of Nanofluids: A Review 343 Fig. 15.5 Bottom-up method of nanofluid synthesis into the heat transfer fluid may be the reason for the development of nanoparticles. The base liquid, which might be ethylene glycol or oil, is put in a revolving cylindrical drum with a regulating heater-boat-evaporator, as well as heat exchanger cooler device. The internal surface of the drum develops a thin liquid layer as it rotates. An adjustable heater-boat-evaporator with a nanomaterial source put within its evaporator is situated close to a section of the spinning thin liquid layer. The evaporant matter is heated, causing some of it to evaporate and form nanoparticles, which are afterwards absorbed by the liquid film to produce nanofluid. In their modified VEROS method, Wagener et al. (1996) synthesized dispersions with Fe and Ag nanoparticles utilizing high pressure magnetron sputtering. Eastman et al. (1996) used a customized VEROS approach to efficiently condense Cu vapour with a moving low-vapour-pressure EG to generate the Cu/EG nanofluid containing Fe and Ag. Cu nanofluid was produced in one step by Zhu et al. (2004) via a chemical process. Al2O3 nanofluid was created by Tran and Soong (2007) using high-power optical vaporization in one-step process. Although the single-step approach has the benefit of creating the least amount of nanoparticle agglomeration, it is expensive and hence probably not practicable on an industrial scale (Kaufui 2009; Yu and Xie 2012; Sawant et al. 2021). 15.3.2 Two-Step Method Because the single-step approach is more expensive than the two-step procedure, the two-step technique is more commonly used. In comparison with the one-step strategy, the two-step approach is best method for creating nanofluids because it has small processing charges and a large supply of commercially available nanoparticles from several businesses. A schematic representation of the two-step method utilized to create nanofluid is shown in Fig. 15.6. The creation of nanoparticles, nanofibers, nanotubes, and other nanomaterials using this method occurs in a separate step prior to their addition to the base fluid or purchased as dry powder (Ali et al. 2018; Kaggwa and Carson 2019). Magnetic stirrers, homogenizers, ultrasonic baths, high-shear mixers, and bead mills are some of the frequently used apparatus 344 C.-Y. Hsu et al. Fig. 15.6 Top-down method of nanofluid synthesis for dispersing nanoparticles in base fluid. By using chemical or physical processes, the nanoparticles employed in this procedure are initially created as dry powders. Higher magnetic force agitation, ultrasonic agitation, high-shear mixing, homogenizing, and ball milling will next be used to disperse the nanosized powder into a fluid in the second processing stage. The interfacial forces between the molecules of the base fluid as well as the nanoparticles can be reduced by adding stabilizing agents, such as surfactants. Figure 15.7 illustrates the key procedures for creating nanofluids. The two-step method has a significant amount of particle aggregation as a disadvantage in comparison with the one-step method. In spite of these drawbacks, this approach remains the well-familiar method for creating nanofluids in lesser or greater quantities and may be employed to generate almost any variety of nanofluid (Haddad et al. 2014; Ali et al. 2018; Kaggwa and Carson 2019). 15.4 Thermophysical Characteristics A broad variety of commercial, industrial, and domestic processes and appliances, such as those used in the chemical processing, solar energy generation and conversion, air conditioning and refrigeration, the gas and oil industries, and electronics cooling, require familiarity and thoughtful understanding of heat transfer. The phrase “heat transfer enhancement” in thermal engineering refers to the augmentation of a system’s thermal performance. Several methods have been suggested as strategies to improve heat transmission during the past 10 years (Siddique et al. 2010; Léal et al. 2013; Sheikholeslami et al. 2015). The base fluid’s thermophysical characteristics, for example, specific heat, viscosity, and thermal conductivity, alter as a consequence of the integration of nanoparticles, which has an impact on convective heat transmission. The degree to which various nanomaterials alter their parameters varies. Some of the key variables that greatly affect the thermophysical characteristics include the nanoparticle concentration, purity level, size, and form of the nanomaterials. Viscosity, density, thermal conductivity, and specific heat are known to have an impact on conduction and convection. Therefore, correct thermophysical property data are necessary for any heat transport model. To ascertain these characteristics of the nanofluids under various operating circumstances, several 15 Thermophysical Characteristics of Nanofluids: A Review 345 Fig. 15.7 Main steps in preparing nanofluids investigations have been conducted. Some of them claimed that as compared to normal fluids, nanofluids had improved thermal characteristics (Das 2017; Chen et al. 2019). The performance of heat transmission is closely correlated with the hybrid nanofluid’s characteristics (Hamzah et al. 2017; Chen et al. 2019). The flow of these dual phases of nanofluids in the evaporation and the condensing areas must thus be analysed in terms of their thermophysical properties. It is crucial to explore the fundamental thermophysical features of the nanofluids consecutively to maximize the performance of the nanofluids in the systems. The most recent review of the thermophysical characteristics of several nanofluids is provided in this section. 15.4.1 Thermal Conductivity From the available literature, the most researched aspect of nanofluids is their improved thermal conductivity, which is generally accepted to be the primary cause for the considerable improvements in heat transfer rates that have been reported. To look at how nanofluids’ thermal conductivity changes, several theoretical and experimental studies have been undertaken on diverse base fluids (propylene glycol, ethylene glycol, methanol, glycerol, paraffin, gear oil, engine oil, etc.) using a variety of nanoparticles. Figure 15.8 shows the thermal conductivities of different solids as well as liquids. Addition of NPs to the base fluids increases their thermal conductance. Brownian motion and the interfacial nanolayer are the key phenomena that govern the thermodynamic activity of nanomaterial solutions that are adjacent to a solid particle surface generate layered structures that are crucial for regulating the thermal characteristics of nanoparticles–fluid dispersions. Choi and Eastman (1995) 346 C.-Y. Hsu et al. claimed that adding a minute amount of nanoparticles might improve the thermal conductivity of the base fluid. Hemmat-Esfe et al. (2014) looked into ferromagnetic nanoparticle suspensions in ethylene glycol. These researchers focused on the effects of temperature, concentration as well as particle size on the viscosity as well as thermal conductivity of the nanofluids having a concentration up to 3% in the temperature gradient of 26–55 °C. According to their findings, as temperature and the volume proportion of solids rose, so did the efficiency of nanofluids. Based on multi-level homogenization and particle size distribution, Deepak and Dhinakaran (2016) gave a concept to forecast the thermophysical properties of nanofluids especially the thermal conductivity. They mostly concentrated on the consequences of Brownian motion, the creation of interfacial layers, and particle clustering. Similar to this, Lee et al. (2016) showed that raising the temperature as well as particle size increased the efficiency of nanofluids. Thermal conductivity and viscosity studies, however, showed that particle size fluctuation was more pronounced than temperature variation. An investigation on the thermophysical characteristics of a nanofluid made of carbon-based material was done by Ueki et al. (2017). They came to the conclusion that thermal conductivity was impacted by temperature and nanoparticle shape. They also discovered that carbon nanopowder increased the conductance of heat by 7% while carbon black by 19%. According to Lenin and Joy (2017), different surfactants have different threshold concentrations for improving thermal conductivity. This variation may be caused by changes in the degree of nanoparticle accumulation and surfactant molecule conformation on the surface of the nanoparticles. According to them, base fluids with inferior thermal conductivities as well as dielectric constants exhibited greater rises in thermal conductivity when compared with the base fluids with higher thermal conductivities. According to Yang et al. (2017), environmental elements (temperature, pH value, base fluid, and the standing duration) and particle parameters (particle type, shape, loading, and size) have an impact on thermal conductivity. The Fig. 15.8 Thermal conductivities of different solids and liquids 15 Thermophysical Characteristics of Nanofluids: A Review 347 variance in thermal conductivity improvement reported in the literature may be significantly attributed to the aforementioned parameters as well as preparation techniques. 15.4.1.1 Factors Controlling the Thermal Conductivity of Nanofluids The thorough analysis leads to the conclusion that base fluid material, temperature, particle dimensions, form, and composition, among other variables, impact the thermal conductivity of nanofluids (Ranakoti et al. 2012; Pastoriza-Gallego et al. 2014). The various elements controlling the thermal conductivity of nanofluids are depicted in Fig. 15.9. Size of Particles The development of nanofluids’ thermal conductivity is significantly influenced by particle size. While the thermal conduction of nanofluids rises with particle size, the consistency of the suspension declines. The size range of the produced nanoparticles is between 5 and 100 nm. The impact of nanoparticle dimension on the base fluid was explored by Paul et al. (2010). The impact of particle dimension on the thermal conduction quotient of alumina (Al2O3)/water nanofluids was investigated by Teng et al. (2010). The findings of the experiment demonstrated that decreasing particle size enhances the thermal conductivity of nanofluids. Shape of Particles In nanofluid research, cylindrical and spherical particles are typically utilized as particle types. Nanoparticles’ cylinder forms feature a significant length to diameter ratio. The impacts of the form of the nanoparticles, such as cylindrical or spherical, on the improvement of thermal conductivity of SiC nanofluids were initially examined by Xie et al. (2002a, b). Murshed et al. (2008a, b) also evaluated at how cylindrical and spherical nanoparticles affected the thermal properties included conductivity of nanofluids. Composition of Particle and Base Fluid The several types of particle materials utilized for the formation of nanofluids include metal carbides, oxide ceramics, metals, nitrides, and nonmetals. Both single and multiwalled carbon nanotubes are having high thermal conductance and are thus employed as particle material. Base fluids including water, ethylene/propyleneglycols, biofluids, and motor oil are used to create nanofluids for thermal energy transfer processes. 348 C.-Y. Hsu et al. Fig. 15.9 Factor affecting the thermal conductivity of nanofluids Temperature Both the temperature of the base fluid and the thermal conductivity of the particles affect the thermal conductivity of the nanofluids. The clustering of nanoparticles and Brownian motion are impacted by temperature variations, which also reveal deviations in the thermophysical properties and the heat transfer of nanofluids. Yu et al. (2009b) noted the temperature-dependent increase in thermal conductance for nanofluids encasing ZnO nanoparticles. With rising temperatures, nanofluids’ thermal conductivity increased. The thermal conductivity of nanofluids was experimentally investigated by Duangthongsuk and Wongwises (2010). TiO2 nanoparticles dispersed in water at a volume concentration of 0.2 to 2 vol% were employed in this investigation. The findings demonstrated that when the temperature of the nanofluids improved, so did their measured thermal conductivity. Kole and Dey (2011) inspected the thermal effects on the improvement of heat transfer characteristics for CuO-gear oil nanofluids. The results specified that at ambient temperature, the greatest boost of 10.4% was acquired with 0.025 volumes present of CuO nanoparticles, and it rises up to 11.9% at 80 °C temperature. According to research by Suganthi et al. (2014a, b), the maximum thermal conductivity improvement was attained at the lowest temperature, and it declined as temperature increased (10–30 °C). According to the logic, the ethylene glycol molecular layer’s thickness rose as temperature decreased and its well-ordered organization of liquid molecules, in comparison with the bulk liquid, had a greater thermal conductivity while as from 30 °C up to 60 °C, no improvement was seen. This was explained owing to the drop in thermal conductivity caused by the thinner liquid layers counteracting the upsurge 15 Thermophysical Characteristics of Nanofluids: A Review 349 in thermal conductance caused by Brownian movements of particles at maximum temperatures. Manikandan et al. (2014) also showed that sand in propylene glycol had a higher thermal conductivity of 46.2% for 2 vol% nanofluids around 10 °C. Additives The introduction of additives keeps the nanoparticles in suspension and stops them from clumping together. As a result, it is anticipated that they will increase the thermal conductivity of nanofluids. Both with and without additions, Eastman et al. (2001) conducted various experiments on Cu in ethylene glycol. According to the findings, additives can significantly boost the thermal conductivity of nanofluids. Cu-gear oil nanofluids with 0.1–2% volume concentration of Cu nanoparticles were created by Kole and Dey (2013) using oleic acid surfactant. 2 vol% of the Cu nanoparticles increased thermal conductivity by 24% at room temperature. Li et al. (2008) examined the impact of adding sodium dodecyl benzene sulfonate as a surfactant on the thermal conductivity of Cu-H2O nanofluids. The findings showed that the SDBS surfactant concentration of nano-suspensions had a substantial control on the improvements of Cu-H2O nanofluids with regard to their thermal characteristics and conductivity. Acidity (pH) There are not many studies looking at how base fluid pH affects the thermal conductivity of nanofluids. After doing various studies on Al2O3/water nano-based fluids, Xie et al. (2002a, b) were the pioneers to study how a rise in pH resulted in a fall in thermal conductivity ratio. The findings revealed that the thermal conductance increase of Al2O3/water nano-based fluids varied from 23% up to 19% when the pH rises to 11.5, from 2. Additionally, Li et al. (2008) demonstrated how pH affects the thermal conductivity of Cu-H2O nanofluids. The impact of pH levels on the thermal conductivity of the nanofluids was also investigated by Zhu et al. (2009). In this work, Al2O3-H2O nanofluids were created, and water’s thermal conductivity was examined at various pH levels. The combined use of a chemical dispersant and pH treatment is suggested to improve thermal conductivity while maintaining the practical use of nanofluids. Clustering Another factor that may impact a nanofluid’s thermal conductivity is clustering. The clustering of nanofluids at high concentrations and for longer periods of time reduces the effective area of thermal contact between particles, lowering the fluid’s thermal conductivity. Researchers Keblinski et al. (2002) and Hong et al. (2006) explored how clustering affected the thermal conductivity of nanofluids. Testing 350 C.-Y. Hsu et al. Fe3O4/water nanofluids, Zhu et al. (2006) witnessed that the raise in thermal conductance was mostly caused by clustering and nanoparticle alignment. Several theoretical models were created by researchers to forecast the rise in thermal conductivity, and numerous tests were performed to relate the results with the models’ predictions. 15.4.2 Thermal Diffusivity The term “thermal diffusivity” refers to a substance’s thermal conductivity divided by the product of that substance’s density and specific heat capacity. The effective thermal diffusivity of nanofluids has only been briefly discussed in a very small number of published works (Eggers and Kabelac 2016). Murshed et al. (2006) documented the augment in effective thermal diffusivity of nanofluids through experimentation and discovered that it was more than what Maxwell (1891) had predicted. The increase in effective thermal diffusivity of nanofluids was seen experimentally by Murshed et al. (2006), and they discovered that it was more than what Maxwell (1891) had predicted. It is important to note that they used observed data to determine the effective thermal diffusivity. The laser flash method (Agresti et al. 2013), the temperature-regulated photoacoustic device created by Agresti et al. (2015), the thermal wave cavity technique, the hot-wire method, as well as the temperature oscillation method (Rodriguez et al. 2016) are all experimental methods that can be employed to measure the actual thermal diffusivity. The absolute thermal diffusivity value of liquids is measured using the thermal lens method, which is a sensitive approach. This approach is advantageous because of its extremely high sensitivity, minimal sample volume required, and reliance on the thermo-optical characteristics of the solvent (Joseph et al. 2010). 15.4.3 Viscosity Viscosity is yet another critical component in heat transfer applications owing to its reliance on the pressure loss and the pumping power. The tendency of the solution to oppose the flow is called as viscosity of nanofluid. It can also be described as the relationship between shear stress and shear rate. Viscosity is the thinness or thickness of a fluid in non-scientific terminology. It is determined by dividing the tangential tension on a liquid in streamlined flow by the velocity gradient. The increase in effective viscosity brought on by the more nanoparticles in the base fluid negates the advantage of nanofluids’ enhanced heat transfer. The demands for pumping power rise as a result of the increase in viscosity, which causes greater pressure losses. The viscosity of base fluid, nanoparticle concentration, shape of particles, diameter, type, temperature, pressure, pH value, and shear rate are the primary factors that 15 Thermophysical Characteristics of Nanofluids: A Review 351 affect the effective viscosity (Eggers and Kabelac 2016). Other important factors are temperature, pressure, and pH value. The seminal study by Choi and Eastman (1995) made the important assumption that the viscosity of the nanofluids would not be considerably affected by the addition of nanoparticles, yet this is not always the case. For instance, Namburu et al. (2007) tested the viscosity of copper oxide nanoparticles distributed in a solution of water and ethylene glycol and discovered that it was four times as thick as the base fluid at a volume concentration of 6.12%. They also came to the conclusion that as the concentration of nanoparticles grows so does the viscosity of nanofluids. CuoH2O nanofluids’ apparent viscosity was studied by Li et al. (2002), and the findings revealed that viscosity declines with inclining temperature and rises with increasing concentration. Al2O3-H2O nanofluids produced comparable findings for Wu et al. (2009). Hemmat-Esfe and Saedodin (2014) made a similar discovery with zinc oxide/ethylene viscosity. Glycol nanofluids rose dramatically with particle volume concentration, as did Mariano et al. (2015) and Yu et al. (2009a), who also highlighted the point that the impacts of the nanofluid’s augmentation of heat transmission were counterbalanced by the need for more power to pump the fluid. 15.4.3.1 Effects of Various Factors on the Viscosity of Nanofluids The viscosity of nanofluids is influenced by a variety of variables, including temperature (Jiang et al. 2017), volume concentration, particle dimension, form, and shear rate (Fig. 15.10). Volume Concentration The most significant component of cooling medium is the volume concentration of nanofluids, which has a direct impact on the viscosity of nanofluids. Numerous studies have demonstrated that the viscosity of nanofluids is influenced by the weight percentage of nanoparticles (Das et al. 2003; Pastoriza—Gallego et al. 2011) and that it rises as particle concentrations rise. The increase in viscosity, according to Sundar et al. (2013) and Abareshi et al. (2011), is brought on by the particle volume concentration. Hung and Chou (2012) examined the MWCNTs nanofluid’s viscosity and examined at how the additive amounts of the MWCNTs and the chitosan in the water affected the suspension performance. Rashin and Hemalatha (2013) investigated the viscosity of a new, two-step-prepared, copper oxide-coconut oil nanofluid of varied concentrations. According to Namburu et al. (2007), raising the volume concentration of nanofluids causes it to become more viscous. Saeedinia et al. (2012) examined the viscosity of stable CuO-base oil nanofluids with various particle weight fractions of 0.2–2% at various temperatures. The viscosity of nanofluids with various nanoparticle volume fractions was tested by Murshed et al. (2008a, b) and discovered to be much greater than base fluids while Manikandan 352 C.-Y. Hsu et al. Fig. 15.10 The viscosity of nanofluids is affected by a number of factors et al. (2014) and Suganthi et al. (2014a, b) demonstrated a decrease in viscosity with the inclusion of nanoparticles. Morphology The viscosity as well as pumping capacity of the cooling system can be influenced by the particle size and shape. In their study of the two distinct Al2O3 particle sizes, 36 and 47 nm, Nguyen et al. (2008) demonstrated that the latter had greater viscosity. According to Chevalier et al. (2007), the size of nanoparticles decreased as shear viscosity increased. SiO2/water and SiO2/EG nanoparticles with diameters of 35, 94, and 190 nm were measured in various volume fractions. Shear Rate Another factor that can affect the viscosity of non-Newtonian nanofluids is shear rate. Experimental research on the viscosity of water-based nanofluids containing carbon nanotubes was conducted by Halelfadl et al. (2013). The outcomes demonstrated that at large particle loadings, the nanofluids functioned as shear-thinning 15 Thermophysical Characteristics of Nanofluids: A Review 353 materials. The nanofluids behaved like Newtonian fluids at lower particle concentrations. The relative viscosity of nanofluids at high-shear rates was also shown not to vary with temperature. The impact of shear rate on the viscosity of EG-based nanofluids containing ZnO nanoparticles was investigated by Yu et al. (2009b). The findings showed that ZnO-EG nanofluids behave in a Newtonian manner at low volume concentrations (0.02). ZnO-EG nanofluids exhibit non-Newtonian characteristics at greater volume concentrations (0.03). Temperature One of the most significant factors determining viscosity is temperature. The influence of temperature on the viscosity of the water-Al2O3 nanofluid with 36 and 47 nm particle dimensions was investigated by Nguyen et al. (2008). The findings showed that while temperature rises obviously lower nanofluids’ dynamic viscosity, it does so considerably with particle volume fraction. Al2O3-water nanofluid viscosity was determined by Mena et al. (2013) in the temperature range of 5–20 °C with nanoparticle volume fractions <1 vol%. The findings showed that within this range of low temperature and volume percentage, the viscosity of Al2O3-water nanofluid presented a good agreement. Suganthi and Rajan (2012) conducted an experimental study on ZnO nanoparticle-water nanofluids. This study looked at how temperature affected the zeta potential and hydrodynamic size distribution during a heating as well as cooling cycle. The findings demonstrated that when temperature (35–55 °C) increased, the relative viscosity decreased. The findings of a study by Turgut et al. (2009) on the thermal conductance as well as viscosity of TiO2-deionized water at temperatures from 13 to 55 °C revealed that viscosity declined with temperature. 15.4.4 Density An essential thermophysical characteristic is density, which is crucial for assessing the heat transmission capabilities of nanofluids. The Nusselt number, pressure loss, friction factor, as well as Reynolds number are all directly impacted. This characteristic of nanofluids has not been extensively examined by researchers. Experimental research on the densities of zinc oxide (ZnO), antimony-tin oxide, and alumina (Al2O3) was conducted by Vajjha et al. (2009). The densities of nanofluids at various temperatures were calculated by Sundar et al. (2007). With rising temperatures, it was discovered that density decreased. In a similar manner, Harkirat (2010) determined the density of Al2O3 nanoparticles distributed in water utilizing specific gravity bottles at various temperature ranges (30–90 °C) and nanofluid concentrations (1–4%). He noted that the density of nanofluids is greater than that of base fluids and rises as the volume percentage of nanoparticles rise from 1% to 4%. Up to roughly 80 °C, increase in temperature caused a drop in the density of nanofluids. 354 C.-Y. Hsu et al. Beyond this point, densities of 1–4% nanofluids were essentially constant but were still higher than those of water. In an experiment, Pastoriza-Gallego et al. (2009) evaluated three distinct nanofluids containing dispersions of Al2O3 nanoparticles at varying concentrations (0.5–7% in weight fraction) (water). For three different temperatures (283.15, 298.15, and 313.15) as well as pressure range (atmospheric to 25 MPa), density was measured. According to observations, smaller particles have a more pronounced impact on volumetric behaviour. Despite this, it was discovered that size had a minimal overall impact. As the number of nanoparticles grows, so does the density. When compared to the other two samples with lower size nanoparticles, the departure from the equation rises with particle loading. The density of an alumina (Al2O3)/water nanofluid was estimated experimentally by Teng and Hung (2014). The findings demonstrate that the variation of the density with equation falls between 1.50% and 0.06% and between 0.25% and 2.53%, respectively. Furthermore, when the particle loading of the nanofluid increased, the computed findings of density showed a higher divergence. The analysis of the literature reveals that there has been very little experimental research on the density of nanofluids. In order to achieve the precise values of density, more study is thus necessary to define the correction factor and correlations dependent on the fluctuation of nanofluids, particle size, temperature, and particle loading, among other factors. 15.4.5 Specific Heat One of the fundamental characteristics of nanofluids is specific heat, which plays a crucial part in determining how quickly they transport heat. Specific heat is the total amount of heat requisite to raise the temperature of a unit gram of nanofluids by one degree centigrade. In comparison with research on thermal conductivity and viscosity, there is less on the specific heat of nanofluids. The purpose of a nanofluid’s specific heat capacity is to control the temperature variance that affects the efficiency of heat transmission and flow (Wang et al. 2010). The specific heat capacity values of nanofluids can be considerably changed by a small addition of nanoparticles (Shin and Banerjee 2014). The literature reveals that the specific heat of nanobased fluid declines with an upsurge in the volume concentration as well as rises with temperature. It relies on the specific heat of the base fluid and nanoparticle, the volume concentration of nanoparticles, and the temperature of the fluids (Nelson et al. 2009; Zhou et al. 2010). The specific heat capacity of nanoparticles was studied by Wang et al. (2006) in relation to size and temperature. Yang et al. (2008) investigated the specific heat of super carbon nanotubes (ST) using the molecular structure, mechanics technique. The outcome demonstrated that the specific heat of the ST was almost independent of the size, chirality, and length of the ST at a given temperature. Tiznobaik and Shin (2013) assessed the specific heat capacity of nanofluids based on high temperature molten salts. To generate high temperature operating fluids, a molten salt eutectic was mixed with four different sized silicon-dioxide nanoparticles (5, 10, 30, and 60 nm in diameter). The specific heat capacity of 15 Thermophysical Characteristics of Nanofluids: A Review 355 nanofluids was measured using a differential scanning calorimeter. Regardless of the size of the incorporated nanoparticles, the findings showed that the specific heat capacity of nanomaterials was increased by 25%. The specific heat capacity of exfoliated graphite nanoparticle fibres dispersed in polyalphaolefin at mass concentrations of 0.6% and 0.3% was measured by Nelson et al. (2009) using a differential scanning calorimeter. They discovered that as the temperature rose, the nanofluid’s specific heat augmented as well. The nanofluid was shown to have a 50% higher specific heat capacity than PAO at a concentration of 0.6% by weight. According to Zhou et al. (2010), the base fluids, nanoparticle size, and volume concentration significantly affect the specific heat capacity of nanofluids. The specific heat of composites, including aligned multiwall carbon nanotubes, graphite powder, and oriented single-wall and multiwall carbon nanotubes contained in a porous aluminium matrix, was investigated by Pradhan et al. (2009) between 300 and 400 K. According to the findings, MWCNTs and SWCNTs behave similarly in terms of specific heat when compared to bulk graphite powder. Above room temperature, the aligned MWCNTs’ specific heat was less intense and less temperature-dependent than the bulk. The specific heat of three nanofluids including Al2O3, SiO2, and ZnO nanoparticles was studied by Vajjha and Das (2012). The base fluid for the first two was a 60:40 mass ratio of ethylene glycol to water, while the base fluid for the third was deionized water. Different particle volume concentrations and temperatures were used in the experiments. Experimental research on the specific heat capacity of CuO-base oil nanofluids with particle weight fractions of 0.2–2% at various temperatures was done by Saeedinia et al. (2012). In this experiment, nanofluids demonstrated a lower specific heat capacity than the base fluid, and this specific heat capacity falls as the concentration of nanofluids rises. The outcome showed that at 40 °C, the specific heat of nanofluids containing a 2 wt% fraction was approximately 23% lower than that of the basic fluid. 15.4.6 Surface Tension Very little study has been done on surface tension. There are no specific studies on the surface tension of nanofluids in the literature. It can be defined as the force acting over the surface of the liquid per unit length of the surface perpendicular to the force. Since bubble departure and interfacial equilibrium depend on surface tension, it has a substantial impact on the boiling process (Das et al. 2008). Surface tension of nanofluids created without the addition of any surfactant was observed to alter just little, in contrast to the major effects of surfactant addition during nanofluid synthesis (Das et al. 2003; Pantzali et al. 2009). The surfactant affects the surface tension of nanofluids by acting as an interfacial shell between the nanoparticles and base fluids (Kathiravan et al. 2009; Ramesh and Prabhu 2011). As temperature and nanoparticle concentration rise, surface tension drops (Murshed et al. 2008a, b; Jeong et al. 2008; Peng et al. 2010). Thus, it is evident from the aforementioned study that the addition of nanoparticles to the base fluids will modify their 356 C.-Y. Hsu et al. thermophysical characteristics. According to an investigation of the standard cooling curve by Gestwa and Przyecka (2010), adding 1% of Al2O3 nanoparticles to a 10% polymer water solution cause cooling rates to rise from 98 to 111 °C/s. When quenching a stainless-steel probe, Babu and Kumar (2011) also noticed variable cooling speeds when adding varying concentrations of CNT to the water. By adjusting the particle volume concentration, particle volume, particle material, particle size, particle shape, and base fluid, it is possible to generate fluids with variable cooling capabilities by altering the thermophysical properties of the base fluids with the addition of nanoparticles. The heat treatment industry would benefit enormously from the synthesis of quenching media with different cooling intensities. 15.4.7 Pressure Drop Characteristics Pressure drop characteristics have received very little research attention. The literature does not contain any particular studies on pressure drop of nanofluids. This characteristic is also anticipated to play a crucial role in regulating how well the nanofluids function in systems with high heat conductivity and viscosity. However, it is believed that the quantity of studies on this feature in the literature is insufficient to draw a general conclusion regarding its influence on the properties of nanofluid heat transfer. The viscosity of nanofluids will get more viscous after being added nanoparticles, and they will also lose pressure as they flow (Alawi et al. 2015; Feroskhan et al. 2022). Identical research on nanofluids containing water as a based fluid was conducted by Madhesh et al. (2014), and it was discovered that the friction factor and consequent pressure drop of the nanofluid were significantly influenced by the concentration of hybrid Cu-TiO2 nanoparticles. They said that because hybrid nanoparticles have a large volume fraction (let’s say, 2 vol%) and a density gradient that increases friction, the pressure drop of nanofluid during its flow is caused by increasing the friction factor. The influence of particle concentration, temperature, as well as surfactants on the surface tension of nanofluids might be comprehensively represented by Khaleduzzaman et al. (2013). They proved that raising the concentration of nanoparticles raised the surface tension of nanofluids, but increasing the temperature of the nanofluids and the amount of surfactants caused the surface tension of the nanofluids to decrease. Further, understanding how surface tension of nanofluids affects their thermal performance, more research on this topic has to be done. Similar studies (Mahbubul et al. 2013, 2015) concerning the pressure decline, density, and specific heat of nanorefrigerants that are existing in the literature have demonstrated that the properties of nanorefrigerants are altered by the nanoparticles that are seeded within them. However, it is urgently important to conduct more tests for the identical combination of base fluids and nanoparticles in order to clarify the order of physical mechanisms responsible for the improved thermal performance of nanofluids. 15 Thermophysical Characteristics of Nanofluids: A Review 15.5 357 Conclusion The majority of the literature on thermophysical properties is included in this current study. The thermophysical features of the basal fluids are anomalously changed when nanoparticles are added. This review reveals that there are still discrepancies, necessitating a thorough experimental investigation of the thermophysical characteristics of nanofluids. Before being employed for commercial purposes, several things must be addressed. The key findings from this review study are as follows: • Data on enhanced heat transfer reported by several reports vary considerably. • While preparing nanofluids and enhancing their qualities, there are a few obstacles that must be addressed. • Since nanofluids are expensive and challenging to manufacture, efforts should be made to develop nanofluids that are efficient and affordable. • New varieties of nano-based fluids with increasing thermal conductivity and decreasing viscosity must be developed to full fill the demands of heat transfer applications. • There is little study utilizing solid, liquid, and hybrid nanofluids; nonetheless, this subject might be further investigated for developing nanotechnology in the next decades. • These are some prospective research directions for nanofluids that might be pursued in order to expand their expenditure in diverse fields. • Finally, in order to expound the sequence of physical principles for the improved thermal performance of nanofluids, more experiments for the identical combination of base fluids and nanoparticles must be conducted immediately. 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