Open access peer-reviewed chapter

Review on the Stability of the Nanofluids

Written By

Sumit Kumar Singh

Submitted: 06 July 2022 Reviewed: 17 August 2022 Published: 27 October 2022

DOI: 10.5772/intechopen.107154

From the Edited Volume

Pipeline Engineering - Design, Failure, and Management

Edited by Sayeed Rushd and Mohamed Anwar Ismail

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Abstract

Both mono and hybrid nanofluids, the engineered colloidal mixture made of the base fluid and nanoparticles, have shown many interesting properties and become a high potential next-generation heat transfer fluid in various engineering applications. The present review focuses on improving the stability of the nanofluids. For this, the present review briefly summarizes the impact of nanofluid preparation on the stability of various nanofluids and described in the following classification; (a) Nanofluid constituent, (b) Nanomaterial synthesis, and (c) Nanofluid synthesis techniques which are well-grouped and thoroughly discussed. Physical mechanisms for heat transfer enhancement using nanofluids are explored as well. Most of the studies reveal that there are significant improvements in the stability of the nanofluids. Hence, there is an excellent opportunity to use stabled nanofluids in various engineering applications. Finally, some useful recommendations are also provided.

Keywords

  • nanofluid
  • stability
  • nanomaterial synthesis
  • surfactant

1. Introduction

Nanofluids are engineered by dispersing nanoparticles, having average sizes below 100 nm, in conventional heat transfer fluids. Proper and stable dispersion of even a negligible fraction of particles in nanofluids can offer significant enhancement in the heat transfer properties. Various types of nanoparticles like metals, metal oxides, alloys, allotropes of carbon, ceramics, phase change materials, and metal carbides are being used for preparing nanofluids. In addition to nanofluids, hybrid nanofluids have also gained attention recently due to significant improvement in heat transfer characteristics and stability may be caused by the synergistic effect of hybridization. Heat exchangers that use tubes or pipes often have a circular, rectangular, or elliptical cross-section and are easier to design. Tubular heat exchangers are fairly prevalent in pipeline engineering applications. These heat exchangers might be built to handle fluids under high pressure or to handle pressure differentials between cold and hot fluids. Double-pipe and shell-tube heat exchangers are additional categories that apply to these heat exchangers. Modifying the fluids’ characteristics can also increase the heat exchange rate of a heat exchanger. Due to the fact that stable nanofluids have significantly improved heat transfer characteristics, particularly in terms of thermal conductivity, slip mechanisms, and the nanofin effect, they may be employed in tubular heat exchangers to increase energy efficiency. For preparing mono or hybrid nanofluids, the two-step method is generally used where firstly different nanoparticles or nanocomposites are prepared. Then they are mixed in the base fluid through magnetic or mechanical stirring. After that, the solution is sonicated and then characterized using different techniques to assure the proper (homogeneous) mixing and stability of the hybrid nanofluids. Both mono and hybrid nanofluids are thus prepared to provide improved heat transfer characteristics due to an increase in thermal conductivity, Brownian motion, proper dispersion, agglomeration, solid/liquid interface layering, thermophoresis, the improved thermal network between the solid nanoparticle and fluid molecules, nanofin and nanoporous effects at the heat transfer surface. The reason behind this improvement can be summarized as: (i) More heat transfer surface between nanoparticles and fluid, (ii) Collision between the nanoparticles, (iii) Increment in the thermal conductivity due to the interactive effect of different nanoparticles, and (iv) Proper dispersion of the nanoparticles in the base fluid, creating micro turbulences. Therefore, in hybrid nanofluids, both nanoparticles compromise their properties and provide better thermo-physical, chemical, and rheological properties within the low cost that makes it preferable over nanofluids for different applications. Stability is the main key factor for the performance of nanofluids in various engineering applications. All the thermo-physical properties of nanofluid are dependent on its stability. The unstability of nanofluid can inhibit its performance in several applications such as heat exchangers, chemical industry applications, enhanced oil recovery etc. The unstability of nanofluid is caused due to the propensity of nanoparticles to form a cluster in the fluid. The nanofluids may be broadly categorized into three groups based on the nanoparticle composition, namely: (i) mono-nanofluids (made from one type of nanoparticles), (ii) hybrid nanofluids containing different nanoparticles, and (iii) hybrid nanofluids consisting one solid covered by a layer of another solid (composite nanoparticles).

The current review emphasizes the impact of nanofluid preparation on the stability of various nanofluids and is described in the following classification; (a) Nanofluid constituent, (b) Nanomaterial synthesis, and (c) Nanofluid synthesis techniques.

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2. Literature review

The available literature on the preparation, characterization, and stability of mono/hybrid nanofluids are discussed in three sections. In all section, it summarizes the impact of nanofluid preparation on the stability of various nanofluids and described in the following classification; (a) Nanofluid constituent, (b) Nanomaterial synthesis, and (c) Nanofluid synthesis techniques. Exclusive reviews on the heat transfer, pressure drop characteristics, and energy performance of both double-tube and shell-tube heat exchangers using nanofluids are presented in the third and fourth sections.

2.1 Impact of nanofluid preparation

There are two main approaches to synthesize nanofluids: the single-step method and the two-step method. In the one-step method, nanofluid is prepared directly by dispersing nanoparticles in the base fluid without the requirement of numerous steps such as particle drying, storage, etc. Using this method, the stability of nanofluid exhibits most superior compared to the two-step method. But this technique is not beneficial for large scale because of its high production cost. Therefore, the two-step method is the more effective and generally common method of nanofluid preparation. The foremost disadvantage of this process is the control of particle agglomeration tendency. The common application of wide ultrasonication and stirring is the most frequently used method to control agglomeration. Several forces such as Van der Waal attractive force, gravitational force, buoyancy force, and electrostatic repulsive force are acted which lead to destabilization and form sediments. The Van der Waal attractive force and gravitational force work against the stability of any colloidal suspension. Stability is the main key factor for the performance of nanofluids in various engineering applications. All the thermo-physical properties of nanofluid are dependent on its stability. The unstability of nanofluid can inhibit its performance in several applications such as heat exchangers, chemical industry applications, enhanced oil recovery etc. The unstability of nanofluid is caused due to the propensity of nanoparticles to form a cluster in the fluid. For considering a stable nanofluid, agglomeration propensity has to be removed. Some stability evaluation methods are used in literature i.e., sedimentation and centrifugation method, zeta potential measurement, spectral absorbance and transmittance measurement, and dynamic light scattering. Numerous efforts have been made to prepare long-time stable and homogenous nanofluids using various techniques. The current review emphasizes the impact of nanofluid preparation on the stability of various nanofluids and is described in the following classification; (a) Nanofluid constituent, (b) Nanomaterial synthesis, and (c) Nanofluid synthesis techniques.

2.1.1 Nanofluid constituent

2.1.1.1 Nanomaterial type

There are several types of nanofluids: metallic nanofluids (Al, Ag, Cu, Fe, Au), metal oxide nanofluids (Al2O3, CuO, Fe3O4, SiO2, TiO2, ZnO, etc.), and non-metallic nanofluids (SiC, TiC, graphite, diamond, SWCNT/MWCNT, graphene, etc.). Several studies on the impact of the nanofluid constituents on its stability are shown in Table 1. Xu et al. [4] prepared hybrid nanofluids with nanoparticles of different masses added with a small amount of SDBS and PEG into DW and observed that 25% Al2O3 + 75% TiO2 hybrid nanofluid shows good suspension stability. The zeta potential value for the 25% Al2O3 + 75% TiO2 hybrid nanofluid is found 42.6 mV indicating high stability. Zeta potential means electrostatic repulsion force between nanoparticles and base fluid. High repulsion force indicates high stability of nanofluid, whereby 30mV is generally considered as a benchmark for a stable nanofluid and excellent nanofluid stability may exceed 60 mV. Some studies investigated the impact of functionalizing the nanoparticles surface which reduces aggregation and improves dispersion. Said et al. [5] studied the stability of Carbon nanofiber (CNF), Functionalized Carbon nanofiber (F-CNF), Reduced graphene oxide (rGO), and F-CNF/rGO nanofluids. The results indicated that hybrid (FCNF/rGO) nanofluid shows higher stability than as compared to CNF, F-CNF, and rGO nanofluids. Also, the sample of CNF almost completely sedimented on 2nd day as shown in the Figure 1. It is due to the low charge density on the surface of the CNF nanoparticle which leads to the tendency of agglomeration. Said et al. [15] used acid treatment of CNF to examine the stability. The zeta potential of 0.02 vol. % F-CNF nanofluid was −42.9 and − 41.8 mV after 2 and 90 days which indicates that the stability was improved while the zeta potential of CNF was −16.3 and − 15.5 mV, indicating a relatively unstable dispersion. One way to achieve long-term stability is to adjust the nanofluid pH, away from the isoelectric point (IEP). Thus, IEP differs from one sample to another. These values were prepared in acidic and alkaline ranges using HCl and NaOH solutions and adjusted by pH meter. Kazemi et al. [6] used two different nanoparticles (GnP, SiO2) with the same base fluid (water) as well as different pH values (3,6,9, and 12) to study the stability of the nanofluids. The results found that SiO2/Water nanofluids have good stability at all pH values, especially for samples with pH >3 and GnP/water achieve better stability at higher pH values. Akhgar and Toghraie [9] examined the stability of water-based MWCNT and TiO2 nanofluid at different pH (3, 6,9, and 12). The results observed that the nanofluid containing water/TiO2 with pH = 9 had more stability than the rest of the samples. On the other hand, MWCNT particles are not dispersed in water and are not stable in any pH without any surfactant. Kazemi et al. [21] compared the stability of three types of nanofluids, G/Water, SiO2/Water, and G-SiO2/Water and found that SiO2/Water nanofluid shows excellent stability at all pH values while G/Water sustainability is poor in lower pH value. Due to better stability in higher pH values, the CMC surfactant can be used to increase pH by creating a negative charge surface for graphene nanoparticles and developing functional groups. Siddiqui et al. [8] performed a stability study with metal (Cu), oxide (Al2O3), and meta-oxide Cu- Al2O3) nanofluid containing the same base fluid (DI water). Al2O3 nanofluid exhibits better stability between 0 and 6 h followed by good stability with little particle settling between 6 and 240 h. Cu nanofluid shows poor dispersion stability after 1 hour of preparation. In case of hybrid nanofluid, nanofluid with optimum mixing ratio exhibits relatively better stability. Muthoka et al. [14] used PCM-DI water as the base fluid and two different nanoparticles (MgO and MWCNT) to examine the stability. They observed that the stability of MgO and 24 wt% base fluid without surfactant showed poor stability after only 24 h while the functionalized MWCNT nanofluid showed no separation after 24 h. Also, it was concluded that the stability of the nanofluid at low temperatures is increased by the use of surfactant. Alawi et al. [16] synthesized PEG-GnP, PEG-TGr, Al2O3, and SiO2 water-based nanofluids. They observed the dispersion stabilities of carbon-based nanofluids and metallic oxides nanofluids for 30days, and the results showed the higher dispersibility of the PEG-GnP, PEG-TGr nanofluids in an aqueous media with very low sedimentation. Akbari and Saidi [17] observed TiO2/DW nanofluid shows good stability as compared to GnP/DW nanofluid. Since graphene is an inherently hydrophobic material and the stability of graphene/water nanofluid is not favorable without any surfactants. Boroomandpour et al. [22] studied the stability of ternary hybrid nanofluids containing MWCNT-TiO2-ZnO/DW-EG (80:20) as well as binary and mono nanofluids. They found that all nanofluids have good stability up to 48 h after fabrication and the addition of CTAB surfactant lead to better stability.

YearAuthorBase fluidNanomaterialSurfactantImportant resultsRemarks
2020Xian et al. [1]DW-EGCOOH-GnP, TiO2SDC CTAB
SDBS
CTAB shows highest degree of stabilityUse of ionic surfactant results in higher stability
2019Almanassra et al. [2]WaterCNTGA
PVP
SDS
GA and PVP show stablility for more than 6 monthsGA can be a promising surfactant for stabilizing the CNT/W nanofluids
2020Cacua et al. [3]DI waterAl2O3SDBS CTABAl2O3 nanofluid with SDBS at 1 CMC and CTAB at 0.5 CMC selects as the most stable and unstable nanofluids, respectively.Anionic SDBS allows to have high repulsive forces between nanoparticles
2020Xu et al. [4]DWAl2O3, TiO2SDBS
PEG
25%Al2O3 + 75%TiO2 shows better stability than other mono/hybrid nanofluidZeta potential values for the hybrid nanofluid and TiO2/W nanofluids are 42.6 mV and 40.8 mV, respectively.
2020Said et al. [5]DWCNF, F-CNF, rGO, F-CNF/rGOHybrid (FCNF/rGO) nanofluid shows a higher stabilityCNF nanofluids can be better for high-temperature applications
Hybrid nanofluid (FCNF/rGO) can be ideal working fluid for lower temperature
2020Kazemi et al. [6]WaterSiO2, GrapheneCMCG/water achieves better stability at higher pH valuesAdjusting the nanofluid pH may lead to better stability
2019Ouikhalfan et al. [7]DWTiO2CTAB, SDSUsing CTAB surfactant, nanofluid shows better stability
2019Siddiqui et al. [8]DI WaterCu, Al2O3, Cu-Al2O3Al2O3 nanofluid shows excellent stability between 0 and 6 h.
Cu nanofluid shows very low dispersion stability at time t = 1 h
Hybrid nanofluid with optimum mixing ratio exhibits relatively better stability
2018Akhgar and Toghraie [9]WaterTiO2, MWCNTCTABTiO2 / Water with higher pH had more stability
MWCNT particles are not dispersed in water and are not stable in any of pH
Low amount of surfactant gives better nanofluid stability.
2018Choi et al. [10]DI WaterMWCNTSDBS, CTAB, SDS, TX-100For short-term time frames (3 h) nanofluids manufactured with SDBS, CTAB, and TX-100 show better stability
For long-term time frames (1 month), the SDBS and TX-100 nanofluids have the highest suspension stability
TX-100, CTAB, and SDS are not suitable surfactants for nanofluids operating from 10–85°C
2018Das et al. [11]DWTiO2 (Anatase)SDBS, CTAB, SDS, Acetic acidSDS and CTAB results show excellent stabilization (stable for exceeding 12 h and 24 h)
2018Gao et al. [12]DW, EG, EG/DWGNPStability of GPN/DW nanofluid is worse than that of GNP/ EG.
2018Kuang et al. [13]BrineSiOx
Al2O3
TiO2
OA, PAA, Cationic, Anionic, NonionicSiOx nanofluids exhibit stable in all cases
Al2O3 + PAA and Al2O3+ cationic surfactant show the most stability
Among all surfactants, TiO2 + PAA show the most stability
2018Muthoka et al. [14]PCM-DI WaterMgO, MWCNTSDSStability of MgO and 24 wt% base fluid without surfactant showed poor stability after only 24 h
Fuctionalized MWCNT nanofluid showed no separation after 24 h
Stability of the nanofluid at low temperatures is increased by the use of surfactant
2018Said et al. [15]DI waterCNF, F-CNFF-CNF-based nanouids exhibit superior stability in water for 90 daysAcid treatment of CNF at relatively low temperatures resulted in stable suspension
2019Alawi et al. [16]DI WaterPEG-GnP, PEG-TGr, Al2O3, SiO2PEG-GnP shows higher dispersibility and the relative concentration of PEG-GnP based water reported very low sedimentation
2019Akbari and Saidi [17]DWTiO2
GnP
TiO2/DW nanofluid shows the good stabilityGraphene is an inherently hydrophobic material and the stability of graphene/water nanofluid is not favorable
2019Cacua et al. [18]DIAl2O3SDBS
CTAB
Al2O3-SDBS exhibits lower rate of sedimentation compared to Al2O3 and Al2O3-CTAB.
2019Etedali et al. [19]DISiO2Ps20, CTAB, SLSNanofluid with all surfactant show excellent stabilityMaximum surface charge for the nanofluids with SLS, CTAB, and Ps20 surfactants are −87.4, 74.2, and − 97.9, respectively
2020Giwa et al. [20]DW, EG-DWAl2O3-Fe2O3SDS, NaDBSDW-based Al2O3-Fe2O3 exhibited more stable than the EG-DW based Al2O3-Fe2O3Absorbance value of the DW-based Al2O3-Fe2O3 displayed better horizontal straight lines than those of the EG–DW Al2O3-Fe2O3
2020Kazemi et al. [21]DWG, SiO2, G-SiO2CMCSiO2/Water nanofluid shows excellent stability at all pH value
G/Water sustainability is poor in lower pH values
Due to better stability in higher pH values, the CMC surfactant can be used to increase pH by creating a negative charge surface for graphene nanoparticles and developing functional groups
2020Boroomandpour et al. [22]Water-EGMWCNT, ZNO, TiO2
MWCNT-ZNO-TiO2
CTABAll nanofluids have good stability 48 h after fabricationAddition of CTAB surfactant lead to better stability

Table 1.

Synopsis of the investigations about the impact of the nanofluid constituents on its stability.

Figure 1.

Photographs of vials showing the stability of nanofluids for: (a) 1 st day, (b) 2nd day, (c) 30 days, (d) 45 days, (e) 60 days, and (f)180 days [5].

From the literature reviews on the preparation of nanofluids with different particles, it is found that the stability of water mono/hybrid nanofluid is strongly dependent on the particle shape and size. It is found that the propensity of aggregation is increased with the reduction in particle size and isoelectric point (pH value) decreases with the decrease in particle size. Therefore, the agglomeration process moves toward lesser pH value. The cylindrical-shaped particles sediment faster than spherical and platelet-shaped particles. High aspect ratio nanoparticles are more susceptible to agglomeration.

2.1.1.2 Surfactant type

Addition of different surfactants such as: Anionic (Sodium Dodecyl Sulfate (SDS), Sodium Dodecyl Benzene Sulfonate (SDBS)), Cationic (Cetyltrimethylammonum Bromide (CTAB)), Non-ionic (Span 80, Tween 20) and polymer (Polyvinyl Pyrrolidone (PVP), Poly Vinyl Alcohol (PVA), Gum Arabic (GA)) during nanofluid preparation is an additional way of controlling particle aggregation. A negatively charged suspension may be obtained by using anionic surfactants (SDC, SDS, and SDBS) while a cationic surfactant (CTAB) may contribute a positive charge. The augmentation in stability will achieve by the coating of surfactant on nanoparticles, which leads to a dominating electrostatic repulsion over the van der Waals force and thus prevent nanoparticles from agglomerating. Also, the stability of the nanofluid can be improved by decreasing the sedimentation velocity of the nanoparticles. According to Stokes law, the sedimentation velocity can be reduced by using nanoparticles with smaller diameters. However, when the nanoparticles diameter decreases, the surface energy will be increased which leads increase possibility of agglomeration. The best way to suppress the agglomeration without disturbing the sedimentation velocity is the usage of surfactants. While surfactant addition is an active way to improve the stability of the nanofluids but surfactants may lead to cause some problems. Surfactants may contaminate the heat transfer media. Surfactants may produce foams while heating and cooling are regular processes in heat exchange systems. Additionally, surfactant molecules attributed to the surfaces of nanoparticles might increase the thermal resistance between the nanoparticles and the base fluid, which may hinder the augmentation of the thermal conductivity.

Xian et al. [1] used three different surfactants, i.e., SDS, CTAB, and SDBS to stabilize the COOH-TiO2 hybrid nanofluid. They observed that COOH-TiO2 hybrid nanofluid with CTAB surfactant exhibited the best surfactant to stabilize this hybrid nanofluid. The visual inspection of sedimentation of nanofluids with different surfactants after 40 days is shown in Figure 2. Almanassra et al. [2] compared the effect of different types of surfactants on the stability of CNT/water nanofluids. They investigated with three types of surfactants namely, GA, PVP, and SDS and found that the nanofluids with GA as well as PVP surfactants were more stable for more than 6 months. Gum Arabic can be a promising surfactant for stabilizing the CNT in water-based nanofluids. Cacua et al. [3] found Al2O3 nanofluid with SDBS at 1 CMC and CTAB at 0.5 CMC were the most stable and unstable nanofluids, respectively. Anionic SDBS provides high repulsive forces between nanoparticles. Ouikhalfan et al. [7] prepared surface-modified TiO2 nanofluid with two different surfactants (SDS and CTAB). The quick sedimentation was found in non-treated TiO2 nanofluid after 24 hours of the preparation as shown in Figure 3. TiO2 nanofluid with CTAB showed better stability up to several days while the nanofluid with SDS surfactant shows less but overall better dispersion compared to nanofluid with non-treated TiO2. Choi et al. [10] studied the effect of various surfactants as well as the temperature on the stability of water-based MWCNT nanofluid. They prepared nanofluid with four different surfactants, i.e., SDBS, CTAB, SDS, and TX-100 between the temperatures 10°C–80°C. It was observed that for short-term time period (3 h), nanofluids prepared with SDBS, CTAB, and TX-100 show better stability while for long-term time period (1 month), the SDBS and TX-100 nanofluids have the highest suspension stability. On the account of temperature, TX-100, CTAB, and SDS are not suitable surfactants for nanofluids operating from 10 to 85°C. Das et al. [11] found TiO2 (Anatase) with SDS and CTAB show excellent stabilization (stable for exceeding 12 h and 24 h) as compared with nanofluid with SDS and acetic acid surfactant. Kuang et al. [13] prepared nanofluids by dispersing three nanoparticles (i.e., SiOx, Al2O3, and TiO2) and five different chemical agents i.e., oleic acid (OA), polyacrylic acid (PAA), a cationic, an anionic, and a nonionic surfactant) in base brine solutions. Nanofluids made with the anionic surfactant made the surface slightly more water wet. The results revealed that SiOx nanofluids exhibit stability in all cases while Al2O3 + PAA and Al2O3 + cationic surfactant show the most stability. In case of TiO2 nanofluid, TiO2 + PAA show the most stability among all surfactants. Cacua et al. [18] used UV–vis spectroscopy to examine the stability of Al2O3 with two different surfactants (SDBS and CTAB). The outcome reveals that the nanofluid with SDBS at 1 CMC and that with CTAB at 0.5 CMC achieved the lowest and highest absorbance variation, respectively. Low absorbance variation over time indicates high nanofluid stability. Etedali et al. [19] investigated the stability of SiO2 nanofluids with different surfactants, i.e., SLS, CTAB, and Ps 20 through the Zeta potentials test. The results of the Zeta-potential test found that the maximum surface charge for the nanofluids with SLS, CTAB, and Ps20 surfactants were − 87.4, 74.2, and − 97.9, respectively, confirming the stability conditions.

Figure 2.

Visual inspection of sedimentation of nanofluids with different surfactants and ultra-sonication time after 40 days. [1].

Figure 3.

Sediment photograph capturing of the nanofluid with (1) nontreated TiO2, (2) CTAB-treated TiO2 nanofluid, and (3) SDS-treated TiO2 nanofluid [7].

2.1.1.3 Base fluid type

Gao et al. [12] prepared GNP nanofluid with three different base fluids namely, EG, DW, and EG/DW and reported that the stability of nanofluid with EG base fluid is better than that of DW-based nanofluid. Figure 1 shows the visual observation of GNP nanofluid with different base fluids. Giwa et al. [20] used two-step method to prepare Al2O3-Fe2O3 hybrid nanofluid with two type of base fluid viz., DW and EG/DW. SDS and NaDBS were used as a surfactant. Using UV-visible spectrophotometer, they found DW-based Al2O3-Fe2O3 were relatively more stable than the EG–DW Al2O3-Fe2O3 hybrid nanofluid. The absorbance value of the DW-based Al2O3-Fe2O3 displayed better horizontal straight lines than those of the EG–DW Al2O3-Fe2O3.

2.2 Nanomaterial synthesis

Ding et al. [23] prepared the functionalized graphene (ESfG) by adding the graphite powder into the milling jar with steel balls of smaller diameter and the system was filled with SO3 gas. After removing metallic impurities, the samples were then freeze-dried for 36 hours at −120°C to yield black powder as the final ESfG. The prepared ESfG was stable for several months in water. The sulfonic-acid groups can bond with carbon atoms at the edge of graphite which tends to enhance the stability of ESfG water-based nanofluids. Gul and Firdous [24] synthesized the graphene oxide nanosheet by the oxidation of graphite using the Hummers method as shown in Figure 4. In this method the graphite powder was mixed with NaNO3, H2SO4, and KMnO4 and stirred in an ice bath for about 30 min. Finally, the mixture was sonicated and added H2O2 and HCl to quench the reaction and get light yellow graphite oxide. The results found that the highly dispersible nature of GO in water which is fruitful for the preparation of GO nanofluid for multipurpose applications. Li et al. [25] introduced the β-cyclodextrin(β-CD) onto the surface of MWCNTs by a simple chemical synthesis method. It was found that the introduction of β-CD onto the surface of MWCNTs exhibited better stability of nanofluids. The possibility of aggregation between CD-CNTs is significantly decreased due to the Vander Waals force or steric interrupts between β-CD. Rahimi et al. [26] treated the hydrophilization of MWCNTs with different concentrated acids. They added the raw MWCNTs into the mixture of H2SO4 and HNO3 and the mixture was refluxed for 3 hours. The acid-treated MWCNTs were obtained after washing with DI water and dried for 4 hours. Acid-treated MWCNTs suspensions display good stability in water. This is due to the generation of hydroxyl groups on nanotube surfaces. Vozniakovskii et al. [27] synthesized a hybrid nanomaterial composed of nanodiamonds-multi-walled carbon nanotubes (DND-CNT) using a catalyst chemical vapor deposition (CCVD) method. The results showed that DND-CNT hybrid suspension was stable up to 100 hours while the initial DND began to precipitate after 1 hour. The stability of DND-CNT hybrid particles in water is explained by the opening of a previously closed surface covered with groups with a labile proton, which ensures the stability of the particles of the hybrid material in water.

Figure 4.

Synthetic route of graphene oxide by hummers method [24].

2.3 Nanofluid synthesis technique

Numerous nanofluid stabilization techniques are used for reducing the cluster size of nanoparticles i.e. ultrasonic vibration and ball milling etc. The role of ultrasonication is to break the nanoparticle cluster and create a homogenous mixture. Ultrasonic vibration can be employed in two ways; (a) indirect method (ultrasonic bath), and (b) direct method (probe sonicator). Among these two methods, the probe sonicator offers better results in terms of breaking the particle cluster and lowering the average cluster size. Several studies on the impact of the nanofluid synthesis technique on its stability are shown in Table 2. Asadi et al. [28] used two-step method to prepare TiO2-CuO hybrid nanofluid. They applied a magnetic stirrer for 1 hour in order to distribute the nanomaterial in the base fluid. Moreover, for breaking the clusters and uniformly distributing the nanoparticles in the base fluid, a probed ultrasonic device was applied for 1 hour. The DLS results ensured the nanoparticles exist in the base fluid, and the phenomenon of agglomeration did not happen. Chen et al. [29] investigated the impact of sonication time on the stability of the Al2O3/liquid paraffin nanofluid. They used two-step method with varying the magnetic stirrer time from 10 to 40 minutes and sonication time from 1 to 4 hours. It was found that nanofluids prepared using shorter sonication times show stability for a minimum of 1 month. When increase in sonication time, it breaks the bond between the nano additives and the surfactant which leads to be unstable. Asadi et al. [30] varied the sonication time from 10 to 80 minutes to measure the stability of MWCNT/water nanofluid. They reported that after the 30th day, the samples subjected to 10, 20, 40, and 60 minutes of ultrasonication showed good stability while the samples subjected to longer time ultrasonication showed the amount of sedimentation leads to having agglomerated particles. Ranjbarzadeh et al. [31] used magnetic stirrer for 1 hour to mix the SiO2 nanoparticles in the base fluid and then sonicated for 60 minutes. By visual observation, the result found that no sediments were formed after 6 months. Aberoumand and Jafarimoghaddam [32] prepared Ag-WO3/Transformer oil nanofluid using the first step method. They applied Electrical Explosion Wire (EEW) to prepare the nanofluid. The Zeta potential of applied nanofluids in three different concentrations of 1%, 2%, and 4% was measured. The results indicate the excellent stability of applied hybrid nanofluids. Using the same EEW method, Aberoumand et al. [34] prepared Ag/water nanofluid and found that with EEW method, the nanofluid maintained their stability for a long time. Dalkılıç et al. [33] prepared CNT-SiO2/DW using two-step methods and the mixture was sonicated for 3 hours. It was found that the sedimentation was not observed up to 48 hours. The raw CNT particles showed poor dispersion stability in the base fluid. SiO2 particles support and increase the stability of CNTs particles in water. Tests showed that CNT particles exhibit less stability in water without SiO2 particles and surfactants. Kakavandi and Akbari [35] used DLS test to examine the distribution of the MWCNT and SiC nanoparticles in the hybrid nanofluids. The results indicated acceptable stability of nanofluids. The hybrid nanofluid was magnetically stirred for 1 hour and then sonicated for 45 minutes. Keyvani et al. [36] used Ce2O/EG nanofluid to examine the stability. The nanofluid was stirred and then exposed to ultrasonic waves for 2 and 7 hours, respectively. The sedimentation of particles was found after 2 weeks. Nanofluid with a higher concentration of particles, nanoparticles led to agglomerate; therefore, the stability of the nanofluid weakened. It was also reported that the stability of the prepared nanofluid with a lower volume fraction of nanoparticles was stable for a longer period of time compared to the nanofluids with a higher volume fraction [39].

YearAuthorNanofluidSynthesis techniqueImportant resultsRemarks
2020Asadi et al. [28]CuO-TiO2/WaterTwo-step methods
Magnetic stirrer time = 1 hour
Sonication time = 1 hour
DLS results confirmed the nano-dimensions of the particles exist in the base fluid, and the phenomenon of agglomeration has not occurred.Strong Van der Waals attraction forces among the particles, type and size of particles, and features of the base fluid may result in the occurrence of a phenomenon known as clustering of nanoparticles
2019Chen et al. [29]Al2O3/liquid paraffinTwo-step methods
Magnetic stirrer time = 10–40 min
Sonication time = 1–4 hour
Nanofluids prepared using shorter sonication times show stability for a minimum of 1 month
2019Asadi et al. [30]MWCNT/WaterTwo-step methods
Magnetic stirrer time = 2 hour
Sonication time = 10–80 min
Samples subjected to 10, 20, 40, and 60 min ultrasonication showed good stabilityApplying ultrasonication longer than the optimum time leads to having agglomerated particles, which results in increasing the sedimentation rate
2018Ranjbarzadeh et al. [31]SiO2/ WaterTwo-step methods
Magnetic stirrer time = 1 hour
Sonication time = 1 hour
Long term stable nanofluid (for more than 6 months)
2018Aberoumand and Jafarimoghaddam [32]Ag-WO3/Transformer oilOne-step method (EEW)Results show very good stability for all of the test samples
2018Dalkılıç et al. [33]CNT-SiO2/DWTwo-step methods
Sonication time = 3 hour
Sedimentation not observed up to 48 hCNT particles show less stability in water without SiO2 particles and surfactant
2018Aberoumand et al. [34]Ag/WaterOne step method (EEW)Zeta potential index confirmed the stability of the utilized nanofluid even after 1 yearEEW method may maintain the stability of nanofluid for a long time.
2018Kakavandi and Akbari [35]MWCNT-SiC/Water-EGTwo-step methods
Magnetic stirrer time = 1 hour
Sonication time = 45 min
DLS test indicate acceptable stability of nanofluids
2018Keyvani et al. [36]CeO2/EGTwo-step methods
Magnetic stirrer time = 2 hour
Sonication time = 7 hour
Sedimentation of particles occurred after 2 weeks.
2018Liu et al. [37]rGO/DI waterTwo-step methods
Sonication time = 30 min
pH = 10
rGO nanofluids exhibited good stabilitiy for 10 day without the addition of other dispersantsUV–Vis intensity (at 270 nm) changes of rGO nanofluids for 10 days
2018Ranjbarzadeh et al. [38]GO-SiO2/WaterTwo-step methods
Sonication time = 1 hour
pH >7
Chemical reaction of the functional groups on surface of the particles to the base fluid promote stability of the nanofluid
2018Sharafeldin and Grof [39]CeO2/WaterTwo-step methods
Sonication time = 90 min
Higher concentration lead less stabilityMean value of zeta potential for 0.0666% volume fraction was −36.91 mV which indicate physical stability
2018Zeng and Xuan [40]MWCNT-SiO2/AgTwo-step methods
Sonication time = 60 min
No precipitations found till 7 days, indicating their long-term dispersion stability
2019Gulzar et al. [41]Al2O3-TiO2 /Therminol-55Two-step methods
Magnetic stirrer time = 4 hour
Sonication time = 2 hour
Value of zeta potential decreases with the increase in concentration which may cause agglomeration sufficiently after long timeThe introduction of the functional group attached to oleic acid on the surface of both nanoparticles helps to reduce the attractive forces among them which prevents the agglomeration and increases the stability
2019Alarifi et al. [42]MWCNT-TiO2/OilTwo-step methods
Magnetic stirrer time = 2 hour
Sonication time = 1 hour
Stability of the prepared samples was observed over 14 days, and no sedimentation was observed
2019Akram et al. [43]CGNP/DI WaterTwo-step methods
Sonication time = 1 hour
zeta potential values for the CGNP nanofluids are far from the isoelectric point (i.e., point of zero charge)pH range (2.8–10.55) results in strong electric repulsion forces between the particles of CGNPs
2019Sharafeldin and Grof [44]WO3/WaterTwo-step methods
Sonication time = 75 min
Mean zeta potential value for WO3/water nanofluid was −43.12 and a little decrease in the values were observed along the period of 7 days.
2019Chen et al. [45]Raw MWCNT, Acid treated MWCNT
Milling treated MWCNT
Two-step methods, Stirred media mill techniqueExcellent stable of fresh MWCNTs nanofluids is obtained by milling
2019Ali et al. [46]Al/waterTwo-Step Fabrication Approach, Controlled bath temperature two-step methods30 °C nano suspensions showed better short- and long-term stability behavior than the conventionally fabricated nanofluids
2019Mahbubul et al. [47]Al2O3/WaterUltrasonication time = 1–5 hourHigher sedimentation rate observed for the nanofluid prepared by low ultrasonication
Increasing ultrasonication duration decreased the sedimentation rate.
Longer ultrasonication durations are necessary to avoid sedimentation if the nanofluids are stored for longer periods.
2019Mahyari et al. [48]GO-SiC/Water-EGTwo-step methods
Magnetic stirrer time = 1 hour
Sonication time = 45 min
DLS test results with different patterns approved acceptable stability of the nanofluid.
2017Chen et al. [49]Fe3O4-MWCNT/Brine waterTwo step methods
Magnetic stirrer time = 30 min
Sonication time = 2 hour
Magnetic MWCNTs nanofluids have high stability in 1000 ppm saline water, and long-term suspension stability also could be obtained,When the solution salinity increased, the original colloidal structure destroyed by charge ion. Therefore, the salt-resisting surfactant was added to reinforce the double-layer repulsion and remained the system stability
2019Okonkwo et al. [50]Al2O3-Fe/WaterTwo-step methods
Sonication time = 8–9 hour
pH = 2–12
Hybrid nanofluids are significantly more stable at pH values of 12 when compared at any other pH value.High pH value favors the stability of the nanofluids
2019Terueal et al. [51]MoSe2LPE method
Sonication frequency (kHz) = 80 and 130,
Nanofluid with 80 kHz and 130 kHz show the highest extinction coefficients values.Higher extinction coefficient values means highest amount of nanomaterial in suspension
2020Li et al. [52]SiO2-oleic acid/liquid paraffinTwo-step methods
Magnetic stirrer time = 30 min
Sonication time = 1 hour
Large numbers of SiO2 nanosized particles possesses maximum value for total count at values less than−40 mVHigh stability of SiO2 nano sized particles in was found liquid paraffin
2020Geng et al. [53]ZnO-MWCNT/OilTwo-step methods
Magnetic stirrer time = 1 hour
Sonication time = 1 hour
Results show that the nanoparticles are in nanoscale after the construction of nano-oil
2020Li et al. [54]SiO2/EGTwo-step methodsCritical voltage value is −56.28 mV and nanofluid is stable.Greater the number of particles with a smaller diameter, the higher the probability of stability.

Table 2.

Synopsis of the investigations about the impact of the nanofluid synthesis technique on its stability.

Liu et al. [37] prepared rGO by the reduction of graphene oxide with L-ascorbic acid as a reductant in an aqueous solution. To prepare rGO, the graphene oxide solution was dispersed in DI water and ultrasonicated for 1 hour. NH3-water was then added to control the pH to 10 with sonication for 30 minutes. L-ascorbic acid was added and the mixture was maintained at 95°C for 3 hours for the completion of the reaction. The rGO solution was filtered to obtain rGO on the filter paper. Finally, rGO nanofluids were prepared by sonicating the filtered powder in a certain amount of DI water. The whole process is shown in Figure 5. The rGO nanofluids exhibited good stability for 10 day without the addition of other dispersants. Ranjbarzadeh et al. [38] conducted a test to study of pH effects on the stability in acid and alkaline spectrums for GO-SiO2/Water hybrid nanofluids. The results observed that the nanofluid, due to the presence of functional groups on the surface of its nanoparticles, shows acceptable stability in all spectrums; however, in the long term, nanofluids with pH > 7 showed better stability. Zeng and Xuan [40] sonicated the MWCNT-SiO2/Ag binary nanofluids for 1 hour and reported that the stability of the binary nanofluid sustained the dispersion stability for 7 days. Gulzar et al. [41] dispersed hybrid nanopowder (Al2O3-TiO2) in Therminol-55 oil and the mixture was subjected to high shear stirring at 2500 rpm using a magnetic stirrer for 4 hours. The mixture was then sonicated for 2 hours using a high energy probe sonicator. Oleic acid was used as a surfactant as of better miscibility with Therminol-55 oil. They observed that the value of zeta potential declines with the rise in concentration which may cause agglomeration adequately after a long time. The surfactant which changes the surface charge and increases the repulsive forces between the nanoparticles also contributes to improved stability. Same way, Alarifi et al. [42] used magnetic stirring for 2 hours and sonicated for 1 hour to prepare a long-term stable MWCNT-TiO2/oil nanofluid. The stability of the prepared hybrid nanofluid was observed over 14 days and no sedimentation was found. Akram et al. [43] checked the stability of CGNP–water nanofluid by zeta potential at different pH values. They prepared this nanofluid after the sonication for 60 minutes and observed that the CGNP nanofluid had high negative values (− 4.42 mV to −49.5 mV) within the pH variations from 1.84 to 10.55. The zeta potential values for the CGNP nanofluids are far from the isoelectric point (i.e., point of zero charges), which indicates that this pH range (2.8–10.55) results in strong electric repulsion forces. Sharafeldin and Grof [44] did sonication of WO3/water nanofluid continuously for about 75 minutes to break the agglomeration between the nanoparticles which leads to well disperse the particles into water. The mean zeta potential value for WO3/water nanofluid was −43.12 and a little decrease in the values was observed over the period of 7 days. MWCNT nanofluids suffer from low dispersion and short-time stability which inhibit their practical application. Chen et al. [45] used a novel method, i.e., a one-pot method by stirred media mill technique. In this method, raw MWCNTs nanoparticles were treated by ball milling to change their morphology, length, and specific surface first. After centrifuging, dry nanoparticles were purified by acid treatment to improve their dispersion in the solution. Thus, the resulting powder was dispersed again in base fluids by ultrasonication and meanwhile, surfactant was added to improve dispersion. The results showed that the milling-treated MWCNT nanofluid exhibited better stability as compared to raw MWCNT and Acid treated MWCNT nanofluid. Ali et al. [46] investigated the stability of dispersed Al nanoparticles in base fluid (water) prepared by the conventional and the controlled bath temperature two-step methods. The sonication process was taken the same for 4 hours in the range of 10–60°C. The results revealed that the sedimentation behavior of the nanofluids prepared through the controlled bath temperatures of less than 30°C was of dispersed sedimentation type, while those produced by the conventional method and the fixed temperatures of 30°C and higher were of flocculated sedimentation type. Furthermore, increasing the controlled sonication temperature led to an increase in the settling process of the sediments. Also, the rise in nanoparticle concentration was seen to reduce the variation in sedimentation height ratio between the fixed temperature samples. A comparison between the two preparation methods was shown that the 30°C nanofluids had better short- and long-term stability than the conventionally produced suspensions. Mahbubul et al. [47] varied the sonication time from 1 to 5 hours to study the effect of sonication time on the stability of the 0.5 vol% Al2O3 nanofluids. They observed that with low sonication time or no sonication, the sedimentation rate is higher. It can be concluded that longer ultrasonication reduces the sedimentation of nanoparticles and hence, increases the stability of nanofluids. Mahyari et al. [48] used probe-type ultrasonicator to achieve the stability of GO-SiC/water-EG hybrid nanofluid. DLS test results with different patterns approved acceptable stability of the nanofluid. Chen et al. [49] prepared the saline water based magnetic MWCNT nanofluids at different mass concentration from 0 to 0.04 wt% by two-step method. A mechanical stirrer was used at 500 rpm continuously for 30 minutes to mix nanoparticles and water and then the mixture was sonicated thoroughly for 2 hours. Magnetic MWCNTs nanofluids showed high stability in 1000 ppm saline water, and when the solution salinity increased, the original colloidal structure would be destroyed by charge ion. Therefore, the salt-resisting surfactant was added to reinforce the double-layer repulsion and remained the system stable. Okonkwo et al. [50] prepared the Al2O3-Fe/Water using two-step methods and measured the stability of nanofluid through the Zeta potential test. Hybrid nanofluid was found significantly more stable at pH values of 12 when compared at any other pH value. Terueal et al. [51] performed using the liquid phase exfoliation technique starting with bulk MoSe2 to prepare stable nanofluids. Triton X-100 was used as a surfactant. The suspension underwent sonication in an ultrasound bath for 4 hours with two different frequencies: 80 kHz and 130 kHz. The samples were then centrifuged at 1000 rpm for 10 minutes and again at 4000 rpm for 10 minutes. The results showed that the nanofluid prepared with the frequency of 80 kHz and 130 kHz show the highest extinction coefficient values after 30 days. Higher extinction coefficient values mean the highest amount of nanomaterial in suspension. Li et al. [52] analyzed the stability of SiO2-oleic acid/liquid paraffin nanofluid through the Zeta potential test. The nanofluid was prepared with two-step methods (magnetic stirrer for 30 minute and then sonicated for 1 hour). It was found that the large numbers of SiO2 nano-sized particles possess maximum value for the total count at values less than −40 mV indicating high stability of SiO2 nano-sized particles in liquid paraffin. Geng et al. [53] used the DLS test to study the stability of ZnO-MWCNT/Oil nanofluid and found that the nanoparticles are in the nanoscale after the preparation of nano-oil. Li et al. [54] produced SiO2/EG nanofluids by the two-step method with a mass fraction of 0.005–5%. The zeta potential value of the nanofluid was found −56.28 mV and claimed that the nanofluid is stable. Greater the number of particles with a smaller diameter, the higher the probability of stability. Nanofluid cluster formation may lead to the larger diameter of the nanoparticles. As the number of clusters increases, the fluid stability will decrease.

Figure 5.

Schematic graph of rGO nanofluids preparation.

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3. Conclusion

From the literature, it can be concluded that the stability of suspension of nanoparticles in the base fluid is improved when the nanofluid is synthesized by the one-step method as compared to the two-step method but the preparation of nanofluids by one-step method is difficult and expensive relative than two-step method. The literature also reveals that with low sonication time or no sonication, the sedimentation rate is higher. It can be concluded that longer ultrasonication reduces sedimentation of nanoparticles and hence, increases the stability of nanofluids. There are major tasks, which need to be focused on for selection of mono/hybrid nanofluids and their fabrication process, the stability of hybrid nanofluids. The stabilized nanofluids and their characteristics can increase the heat exchange rate of heat exchangers which are generally used in pipeline engineering. In order to help newcomers and researchers in this field recognize the potential research gap, this review study seeks to provide the latest research and development on stable nanofluids and their applications in pipeline engineering. Due to the lack of understanding of the mechanism of nanofluid at the atomic level, many experimental studies are needed to consider several important issues such as particle migration, agglomeration, and stability.

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Written By

Sumit Kumar Singh

Submitted: 06 July 2022 Reviewed: 17 August 2022 Published: 27 October 2022