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International Journal of Thermal Sciences 76 (2014) 168e189

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International Journal of Thermal Sciences

journal homepage: www.elsevier .com/locate/ i j ts

A review on how the researchers prepare their nanofluids

Zoubida Haddad a,b,**, Chérifa Abid b, Hakan F. Oztop c,*, Amina Mataoui d

aDepartment of Electronics and Electrical Engineering, University of Boumerdes, AlgeriabAix-Marseille University, CNRS, IUSTI UMR 7343, 13453 Marseille, FrancecDepartment of Mechanical Engineering, Technology Faculty, Firat University, 23119 Elazig, TurkeydDepartment of Fluid Mechanics, Faculty of Physics, University of Sciences and Technology e Houari Boumediene, Algiers, Algeria

a r t i c l e i n f o

Article history:Received 19 April 2013Received in revised form18 August 2013Accepted 19 August 2013Available online 17 October 2013

Keywords:NanoparticlesStabilityNanofluid preparation

* Corresponding author. Tel.: þ90 424 237 0000x4** Corresponding author. Department of ElectronicUniversity of Boumerdes, Algeria.

E-mail addresses: zoubida.haddad@etu.univ-amugmail.com, hfoztop1@yahoo.com (H.F. Oztop).

1290-0729/$ e see front matter � 2013 Elsevier Mashttp://dx.doi.org/10.1016/j.ijthermalsci.2013.08.010

a b s t r a c t

The past decade has seen the rapid development of nanofluid science in different aspects, where theresearchers focused mainly on the enhancement of heat transfer. However nanofluids preparation alsodeserves the same attention since the final properties of nanofluids are dependent on the stability of thedispersion. In this paper, we summarize the nanofluid preparation methods reported by different in-vestigators in an attempt to find a suitable method for preparing stable nanofluids. In this context,nanofluids are classified according to material type as metallic and nonmetallic nanoparticles sincedifferent nanoparticles need their own stability method. Various types of nanoparticles with differentbase fluids are investigated. Also, the available data for the zeta potential as a function of pH is discussed.

� 2013 Elsevier Masson SAS. All rights reserved.

1. Introduction

Over the last several decades, researchers have attempted toovercome the limited heat transfer capabilities of traditional heattransfer fluids such as water, engine oil, and ethylene glycol (EG), bydeveloping a new class of fluids which offer better cooling orheating performance for a variety of thermal systems. Applyingnanotechnology to thermal engineering, the novel concept of“nanofluid” which was coined by Choi in 1995 [1] has been pro-posed to meet these cooling challenges. Nanofluids, which aresolideliquid composite materials consisting of nanometer sizedsolid particles suspended in different base fluids, provide a prom-ising technical selection for enhancing heat transfer because of itsmany advantages besides anomalously high thermal conductivity.Nanofluids represent improved stability compared with conven-tional fluids added with micrometer- or millimeter-sized solidparticles because of size effect and Brownian motion of the nano-particles in liquids. With such ultrafine nanoparticles, nanofluidscan flow smoothly in a microchannel without clogging and the sizeof the heat transfer system can be reduced for the use of nanofluidswith high heat transfer efficiency.

248; fax: þ90 424 236 7064.s and Electrical Engineering,

.fr (Z. Haddad), hfoztop1@

son SAS. All rights reserved.

Preparation of stable nanofluids is the key issue of nanofluidresearch. The stability of nanofluids refers to several aspects: 1)Nanofluids are multi-phase dispersion system with high surfaceenergies and are, therefore, thermodynamic unstable. 2) Nano-particles dispersed in the nanofluids have strong Brownian mo-tions. The mobility of the nanoparticles can offset theirsedimentation caused by the gravity field. 3) Dispersion of nano-particles in the fluids may deteriorate with time due to the aggre-gation of nanoparticles, which is caused by van der Waals forces. 4)No chemical reactions either between the suspended nanoparticlesor between the base fluid and nanoparticles are desired at workingconditions of the nanofluid. Therefore, there are two phenomenathat are critical to the stability of nanofluid, aggregation andsedimentation.

It was found that only a few review papers have discussed on thepreparation methods for nanofluids [2e4]. In the present paper, weattempt to review the preparationmethods of nanofluids presentedin previously published data with much more details. The purposeof this paper is to understand the lack stability of nanofluids, whichis a key issue that influenced the nanofluid properties for applica-tion, and to propose suggestions that could lead to prepare stablenanofluid over a long time, with negligible agglomeration andwithout chemical change of the fluid properties. The review dis-cussed different types of nanoparticles; nonmetals (Al2O3, ZnO,CuO, TiO2, Fe3O4, CNTs, SiO2 and AlN) and metals (Al, Ag and Cu).Also, we present the available data for the zeta potential as afunction of pH values.

Fig. 1. Particle size distributions of AlN filtered nanofluid [7].Fig. 2. Sedimentation rate of AlNePPG 425/2000 diluted systems [8].

Z. Haddad et al. / International Journal of Thermal Sciences 76 (2014) 168e189 169

2. Preparation of nanofluids

Nanoparticles, the additives of nanofluids, play an importantrole in changing the thermal transport properties of nanofluids. Atpresent, various types of nanoparticles, such as metallic nano-particles and ceramic nanoparticles, have been used in the nano-fluids preparation. In the following part, we will present thenanofluid preparation methods for eleven different nanoparticlesreported in the literature.

2.1. Preparation of non-metallic nanofluids

2.1.1. Aluminum nitride -nanofluidsAluminum nitride (AlN) is a nontoxic newer material in the

technical ceramics family. While its discovery occurred over 100years ago, it has been developed into a commercially viable productwith controlled and reproducible properties within the last 20years. AlN is one of the typical ceramics that have special propertiessuch as high thermal conductivity (8e10 times that of Al2O3), lowdielectric coefficient (about 8.15), high electrical resistance, corro-sion & erosion resistance and low density. Because of these ad-vantageous properties, it is used in various engineeringapplications and has attracted the intense interest of researchers.However, till date very few results concerning AlN nanofluids havebeen reported in the literature. In this context, Hu et al. [5] were thefirst to disperse AlN nanoparticle, produced by plasma arc in the gasphase into ethanol with castor oil as a dispersant to improve sus-pension stability. The suspension was then stirred with a high-speed magnetic stirrer. The resulting suspension was placed in anultrasonic homogenizer for 10 min. It was observed that the pre-pared sample can remain stable for more than 2 weeks withoutsettling. Choi et al. [6] mixed the agglomerated Al2O3 and AlNnanoparticles with n-hexane and a proper amount of oleic acid. Themixturewas subjected to bead-milling with ZrO2 beads in a verticalsuper-fine grinding mill. The powders were hydrophobic modifiedby esterification reaction simultaneously with bead milling bycirculating the suspensions between bead mill and ultrasonic re-action bath. A surface modified solution was then filtered usingultra filtration (UF) membrane to remove excess oleic acid whichdid not form stable chemical bonds with the particle surface. Thefiltered solution was mixed with transformer oil, and then finallydried off the n-hexane using a rotary vacuum evaporator. Theyobserved that for a period of one month, filtered and non-filteredAl2O3 nanofluids are well dispersed at the beginning, while sedi-mentation was very clear for non-filtered nanofluid after onemonth. The settling phenomenon for non-filtered suspension wasinterpreted as the formation of double chain of oleic acid (OA) onthe particle surface, making it hydrophilic again. Yu et al. [7]

prepared AlNeethylene glycol and AlNepropylene glycol nano-fluids by stirring and continuous sonication for 3 h, to ensureuniform dispersion of nanoparticles in the base fluid. As shown inFig. 1, they found that the average particle sizes for AlNeEG andAlNePG nanofluids are 165 and 169 nm, respectively. Wozniak et al.[8] suspended AlN nanopowder in poly propylene glycol PPG 425and PPG 2000, and then the suspensions were homogenized byusing a magnetic stirrer for 3 h. More concentrated AlN dispersionswere prepared using a laboratory dissolver, which had a mechan-ically modulated dispersive disk. The powder was slowly andincrementally added to PPGs. Each addition was followed by ho-mogenization step. When all the powder was added, the disper-sions were stirred for 40 min at 3000 rpm. It was reported that thesuspensions were of high flow-ability and noticeable homogeneity.AlNePPG 425/2000 were of much higher sedimentation in PPG oflower average molecular mass, i.e. 425. In addition, the particlessettled quite rapidly in PPG 425; after 30 h the sedimentation ratewas >90% and afterward it remained at the same level. However,AlNePPG 2000 suspension demonstrated its sedimentationbehavior only slightly, which was confirmed by its low sedimen-tation rate (max. w10% in 72 h), as shown in Fig. 2. The zeta po-tential measurements in PPG media showed that zeta potential isnegative for both liquids; it averaged (�30) mV for AlNePPG 425dispersion and below (�10) mV in case of AlNePPG 2000 system.

Although different surfactants and physical treatments wereused for the preparation of AlNenanofluids, only more than twoweeks stable time of nanofluids was reported [5]. Thus, otherpreparation methods are highly desired to prepare stable AlNenanofluids.

2.1.2. Zinc oxide-nanofluidsZinc oxide is emerging as a material of interest for a variety of

electronic applications such as semiconductor for making inex-pensive transistors and thin film batteries. It can be used in a largenumber of areas, and unlike many of the materials with whichcompetes, is inexpensive, relatively abundant, chemically stable,easy to prepare, antibacterial and nontoxic. One of the first in-vestigations dealt with this type of nanofluids was presented by Yuet al. [9]. They prepared ZnO nanofluids by dispersing ZnO nano-particles in ethylene glycol. The mixture was stirred and sonicated(40 kHz and 150 W) continuously for 3 h to ensure uniformdispersion of nanoparticles in the base fluid. Based on the influ-ence of ultrasonification on the particle size, it was reported thatthe average size decreases rapidly in the first 3 h, after 3 h theaverage size was about 210 nm (w10e20 times the primary size). Itwas concluded that ultrasonification was not effective in avoidingparticle aggregation and producing uniformly distributed andwell-controlled size of ZnO nanoparticles. Moosavi et al. [10] first

Fig. 3. The ZnO cluster size in suspension as a function of sonication time [12].

Z. Haddad et al. / International Journal of Thermal Sciences 76 (2014) 168e189170

synthesized the ZnO nanoparticles, which were then mixed withethylene glycol and glycerol as base fluids with the aid of magneticstirrer. Ammonium citrate was added as dispersant to enhance thestability of the suspension. The weight ratio of dispersant tonanoparticle was 1:1. As a result, the samples were stable forseveral months and no sedimentation and agglomeration of theparticles in the samples was observed. Raykar and Singh [11]synthesized water soluble ZnO nanoparticles. The solutions weresonicated for 1 h and a proper amount of acetylacetone (acac) wasadded as a dispersant to the solutions, which were sonicated againfor 10 min. They showed that the nanofluids were stable over 9months to 1 year, and the size of ZnO nanoparticles was reducedfrom 150 nm to 80 nm due to the reaction with acac, and. Kole andDey [12] dispersed fairly agglomerated ZnO nanoparticles inethylene glycol by intense ultrasonication (200 W). The cluster sizeof ZnO nanoparticles in suspension was plotted against sonicationtime to ascertain the optimum time of sonication. As shown inFig. 3, it was observed that ZnO clusters size rapidly decreases fromw459 nm to w91 nm between 4 and 60 h. However, beyond 60 hof sonication, cluster size increased and for 100 h of sonication ZnOclusters increased to w220 nm. Therefore, an optimum durationfor sonication was chosen to be 60 h. As a result, the suspensionwas stable for 30 days without any trace of visible sedimentation.Chung et al. [13] dispersed two types of zinc oxide powders, pro-duced by solegel and physical vapor synthesis in deionized watercontaining ammonium polymethacrylate as a dispersant. Themixture was stirred at 25 �C for 30 min, and sonicated usingseveral ultrasonic agitation systems, including a single piezoac-tuated bath, a solenoid-actuated bath and a static bath withimmersed horn. They found that dispersion by ultrasonic horn wasmore effective in terms of the size reduction rate, the minimumachievable size, and sedimentation rates. Suganthi and Rajan [14]synthesized zinc oxide nanoparticles by chemical precipitationmethod using zinc nitrate hexahydrate as precursor. The ZnOewater nanofluid was prepared with the aid of ultrasonication for3 h and stabilization using sodium hexametaphosphate (SHMP) asa dispersant. A predetermined quantity of ZnO was added to thesurfactant solution under high shear homogenization and ho-mogenized for 20 min at 7000 rpm, followed by ultrasonication for180 min (750 W and 20 kHz). Surfactant: nanoparticle ratio wasfixed at 1:5. The obtained higher values of absolute value of zetapotential ensured higher colloidal stability of dispersions, whichwas confirmed also through visual observation as well. Zafarani-Moattar and Majdan-Cegincara [15] dried ZnO nanoparticles inan electrical oven at about 110 �C for 24 h prior to use for removingadsorbed water moisture on the surface. The nanoparticles were

dispersed in poly ethylene glycol and its aqueous solutions usingsonication. It was found that the average particle size measured forthe nanofluids investigated was much larger than the size of pri-mary particles. It was also found that ZnOePEG nanofluid wasstable at least for 140 min. Saleh et al. [16] synthesized ZnOpowders using chemical precipitation method. The nanoparticleswere dispersed in ethylene glycol using a magnetic stirrer andultrasonic processor under continuous pulse for 2 h. Lee et al. [17]prepared EG-based ZnO nanofluids using a one step methodknown as pulsed-wire evaporation (PWE). The synthesized nano-particles came into direct contact with EG inside the chamber walland the ZnO nanoparticles were obtained without any surfacecontamination.

From the above ZnO-nanofluid preparation methods, it can beseen that long-term stability was obtained when using acac andsonication for 10 min [11]. This clearly indicates that acac can be asuitable surfactant to stabilize ZnOewater nanofluids.

2.1.3. Titanium dioxide-nanofluidsTitanium dioxide has three types of crystal habits which are

brookite, anatase and rutile. Brookite is one kind of unstablecrystal, with no industrial value, while anatase and rutile all havestable properties, which are very important white pigment.Compared with other white pigments, it is well accepted for itssuper whiteness, tinting strength, covering power, durability, heatresistance, chemical stability, and especially without any toxicity.Titanium dioxide is widely applied in many fields, includingnanofluids preparation. Kayhani et al. [18] functionalized TiO2nanoparticles by a chemical treatment. The TiO2 nanoparticleswere mixed with 1,1,1,3,3,3 hexamethyldisilazane (C6H19NSi2) in amass fraction of 2:1. The resulting mixture was sonicated at 30 �Cfor 1 h using ultrasonic vibration (40 kHz). Then, the soakednanoparticles were dried with a rotary evaporation apparatus. Thenanoparticles were mixed with distilled water and subjected toultrasonic vibration (400 W and 24 kHz) for 3e5 h. The obtainednanofluids were found to be stable for several days without anyvisible sedimentation. He et al. [19] mixed TiO2 nanoparticles inthe form of large agglomerates with distilled water using ultra-sonication for 30 min. Then, the suspension was processed in amedium-mill to reduce the agglomerated nanoparticles. The pHvalue was adjusted to 11 (corresponding to zeta potential ofw40 mV) to prevent re-agglomeration of the milled samples.Therefore, the obtained nanofluids were found to be very stable formonths. Murshed et al. [20] used ultrasonic dismembrator for 8e10 h to ensure proper dispersion of TiO2 nanoparticles withdeionized water. The size of the nanoparticles in the base fluid wasfound to be increased. Therefore, oleic acid and cetyltrimethylammonium bromide (CTAB) surfactants (0.01e0.02%) were addedto ensure better stability and proper dispersion. Duangthongsukand Wongwises [21] used CTAB with very low concentrations(about 0.01%) to ensure better stability and proper dispersion ofthe TiO2 nanoparticles in water, without affecting the thermo-physical properties of the nanofluid. The surfactant was firstmixed with water, and the nanofluids were sonicated continuouslyfor 3e4 h using an ultrasonic vibrator. They observed littleagglomeration after 3 h of sonication. Kim et al. [22] synthesizednanofluids containing Al2O3, TiO2 and ZnO in water and ethyleneglycol. The suspensions were sonicated in an ultrasonic bath for1 h, and then agitated for 10 h by a magnetic stirrer to make thenanofluids homogenous. Sodium dodecyl sulfate (SDS) of 0.05 M(above the critical micelle concentration 0.01 M) was added as asurfactant. They observed that all nanofluids were stable exceptthe 10 nm ZnO/EG nanofluid, where no trial was successful tostabilize the fluid, including addition of different surfactants andadjusting of the pH level. They attributed this to an increase in

Z. Haddad et al. / International Journal of Thermal Sciences 76 (2014) 168e189 171

surface area of the ZnO nanoparticles. Abbasian Arani and Amani[23] dispersed TiO2 nanoparticles in distilled water. They usedultrasonic vibrator with magnetic stirrer for approximately 3 h inorder to break down agglomeration of the nanoparticles. CTABsurfactant was added with very low concentration (around 0.01%)to not affect nanofluids’ thermophysical properties even though itwas reported that sedimentation of nanoparticles is less importantfor turbulent flow regime. Moreover, the pH values were between5.62 and 7 (IEP of TiO2 is 2.9). A minimum of 2.7 l of each con-centrations were prepared and it was observed that the suspen-sions were stable for several hours (days). Utomo et al. [24] diluted30e40 wt.% of alumina and titania suspensions by addition ofdistilled water whilst keeping pH constant, and ultrasonicating thesuspension for 3 min to obtain homogenous mixtures. It was re-ported that according to the manufacturer the alumina and titaniasuspensions were stabilized by using octyl silane and ammoniumpolyacrylate (molecular weight ¼ 3000 g/mol), respectively. It wasobserved that even after sonication, nanoparticles formed rela-tively large aggregates with the size of the order of 200 nm and140 nm for alumina and titania, respectively. Longo and Zilio [25]verified the dispersion efficiency of Al2O3ewater (15 wt.%) andTiO2ewater (25 wt.%) nanofluids, that were prepared and mixed bythe supplier. Each nanofluid was subdivided into two parts: thefirst was subjected to mechanical stirring, and the second wassonicated at 25 kHz for 48 h. It was found that Al2O3ewater simplystirred showed a distribution with two peaks corresponding to anaverage size of 47 nm and 285 nm. Also, the Al2O3ewater soni-cated nanofluid exhibited two peaks distributions correspondingto an average size of 28 nm and 165 nm. The first peak of bothdistributions was close to the declared average diameter of thenanoparticles (30 nm), whereas the second peak indicates thetendency of the nanoparticles to agglomerate into clusters. It wasalso found that TiO2ewater simply stirred nano-fluid shows asingle peak distribution with an average size of 220 nm, whereasthe TiO2ewater sonicated nanofluid presents a single peak distri-bution with an average size of 155 nm. It was concluded that theultrasound treatment shows better dispersion efficiency thansimple mechanical stirring, and that both nanofluids showed sta-bility for more than one month. Chen et al. [26] formulated EGeTNT nanofluids by using dry titanate nanotubes, synthesized basedon the alkali hydrothermal transformation. The nanofluid wasprepared by mixing nanoparticles with ethylene glycol undergentle stirring, followed by sonication for 48 h using ultrasonicbath. It was found that the average size was w260 nm for allnanofluids. The nanofluids produced were stable over the period oftwo months. Mo et al. [27] dispersed rutile and anatase TiO2nanoparticles in deionized water. The pH value of the deionizedwater was adjusted to eight by using ammonia, and sodiumdodecyl sulfate (SDS, C12H25SO4Na) was used as a surfactant toensure better stability and proper dispersion. The suspension waswhisked using magnetic stirrer for 10 min and sonicated for40 min. They estimated the stability of the TiO2ewater nanofluidsby the changes in the weight concentrations of the TiO2 nano-particles, which were measured using a spectrophotometer. It wasobserved that the 0.30 wt.% and 0.70 wt.% TiO2ewater nanofluidswere kept stable for 286 h (about 12 days). However, the stabilityof the 0.05 wt.% nanofluids was not so good. Bobbo et al. [28] useddifferent dispersants and various physical treatment techniquesincluding ultrasonic agitation, ball milling and homogenization.They found that homogenization method was the best process toimprove the suspension stability. Also, SDS and PEG were identi-fied as the best dispersants for the nanofluids based on single wallcarbon nanohorns (SWCNH) and TiO2, respectively. Hence, thenanoparticles were mechanically dispersed in water. Then, a highpressure homogenizer (up to 1000 bar) was employed to optimize

the dispersion. For the nanofluids based on SWCNH at concen-trations of 0.1% and 1% by mass, the ratio between nanoparticlesand dispersant mass was 1:1. For the lowest concentration (0.01%by mass), the ratio was 1:3. For watereTiO2 nanofluid, the ratiobetween nanoparticles and dispersant mass was 1:2 for eachconcentration. It was found that the nanofluids formed by water,SDS and SWCNH were very stable even after several days. Themeasured nanoparticle average diameter was around 140 nm,188 nm and 120 nm for the 0.01%, 0.1% and 1% mass concentra-tions, respectively. However, solutions of water, PEG and TiO2 wereless stable. It was found that all the measurements provide muchhigher values than the 21 nm correspondent to the nominaldiameter of the nanoparticles: at 0.01%, 0.1% and 1% mass con-centrations the measured average diameter was around 180 nm,121 nm and 132 nm, respectively. The diameter increase indicatesa tendency of titania particles to rearrange in liquid media formingaggregates. Hojjat et al. [29] subjected the suspensions of g-Al2O3,TiO2, and CuO nanoparticles in deionized water to ultrasonic vi-bration for about 1 h. Then, appropriate amounts of carboxymethyl cellulose (05 wt.% of CMC in DIewater) were added to thesuspensions and thoroughly mixed to achieve the desiredcomposition of nanofluids. They observed no sedimentation afterseveral days following the nanofluid preparation. In addition, thepreparation of stable Al2O3 nanofluids was more difficult and, as aresult, Al2O3 nanoparticle dispersions were limited to 1.5 vol.%.Fedele et al. [30] sonicated TiO2 nanofluid at 35 wt.%, then addingbidistilled water in a weighed amount. They analyzed the averagedimension of the nanoparticles in suspensions using dynamic lightscattering method. The TiO2 mean diameter was 76 nm at 1 wt.%,72 nm at 10 wt.% and 73 nm at 20 wt.%. They found that at thesecompositions, the nanofluids were stable with absence of particlesaggregates. To determine the tendency of the particles to settledown along time, they put two samples at 1 wt.% in two differentmeasurement cuvettes. The first sample was measured almostevery day for thirty-five days, without shaking the fluid. The sec-ond sample was also measured almost every day for thirty-fivedays after sonication of the fluid. In the case of static solutions, itwas observed that the nanoparticle mean size slightly decreased toaround 51 nm after 35 days, indicating a partial precipitation.However, after 1 h of sonication, a mean particle size of 76 nmwasalways recorded, suggesting the absence of further aggregationphenomena. TiO2ewater nanofluid zeta potential was around55 mV, which is higher than the empirical limit of 30 mV overwhich a colloidal solution should be stable. The pH values were1.86 for the 35 wt.% solution, 2.24 for 20 wt.%, 2.37 for 10 wt.%, and3.07 for the 1 wt.%. Tajik et al. [31] and Chakraborty et al. [32]produced Al2O3 and TiO2 nanofluids by dispersing nanoparticlesin distilled water, using continuous ultrasonication for 30 min andultrasonic stirring for 20 min, respectively. The prepared disper-sion remains stable for the duration of experiment [28] and noappreciable sedimentation was observed after 24 h [32]. Mitraet al. [33] dispersed TiO2 and MWCNT nanoparticles in water fol-lowed by 30 min of ultra-sonication. Sajadi and Kazemi [34] mixedthe proper amount of TiO2 nanoparticles with distilled water by amixer for 10 min. Then, ultrasonic cleaner was used to dispersenanoparticles for 30 min. Suriyawong and Wongwises [35] diluted40 wt.% of TiO2 nanofluid by adding water. After that, the nano-fluids were treated with ultrasonic vibration for 2 h to distributethe nanoparticles evenly. Other researchers have also preparedTiO2 nanofluids by using only physical methods. However, theydidn’t indicate the time used for nanofluid preparation [36e39].

In this section we mainly discussed the TiO2enanofluidspreparation methods. It can be seen that stable nanofluid was ob-tained using either sonication for 48 h [25,26] or adjusting the pHvalue [19].

Fig. 4. Summary of DLS analysis of diluted SiO2 dispersions in TH66 24 h after ultra-sonication; (a) no surfactant, (b) benzalkonium chloride, (c) benzethonium chloride,(d) cetyltrimethyl ammonium bromide. (Inset, middle) Visual appearance of 1 vol.%SiO2/TH66 with various surfactants. (Inset, right) SEM image of SiO2 powder used inpreparation of nanofluids [40].

Fig. 5. Photos of visualization and Tyndall effect of methanol-based SiO2 nanofluidsjust right after preparation [45].

Z. Haddad et al. / International Journal of Thermal Sciences 76 (2014) 168e189172

2.1.4. Silicon dioxide-nanofluidsSilica is a widely used ceramic material both as a precursor to

the fabrication of other ceramic products and as a material on itsown. Silica has good abrasion resistance, electrical insulation andhigh thermal stability. It is insoluble in all acids with the exceptionof hydrogen fluoride (HF). Timofeeva et al. [40] dispersed silicondioxide nanopowders in non-polarorganic fluid (TH66). Benzalko-nium chloride (BAC), benzethonium chloride (BZC), and cetyl-trimethyl ammonium bromide (CTAB) were tested as surfactantsfor dispersing silica. Surfactants were dispersed into the base fluidfirst, followed by introduction of the nanopowder. The mixture washomogenized by continuous stirring and sonicated 10 times(w80 W) for 5 min each time. Suspensions with 1 vol.% of SiO2

nanoparticles with no surfactant and excess of each surfactant(5 wt.% or w0.12e0.14 M) were prepared using the adsorptionmodel with ‘laying flat’ and compacted ‘standing up layers of thesurfactant molecules. The visual appearance of suspensions 24 hafter the last sonication indicated that BAC surfactant was the beststabilizer for SiO2/TH66, as shown in Fig. 4. Both DLS and SEM re-sults confirmed that the average particle size was the smallest insuspension with BAC, followed by BZC. The suspension with nosurfactant had the largest agglomerates, while CTAB provided justsome reduction in the average agglomerate size. Nanofluidsappeared to be stable without any visual phase separation for atleast aweek. Silica nanoparticles were functionalized using graftingsilanes directly to the surface of silica nanoparticles by Yang and Liu[41]. A silane of (3-glycidoxylproyl) trimethyoxysilane was used forthe functionalizing process. The mass ratio of the reacting silaneand silica nanoparticles was taken as 0.115. The nanoparticles weredispersed into water and the solution was kept at the environ-mental temperature of 50 �C for 12 h. It was found that function-alized nanoparticles can still keep dispersing well after thenanofluid has been standing for 12 months even at the mass con-centration of 10%. Moreover, no sedimentation was observed. Theyalso prepared traditional nanofluid by dispersing and oscillatingnanoparticles into water. Silica nanoparticle powders were firstlydispersed into deionized water and the suspension was thenoscillated in an ultrasonic bath for 12 h. It was observed thatsedimentation occurred after several days. Anoop et al. [42]dispersed an appropriate amount of SiO2 nanoparticles in

deionized water using an ultrasonic bath for 30 min. Further, thiscolloidal suspensionwas subjected to intensified ultrasonication byimmersing a probe type sonicator in the nanofluids. Cyclic ultra-sonic pulses for about 15 min were given to the suspension toachieve maximum possible de-agglomeration of particles. The pHvalue of the nanofluid suspension was kept away from the iso-electric pH value, at a magnitude of 4.5 by adding reagent gradenitric acid. It was observed that nanofluids exhibited good stabilityover time. Qu and Wu [43] prepared Al2O3 and SiO2ewater nano-fluids. They first adjusted the pH value of the nanofluids to a certainvalue (with pH ¼ 9.7 and 4.9 for the silica and alumina nanofluids,respectively), which was far away from the corresponding iso-electric point (IEP) of silica (with pH w 3) or alumina (withpH w 9), and then nanoparticles were added into water. Thedispersion solution was subsequently vibrated for about 4 h in anultrasonic bath. It was found that alumina nanoparticles werebetter dispersed. Fazeli et al. [44] dispersed SiO2 nanoparticles indistilled water, and then the suspension was sonicated by an ul-trasonic bath for at least 90 min. They found that silica nanofluidsstayed stable for a period of 72 h without any visible settlement.Pang et al. [45] mixed nanoparticles in pure methanol by usingultrasonic vibration (750 W, 20 kHz) for 2 h to break down theagglomeration. They studied the effect of nanoparticle concentra-tion on the zeta potential and pH of methanol-based nanofluids.They showed that zeta potential is highly related to the pH of thesuspension. The measured zeta potential of Al2O3 nanofluids wasover 60 mV, and the zeta potential of SiO2 nanofluids was over30 mV, which indicates the good stability of both nanofluids.However, it was observed that the nanoparticles in nanofluidscontact each other and form some clusters, and the cluster size ofSiO2 particlewas larger than that of Al2O3 particle (clustering size ofSiO2: 280e401 nm; clustering size of Al2O3: 120e148 nm). Thephotos of visualization and Tyndall effect showed that methanol-based nanofluids were well dispersed, as shown in Fig. 5. Boluk-basi and Ciloglu [46] prepared SiO2 nanofluids by using magneticstirrer. Then, the suspensions were transferred into an ultrasonicvibrator and sonicated continuously for 2 h (600 W and 40 kHz).They reported that no sedimentation was observed during theperiod of experiment. Darzi et al. [47] added distilled water to aspecified amount of SiO2 nanoparticles and mixed together bymagnetic stirrer for 2 h. Kulkarni et al. [48] synthesized nanofluidscontaining Al2O3, CuO and SiO2 nanoparticles. The nanoparticleswere obtained as colloidal dispersion in 50% water by weight. Thenanofluids were prepared in 60:40 ethylene glycol and water so-lution (binary fluid). The samples were placed in a sonicator bathfor approximately 2 h to ensure proper dispersion and preventagglomeration. They confirmed from the DLS results that thenanoparticles were dispersed uniformly in the suspension. Hwang

Table 1The pH value and zeta potential of the nanofluids with various solid contents in thisexperiment.

CuOconcentration(vol.%)

NaHMPconcentration(g/100 ml nanofluid)

Wt ratioNaHMP/CuO

pH value(e)

Zetapotential(mV)

0.01 0.2 3.12 6.66 �59.80.02 0.4 3.12 6.66 �51.30.07 1.4 3.12 6.64 �37.50.10 2.0 3.12 6.68 *0.20 2.0 1.56 6.70 *0.40 2.0 0.78 6.67 *

*The high solid content turned the sample solution opaque so that the zeta potentialcould not be measured using the dynamic light scattering analyzer [54].

Z. Haddad et al. / International Journal of Thermal Sciences 76 (2014) 168e189 173

et al. [49] used an ultrasonic disruptor to produce CuO, MWCNTsand SiO2 nanofluids. They obtained stable suspensions for the caseof SiO2 and CuO nanoparticles. However, sodium dodecyl sulfate(SDS) was used as a surfactant when producing MWCNT nanofluidssince MWCNTs are entangled and agglomerated in aqueoussuspension.

From the aforementioned preparation methods of nanofluidscontaining mainly silica nanoparticles, it can be noticed that SiO2e

water nanofluid can be kept stable for 12 months even at 10% massconcentration [41], which indicates that Surface-functionalizedsilica nanoparticles can be competitive in the preparation of silicananofluids.

2.1.5. Copper oxide-nanofluidsNano-copper oxide is awidely usedmaterial. It has been applied

to the catalyst, superconducting materials, thermoelectric mate-rials, sensing materials, glass, ceramics and other fields. RohiniPriya et al. [50] prepared CuO nanoparticles by a sol gel route usingcupric nitrate and sodium hydroxide. The synthesized particleswere dispersed in water with aid of ultrasonic treatment for about6 h, followed by addition of tiron (4,5-dihydroxyl-1,3-benzenedisulfonic acid disodium salt) as dispersant. The opti-mum ratio of CuO to tironwas determined by studying the colloidalstability of dispersions with different concentrations of surfactant.The minimum concentration of surfactant required to ensurecolloidal stability corresponded to CuO:Tiron ratio of 2.5:1. Theprepared nanofluids were found to possess a maximum absolutezeta potential of 30 mV, which was considered sufficient to ensurecolloidal stability. In addition, the stability was also confirmedthrough visual observation. Suresh et al. [51] also synthesizedcopper oxide powder by solegel method. The nanoparticles weredispersed in water by using an ultrasonic vibrator (100 W and36� 3 kHz) for 6 h. The pH of the prepared nanofluidwasmeasuredand found to be around 4.83. Lee et al. [52] used in-house preparedCuO nanofluids in two ways such as one step method and two stepmethod. For the two step method, CuO/DIewater nanofluid wassonicated continuously for 6 h while the process of preparation ofCuO nanofluids in one step method was carried out by pulsed laserablation in liquids (PLAL) using a single-pulsed laser beam(k ¼ 532 nm) as follows: first A Cu pellet was put at the bottom of abeaker filled with DIW, and then a Nd:YAG laser was used to pro-duce CuO/DIW nanofluid for 8 h. As shown in Fig. 6, the size ofspherical CuO nanoparticle of nanofluid fabricated by one-stepmethod is much smaller than that used in two-step method. Theychecked the dispersion stability of nanofluids by measuring zeta

Fig. 6. TEM images of CuO nanoparticles according to manufacturing

potential and pH. It was found that dispersion stability of CuO/DIewater nanofluid fabricated by one-step method (zetapotential ¼ 30 mV) of the PLAL is much better than that preparedby two-step method (zeta potential ¼ 15.8 mV). They attributedthis to the ions dissociated from water molecules during the laserablation process. Yang and Liu [53] dispersed CuO nanoparticleswhich were commercial products of gas condensation into deion-izedwater, then oscillated in an ultrasonic bath for 12 h. They foundthat the particle diameter distribution in suspension is almost threetimes larger than the nominal diameter. Chang et al. [54] synthe-sized copper oxide nanoparticles using spinning disk reactor (SDR).The nanoparticles were agitated in jacketed vessel containingNaHMP as dispersant, using sonication with a power intensity of165 W. The weight ratio of NaHMP to CuO was first kept at 3.12:1while the CuO content increased from 0.01 to 0.10 vol.%. Then, toreduce the extent of CuO dissolution caused by adding NaHMP, theconcentrations of NaHMP were fixed at 2 g/100 ml, but the CuOcontent was varied from 0.10 to 0.40 vol.%. They observed thatwhen the CuO content was higher than 0.40 vol.%, the suspensionwas very unstable and the CuO nanoparticles tended to settlewithin several minutes, which rendered the experiment impos-sible. It was reported that the high solid content turned the samplesolution opaque so that the zeta potential could not be measuredusing the dynamic light scattering analyzer, as shown in Table 1.Harikrishnan and Kalaiselvam [55] synthesized CuO nanoparticlesby precipitationmethod. The preparation process of CuOeoleic acidnanofluids were performed using an ultrasonic vibrator at a fre-quency of 40 kHz and the residing times of nanofluids in thevibrator were varied for different mass fractions and they were30, 35, 40 and 45 min for 0.5, 1.0, 1.5 and 2.0 wt.%, respectively.

methods: (a) one-step method and (b) two-step method [52].

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They found that nanoparticles were stably dispersed and uniformlydistributed in base fluid. It was reported that CuO nanoparticleswere not dissolved in oleic acid, but dissociative adsorption mightexist between oleic acid and CuO nanoparticles. Saeedinia et al. [56]dispersed CuO nanoparticles in oil by using an ultrasonic processor(400Wand 24 kHz) to break large agglomerates of nanoparticles inthe fluid and make stable suspension. It was observed with nakedeyes that the nanofluids were uniformly dispersed for 24 h and thecomplete sedimentation occurred after a week. Selvakumar andSuresh [57] dispersed specified amount of nanoparticles in deion-ized water by using an ultrasonic vibrator for 6 h. They measuredthe pH of prepared nanofluids and the values were found to bearound 4.8, which is far away from the isoelectric point. It wasobserved that the prepared nanofluids were stable even after aweek except very little sedimentation. Kannadasan et al. [58] pro-duced CuO nanoparticles by chemical precipitation method, theyprepared CuOewater nanofluids using an ultrasonic bath (100 W,36 � 3 kHz) for 4 h to get stable suspension. It was observed thatthere was a very little settlement of nanoparticles even after 25days. Byrne et al. [59] prepared CuOewater nanofluids with andwithout surfactant. The amount of surfactant (CTAB) used wasequal to the volumetric amount of nanoparticles added to the basefluid. Once the components were mixed, the batch was sonicatedusing a high intensity ultrasonic processor (100 W) for 7e8 h toensure an adequate dispersion of particles. From the DLS resultswith concentration of 0.1% and no surfactant, it was observed thatthe average particle size in the quiescent fluid decreases steadilyfrom roughly 3000 nm at time zero to approximately 300 nm afternearly 4 h, indicating the presence of agglomerates in the samplesince no surfactant was used. Also, it was observed that the 0.1%nanofluid with surfactant had an average particle size of about200 nmwith almost no variation over 7 days. In addition, the rapidinitial settling of large agglomerates seen in the sample withoutsurfactant was not observed. They indicated that the use of a sur-factant reduces the average particle size, improves the dispersion ofthe particles and makes a suspension much more stable. Liu et al.[60] dispersed CuO nanoparticles made by the gas-condensationmethod into deionized water. The suspension was then oscillatedfor about 10 h in an ultrasonic water bath with aworking frequencyof 25e40 kHz to ensure full dispersion. They reported that the pHvalue of the nanofluid relied weakly on the concentration and itwas fixed at about 7. Fotukian and Esfahany [61] prepared CuOnanofluids by gradually adding nanoparticles to distilled waterwhile agitated in flask. The suspensionwas then vibrated for 10 h inultrasonic mixer. As a result, no precipitation was observed after5 h. Liu et al. [62] produced CuO/ethylene glycolewater nanofluids.They put the nanoparticles and water into a super-sonic water bathand surged for about 12 h to form stable suspension. The resultsshowed that the stability and uniformity of nanoparticle suspen-sions were poor after several days. Zeinali Heris [63] first usedmechanical agitator for 6 h to disperse the nanoparticles, then thesuspension was subjected to ultrasonication for about 2 h. Noprecipitation of nanoparticles was observed after 22 h. Namburuet al. [64] stirred and agitated the CuOenanofluid mixture thor-oughly for 30 min using an ultrasonic agitator. Wongcharee andEiamsa-ard [65] dispersed CuO nanoparticles in water using anultrasonic vibrator for 5 h.

It can be clearly seen that NaHMP can be used for the prepara-tion of copper oxide nanofluids at low volume fraction that is lessthan 0.4 vol.%. In addition, CuO nanofluids showed short timestability.

2.1.6. Aluminum oxide-nanofluidsAlumina is the most cost effective and widely used material in

the family of engineering ceramics. The raw materials from which

this high performance technical grade ceramic is made are readilyavailable and reasonably priced, resulting in good value for the costin fabricated alumina shapes. With an excellent combination ofproperties and an attractive price, it is no surprise that fine graintechnical grade alumina has a very wide range of applications.

Sonawane et al. [66] performed a series of trials to determinethe exact proportion of surfactants to be added to the aviationturbine fuel (ATF) to make a stable Al2O3 nanofluid. Initially, theymixed 1 L of ATF with 1 vol.% of Al2O3 and ultrasonication wascarried out for 1 h, 2 h and 12 h, in three different experiments.However, irrespective of the sonication time, the nanoparticleswere found to settle within an hour of completion of sonication. Inthe next set of trials, six samples with ATF (10 ml) and Al2O3 (1% byvolume) were prepared along with Oleic acid as a surfactant toprevent the nanoparticles from settling. A syringe was used tocarefully measure the addition of surfactants in the samples. Eachof the six samples has a different concentration of surfactant to findthe most appropriate combination for stability. The mixture wascontinuously stirred while adding the nanoparticles and surfactant.However, Al2O3 particles settle at the bottom in each case. The sametrial was repeated with a different surfactant, PolyoxyethyleneSorbitan Monolaurate (known as ‘Tween’ 20 LR) known to be usedwith organic fluids. Numerous trials were performed usingdifferent concentrations of ‘Tween’ 20 LR. However, in these trialstoo, the Al2O3 particles settle at the bottom. Subsequently, trialswere carried out using different concentrations of Tween 20 andOleic acid in combination, and the appropriate concentration ofsurfactants to be mixed in ATF (10 ml) with 1% Al2O3 (by volume) isfound to be 3 drops (0.026ml) each of Oleic Acid and ‘Tween’ 20 LR.Trials are further extended for 1000 ml of ATF and 1 vol.% of Al2O3and proportionately 300 drops (2.6 ml) of each surfactant wereused. The nanofluid was sonicated for 1 h, 2 h and 12 h. It wasobserved that the solution was stable even after 24 h, and it wasindependent of ultrasonication time. With this appropriate con-centration of surfactants, nanofluids were prepared by suspendingAl2O3 nanoparticles in ATF. For higher nanoparticle volume con-centration, the surfactant amount was proportionately higher.Suresh et al. [67] synthesized Al2O3 nanoparticles by using chem-ical precipitationmethod. They prepared nanofluids by dispersing aspecified amount of Al2O3 nanoparticles in water using an ultra-sonic vibrator generating ultrasonic pulses of 100 W and36 � 3 kHz. The nanofluids were kept under ultrasonic vibratorcontinuously for 6 h. The pH of the prepared nanofluidwas found tobe 4.8, which is far from the isoelectric point. In addition, the zetapotential was expected to be around 45 mV which is an indicativeof good colloidal stability. They found that the Al2O3/water nano-fluid was very stable for several weeks without visually observablesedimentation. Beck et al. [68] subjected a mixture of Al2O3 nano-particles and ethylene glycol to ultrasonic mixing for several mi-nutes to obtain uniform dispersion. They reported that theresulting dispersions were remained uniform for the duration ofthe experiments because of surface charges on the particles.Sharma et al. [69] used SDBS, measuring one tenth of nanoparticlesas a dispersant. The dispersant was mixed with water and thecalculated amount of nanoparticles was added. The mixture wasstirred continuously for 12 h. They observed that a nanofluid havingless than 3% volume concentration was stable for over a week.However, some sedimentation was observed at higher concentra-tion. Sundar and Sharma [70] used the same technique, the onlydifference is the time of stirring which was 10 h. Gharagozloo andGoodson [71] diluted 20% weight concentration Al2O3ewaternanofluid with less than 1% nitric acid with deionized water to thedesired volume concentrations. The measured pH for each of thenanofluid concentration was 5.5. The nanofluid was sonicatedcontinuously for 4 h at 60 Hz and 130 W. They observed that the

Fig. 7. Effect of sonication time (h) on zeta potential of the Al2O3 nanoparticlesdispersed in water [84].

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nanofluid was stable with only minor settling after a week. Junget al. [72] produced two types of water-based alumina nanofluidswith/without polyvinyl alcohol (PVA), using a horn-type ultrasonicdisrupter for 2 h. The PVA concentration was set to be the same asthe nanoparticle concentration. They found that the particles in theprepared nanofluids with/without PVA were below 300 nm, andthe aggregated particles were stably suspended for more than 1month. Soltani et al. [73] first prepared the base liquid by mixingCarboxy methyl cellulose (CMC) aqueous solutions with a concen-tration of 0.5 wt.% with distilled water, using mechanical mixer.Then, the nanoparticles were added thoroughly mixing for 6 h. Theprepared nanofluids were sonicated for 1 h just before the exper-iment to increase the stability of solutions. Ho et al. [74] dispersednanoparticles into the bottle of ultra-pure Milli-Q water whilestirring with a magnetic stirrer for 2 h. They adjusted the pH valueof the nanofluid to pH ¼ 3. It was observed that the suspensionswere stable for at least two weeks. However, poor suspensionstability was observed for particle fraction greater than 4 vol.%.Addition of a suitable amount of Triton X-100 (about 0.021 wt.%) asdispersant to double distilled water to reach a homogenouslydispersed solution after sonication for about 30 min was done byYousefi et al. [75]. It was found that the obtained nanofluid wasstable about three days after sonication. Raveshi et al. [76] addedSDBS dispersant with a mass proportion of one tenth of thenanoparticles to WEG50 as a base fluid, and then the mixture wassonicated in ultrasonic bath for 2 h. Then, the nanoparticles weredispersed in the mixture which was stirred for 5 h. No obvioussedimentationwas observed after 3 days. Hung et al. [77] producedAl2O3/water nanofluid using a homogenizer operating at 8000 rpmfor 30 min, an electromagnetic agitator running at 600 rpm for90 min, and an ultrasonic vibrator operating at 400 W for 60 min.The base liquid was prepared by adding 0.2 wt.% of water-solublechitosan as a cationic dispersant to distilled water. Theyconfirmed that the difference between initial and final concentra-tions (after 2 weeks) of Al2O3/water nanofluid was less than 5%,indicating the stability of the prepared nanofluids. Heyhat et al. [78]put Al2O3 nanoparticles into the weighed distilled water graduallyand sonicate the mixture continuously for 1 h at 400Wand 24 kHz.The measured zeta potential was about 30 mV, which confirmedthe physical stability of the nanofluids. Singh et al. [79] dispersedtwo sizes Al2O3 nanoparticles produced by vapor condensationtechniques in ethylene glycol and water. The aluminaewaternanofluids were stabilized through electro-static method; a fewdrops of hydrochloric acid were mixed to maintain the pH valueand hence the zeta potential. The pH value was fixed at 4 and thezeta potential for these nanofluids was 58.7 mV. They concludedthat a pH of 3.5e5.0 was able to keep the nanofluids stable for along times. For Ethylene Glycol based nanofluids, nothing wasadded and it was found that only sonication is enough to get astable suspension. Pandey and Nema [80] dispersed nanoparticlesin distilled water using ultrasonic processor for 8e16 h. No sedi-mentation was observed in the produced nanofluids after 24 h.Chandrasekar et al. [81] synthesized Al2O3 nanoparticles usingmicrowave assisted chemical precipitation method. The nanofluidswere kept under ultrasonic vibration for 6 h at 36�3 kHz and100W. The pH valuewas found to be around 5which is far from theisoelectric point for alumina nanoparticles. Qu et al. [82] regulatedthe pH value of the base water by adding small amount of HClsolution, and then the nanoparticles were added. The dispersionwas ultrasonic vibrated continuously for 4 h in an ultrasonic bath. Itwas observed that nanoparticles could be stably suspended inwater at least for 3 days when the pH value is 4.9. Anoop et al. [83]used two Al2O3 nanoparticle sizes produced by laser evaporatedphysical methods to make alumina nanofluids. The particles weredispersed using ultrasonication and by keeping the pH value away

from the iso-electric point. The pH values used for 1 wt.%, 2 wt.%,4 wt.% and 6 wt.% were 6.5, 6, 5.5 and 5, respectively. It wasobserved that the suspensions were stable for several weeks. Leeet al. [84] sonicated four dilute Al2O3-nanofluids with pH of 6.04 ata fixed concentration of 0.1 vol.% for 0 h, 5 h, 20 h and 30 h at 30e40 kHz, respectively. It was observed that sonication is needed forlong (w5) h to improve the particle dispersion, as shown in Fig. 7. Itwas also observed that 5 h sonication provides good dispersionwith little aggregates. Kim et al. [85] synthesized spherical/fibrousalumina nanoparticles. The spherical nanoparticles were preparedby pulsed wire evaporation (PWE) while the fibrous nanoparticleswere prepared, using a hydrolysis reaction of spherical Al nano-particles produced by PWE method. The fabricated alumina nano-fibers were mixed with EG by sonication for 1 h. They found thatthe dispersion stability for fibrous alumina nanofluid decreasedwith time due to the aggregation of nanofibers. Hegde et al. [86]stirred Al2O3ewater nanofluids in a sonicator for 3 h. The particlesize range was found to be between 10 nm and 120 nm, and thenanofluid showed no agglomeration of the nanoparticles 2 h aftersonication. Esmaeilzadeh et al. [87] stabilized alumina waternanofluids through a 4 h process of ultrasonicationwith 170W and50 Hz and electromagnetic stirring. No sedimentation wasobserved throughout the testing period. Ali et al. [88] diluted20 wt.% of alumina nanofluid using distilled water. The new dilutedsolutions were ultrasonically vibrated. No precipitation wasobserved two days after the ultrasonic vibration. Jacob et al. [89]suspended alumina nanoparticles and adjusted the pH value ofthe suspensions. The nanofluid was sonicated for 5e6 h. Teng et al.[90] dispersed Al2O3/water nanofluid for several time by ultrasonicvibration and electromagnetic agitation. The nanofluid formed wasaddedwith cationic dispersant (0.3 wt.% of chitosan). They reportedthat all the completed experimental samples have to be staticallyplaced for 1 month until good suspension effect was achieved.

From the above discussion, it can be seen that regardless thebase fluid, a stability of several weeks was obtained using onlysonication [67] or sonication with adjusting the pH value [83],while a stability of a month was found using sonication and PVAsurfactant [72]. In addition, methanol nanofluids showed anexcellent stability (zeta potential over 60 mV) even though themixture was sonicated only for 2 h [45]. It should be mentionedthat some researchers didn’t give details about how the Al2O3nanofluids were prepared for their experiments.

Fig. 8. Stability comparison of nanofluids containing PCNTs or TCNTs [91].

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2.1.7. Carbon nanotube-nanofluidsCarbon nanotubes have strength and stiffness properties

several 1000 times better than that of steel and conductivity betterthan copper, carbon nanotubes have extraordinary intrinsic elec-trical, thermal and mechanical properties making them potentiallyattractive materials for use in different fields. CNTs have attractedmany researchers due to their higher thermal conductivity andvery high aspect ratio, for preparing nanofluids. Chen et al. [91]used commercial multi-walled CNTs, produced by chemical vapordeposition method to prepare their nanofluids. It was revealed thatthe as received pristine CNTs (PCNTs) were not only aggregated,but also entangled. Therefore, they introduced hydrophilic func-tional groups on the surface of the nanotubes by mechanochemicalreactor, and then dispersed the chemically treated CNTs (TCNTs)into a base fluid. Potassium hydroxide was used to modify thesurfaces of CNTs. It was shown that the aggregates and entangle-ments of PCNTs were broken after chemical treatment. The CNTswere cut short by the intensive mechanical forces during the ballmilling process. It was also shown that PCNTs were precipitated tothe bottom when dispersed in most of the fluids even after long-time intensive sonication if surfactant was not added and evenwith surfactant like oleylamine. Almost all the nanotubes weresedimented after 5 min for the suspension with 0.1 vol.% PCNTsdispersed in DW, leaving upper fluid transparent, while the TCNTs/fluid suspensions remained stable for many months with no visibleprecipitation at the bottom, as shown in Fig. 8. Su et al. [92]modified the surface of CNTs with nitric acid; One gram of CNTswas suspended in 40 ml of concentrated nitric acid (z68%) andrefluxed for an hour at 120 �C. After washing with deionized wateruntil the supernatant attained a pH value of around 7, the CNTswere dried at 55 �C. And then, the chemically treated CNTs wereadded directly into the base fluid. The suspension was agitated for2 h by ultrasonic applicator, with the frequency of 100 kHz. It wasfound that almost all of PCNTs with 0.2 wt.% deposited, havingupper fluid transparent. In sharp contrast to PCNTs, TCNTs werewell dispersed in aqueous ammonia. Meng et al. [93] kept 1 g ofCNTs into 60 ml HNO3 at 120 �C for 6 h. The mixture was cooled,filtered and washed to neutral pH with distilled water. Then thewashed product was freeze-dried. Finally, the dried particles wereadded to an appropriate amount of glycol under ultrasonic vibra-tion to get the CNTs glycol nanofluid. They found that an untreatedCNTs tend to aggregate resulting in poor stability. However, the as-prepared nanofluids could remain stable for more than twomonths without sedimentation. Liu et al. [94] added the CNTs in analkali/deionized water mixture. Then, the CNTs alkali/deionizedwater mixture was oscillated continuously for about 10 h in anultrasonic deionized water bath to break up the self-windingstructure of the CNTs and make the steady CNT suspension.

Later, the nitric acid was added into the CNT suspension and the PHvalue was adjusted to a constant of 6.5. Finally, the CNT suspensionwas again oscillated continuously for about 2 h and was then holdin a tank. Before the boiling experiment, the suspension wasoscillated again for about 2 h. Wang et al. [95] used DI water andthe Carboxyl MWNTs to produce nanofluids. A binary mixture ofTriton X-100 and sodium dodecyl benzene sulfonate was used toaccomplish better stability. The mass ratios of Triton X-100/SDBSand mixture/MWNT were about 20:1 and 3:20, respectively. Then,the MWNT and surfactant mixtures were dispersed into a presetamount of DI water, and the agglomerated MWNTs were separatedby a high-shear mixer. Meibodi et al. [96] synthesized carbonnanotubes by catalytic decomposition of 20% methane in hydrogenover CoeMo/MgO catalysts at 800e1000 �C. They reported that atypical process to make nanofluid stable for months involvessonicating a functional SWNT (FSWNTs) sample in ultrasonic bathfor over 2 h, or disruptor 30 min and dispersing the sonicatedSWNTs into a preset amount of distilled water and adjusting thesuspension to a preset pH level. Liu et al. [97] mixed MWCNTs,produced by a catalytic chemical vapor deposition method withethylene glycol or synthetic engine oil base fluids. The suspensionswere then homogenized by intensive ultrasonics. On the otherhand, N-hydroxysuccinimide (NHS) in the solid particle form wasemployed as the dispersant in carbon nanotubeesynthetic engineoil suspensions. NHS was added into CNTs directly. The mixturewas blended by a magnetic stirrer. Synthetic engine oil was thenfilled into the CNTseNHS mixture. The mixture was mixed up byan ultrasonic homogenizer. They reported that the CNTs were welldispersed and the sedimentation of the suspensions was notvisible. Babu and Prasanna Kumar [98] synthesized MWCNTs usingan arc discharge method. Then the nanoparticles were chemicallytreated; soaking the CNTs in an acid mixture containing HNO3 andH2SO4 in the volumetric ratio of 1:3 for 5 h. To study the stability oftreated CNTs and pristine CNTs, the samples were sonicated at20 kHz for 5 min. It was found that nanofluid containing TCNTsexhibited a comparatively better stability over nanofluid contain-ing PCNTs. Therefore, it was decided to sonicate the TCNT nano-fluids for 1 h to disentangle and disperse the TCNTs well.Kumaresan and Velraj [99] used MWCNTs produced by thechemical vapor deposition method (VCD) to prepare nanofluids. Todisentangle the MWCNTs, a ball milling was carried out using10 mm Tungsten Carbide balls for 45 min, followed by ultra-sonication for 60 min under dry condition. The surfactant(0.1 vol.% of SDBS) was dissolved in the base fluid mixture usingmagnetic stirrer, followed by the addition of MWCNT. The mixturewas continuously stirred for 30 min, followed by ultra-sonicationfor 90 min to ensure the proper dispersion of the MWCNT in wa-tereethylene glycol mixture. It was observed that the preparednanofluid was stable for more than 3 months without any visiblesedimentation. Lamas et al. [100] functionalized the MWCNTsproduced by the chemical vapor deposition method. The pristineMWCNTs were refluxed at 413 K in nitric and sulfuric acid at 1:3volume ratio for 30 min, followed by exhausting wash with DWuntil no signs of acidity and dried in an oven at 373 K, for at least72 h, to evaporate the humidity. The functionalized MWCNTs weredispersed in 50 ml of base fluid with a magnetic stirrer combinedwith ultrasonication for 60 min. It was observed that after 24 h, thesedimentation rate was slow and constant. Yousefi et al. [101]added MWCNTs and 1:350 of Triton X-100 to CNT ratio to doubledistilled water. Also, the pH value of MWCNTs suspensions wasadjusted. The mixture was sonicated for 30 min (time chosen afterseveral test). It was observed that the prepared nanofluid wasstable up to 10 days. Indhuja et al. [102] added pristine MWCNTs tothe base fluid during probe sonication, which was carried out for4 h. 0.25 wt.% of gum Arabic was added and stirred in a magnetic

Fig. 9. Sample nanofluid prepared by dispersing MWCNTs in deionized water: withoutchitosan; (b) with 0.2 wt.% chitosan [106].

Z. Haddad et al. / International Journal of Thermal Sciences 76 (2014) 168e189 177

stirrer, followed by probe sonication for 1 h. The zeta potential ofgum arabic stabilized MWCNTewater nanofluids was found tobe �29.1 mV. It was reported that this method could be preparedwith a maximum MWCNTs concentration of 0.5 wt.% only. Garget al. [103] dissolved gum Arabic in DI water using a magneticstirrer, followed by the addition of MWCNT to the solution. Theresulting composition was ultrasonicated for 5 min at 100%amplitude using a 130 W, 20 kHz ultrasonication probe, withvarying ultrasonication times including 20, 40, 60, and 80 min. Thesonication was followed by 5 min of magnetic stirring. The ultra-sonication and magnetic stirring process were alternated every5 min until the sample had been sonicated for the desired amountof time. It was found that the samples prepared by this techniquewere stable for over 1 month with no visible sedimentation orsettling. Ashtiani et al. [104] dispersed MWCNTs in heat transfer oil(HT-B oil) by an electrical mixer for 10 min and then an ultrasoniccleaner for 30 min. No settlement was observed with naked eyeswithin 5 days for 0.4 wt.% nanofluid. Fakoor Pakdaman et al. [105]used the same technique as Ashtiani et al. [104] to prepareMWCNTs/HT-B oil. However, the time of dispersion using an ul-trasonic processor and electrical mixer was 6 h and 1 h, respec-tively. It was observed with naked eyes that the sedimentationstarted after about 24 h of the time that the applied nanofluid wasmade uniform by the ultrasonic device. Therefore, each day beforestarting the experiments, the working fluid was made uniform bythe mentioned ultrasonic device. Phuoc et al. [106] first preparedthe base fluid by mixing an appropriate amount of chitosan intodeionized water having 0.5 vol.% acetic acid then was stirred for24 h using a magnetic stirrer. The nanofluids were prepared bydispersing an appropriate amount of MWCNTs into the preparedbase fluid. The mixture was then ultrasonicated for 10 min at 100%amplitude using a 130 W, 20 kHz ultrasonic processor, and it wasfollowed by 20 min stirring using a magnetic stirrer. The processwas repeated until the total mixing time was 1 h. It was observedthat the nanofluid prepared without chitosan was not stable at all,the solid precipitated quickly and settled down at the bottom ofthe vial about 30 min after preparation, As shown in Fig. 9. How-ever, the nanofluid stabilized by 0.2 wt.% chitosan was stable formonths. Ding et al. [107] dispersed the sonicated CNTs into a presetamount of distilled water containing 0.25 wt.%. Gum Arabicdispersant, adjusted the suspension to a preset pH level, andtreated the mixture with the high shear homogenizer for 30 min. Itwas found that the CNT nanofluids made in this way were verystable for months without visually observable sedimentation.

It can be noticed that CNTs nanofluids may be kept stable formonths if the nanoparticles are functionalized or dispersed withsurfactant.

2.1.8. Iron oxide-nanofluidsIron oxides exist inmany forms innature,withmagnetite (Fe3O4),

maghemite (g-Fe2O3), and hematite (a-Fe2O3) being probably themost common. Magnetic fluid is a colloidal suspension consisting ofmagnetic nanoparticles and carrier liquid. Due to its unique char-acteristics, the magnetic fluid behaves as a smart or functional fluidand has been finding more and more applications in a variety offields such as electronic packing,mechanical engineering, aerospace,and bioengineering. Li et al. [108] prepared two types of nanofluids.One was Fe3O4ewater magnetic fluid prepared by the chemicalprecipitation method in which the oleic acid was added as adispersant. The samples with different particle volume fractionswere obtained by diluting the original sample of the magnetic fluid.Also, Feewater magnetic fluid was prepared by the direct mixingmethod. Fe nanoparticle were mixed with deionized water andstabilized with sodium dodecylbenzenesulfonate. In addition, themagneticfluid samplewas vibrated for several hours in an ultrasonicvibrator. They found that if the suitable percentage of sodiumdodecylbenzenesulfonate is added, the stabilization of the suspen-sion can last about from several hours to one week in the stationarystate. Sundar et al. [109] adjusted the pH value of Fe3O4-waternanofluid at 3, using a small amount of sulfuric acid (H2SO4). Then,themixturewas sonicated for approximately 2 h. They reported thata uniform dispersion was established by measuring the densities ofnanofluid at different locations in the container. Sundar et al. [110]synthesized the magnetic nanoparticles by chemical coprecipita-tion of FeCl3$6H2O, FeCl2$4H2O, sodium hydroxide (NaOH). Theyconsidered three different concentrations of base fluid like 60:40%,40:60% and 20:80% of ethylene glycolewater mixture. The solutionwas kept in ultrasonic bath up to 2 h. They found that all themeasured Zeta potential values were higher than 30 mV; the zetapotential of 1.0% volume concentration of 60:40% EG/W was 54 mV,40:60% EG/W was 45 mV and 20:80% EG/W was 49 mV at the so-lution pH ¼ 5. In addition, no particle sedimentation was observedup to 80 days. Asadzadeh et al. [111] added the nanoparticles to thebasefluid in the presence of vigorousmechanical agitation. Then, thesuspension was sonicated for an hour in the ultrasonic bath. Nosedimentationwas observed in the 12 h subsequent to the nanofluidpreparation. Abareshi et al. [112] synthesized Fe3O4 nanoparticles bya co-precipitation method at different pH values. The nanoparticleswere dispersed in deionized water, and tetramethyl ammoniumhydroxide was used as a dispersant. It was shown that changing thefinal pHof the product (from10.5 to 9.5) and increasing the initial pHof the iron salts solution (from �1 to 1.5) improves remarkably thecrystallinity of Fe3O4 nanoparticles. Also, the measured zeta poten-tial for f ¼ 2% was �41.7 mV at pH ¼ 12.8, indicating that Fe3O4nanofluids have good dispersion and stability. Abareshi et al. [113]synthesized magnetic nanoparticles of hematite, a-Fe2O3 by sol-vothermal method using Fe(NO3)3 as a starting material. The nano-particles were dispersed in glycerol using an ultrasonic processor at20 kHz and 700 W for 30 min. Yu et al. [114] synthesized Fe3O4nanoparticles by coprecipitation. Oleic acid was added tomodify thenanoparticles. After 1 h, kerosene was added to the mixture withslow stirring. The phase-transfer process occurred spontaneously,and there was a distinct phase interface between the aqueous andkerosene. After removing the aqueous phase using a pipette, thekerosene-based Fe3O4 nanofluid with volume concentration 1% wasobtained. It was shown that there is no clear relation between theparticle size and ultrasonication time, as shown in Fig. 10, indicatingthat the phase-transfer method avoids the long time ultrasonicationand oxidation of Fe3O4 in the disperse process. Also, the Fe3O4

Fig. 10. Average particle size as a function of ultrasonification processing time for1.0 vol.% Fe3O4 nanofluid [114].

Fig. 11. Typical samples of Ag-deionized nanofluids generated by multi beam ablationin liquid technique [119].

Z. Haddad et al. / International Journal of Thermal Sciences 76 (2014) 168e189178

nanoparticles modified by oleic acid have good compatibility withkerosene, and the nanofluids were stable. Phuoc andMassoudi [115]mixed the Fe2O3 nanoparticleswith deionizedwater containing 0.2%by weight of Polyvinyl pyrrolidone (PVP) or Poly (ethylene oxide)(PEO) as a dispersant. The mixing was carried out using a magneticmixer and sonicated for 30min using a 130Wultrasonic processor. Itwas found that using these polymers, the prepared nanofluids werestable for about twoweeks when the particle concentrationwas lessthan 2% and less than a week when the concentration was higher.Guo et al. [116] added sodiumoleate as a dispersant to themixture ofethylene glycol and deionized water with volume ratio of 45:55, andthen the nanoparticles were gradually added into the base mixturefluid with violent stirring. Afterward, the suspensions were stirredusing disperse mill (7200 r/min) for 40 min. Nanofluids withdifferent volume fractions were obtained by intensive ultra-sonication for 45 min. It was shown that the average size is about1200 nm without surfactant and about 150 nm with surfactant.Sheikhbahai et al. [117] dispersed Fe3O4 nanoparticles in ethyleneglycol under ultrasonicmixing for anhour. Sonicationwas continuedfor anotherhour to get stable nanofluid. Required amountofDIwaterwas gradually added to nanofluid under vigorous agitation in 30minperiod just before each experiment. No sedimentationwas observedat the bottom or clear layer at the top after 8 h.

It can be seen that the nanoparticles synthesized by chemicalprecipitation method can be well dispersed by using only sonicat-ion for 2 h [110].

2.2. Preparation of metallic nanofluids

2.2.1. Gold & silver-nanofluidsMetal nanoparticles such as gold (Au) and silver (Ag) have

recognized importance in chemistry, physics, and biology becauseof their unique optical, electrical, and photothermal properties.Such nanoparticles have potential applications in analyticalchemistry and have been used as probes in mass spectroscopy, aswell as in the colorimetric detection for proteins and DNA mole-cules. Furthermore, Au nanoparticles have photothermal propertiesthat can be exploited for localized heating resulting in drug release.Thus, increasing their potential for therapeutic applications. Theease of synthesizing Au and Ag nanoparticles and their affinity forbinding many biological molecules, makes them attractive candi-dates for study.

Patel et al. [118] used gold and silver for the first time to preparenanofluids. The gold and silver nanoparticles were prepared by thecitrate reduction route. Also, the gold nanoparticles with thiolatecovering were prepared using two-phase (wateretoluene) reduc-tion of AuC14- by sodium borohydride in the presence of an alka-nethiol. The samples were stable over a period of several monthsand no degradationwas observed during storage or in the course ofthe experiment. Phuoc et al. [119] prepared different samples ofAgedeionized water using multi-pulse laser ablation in liquidapproach. The first sample (sample I, brownish yellow) was pre-pared by ablating the silver target in deionized water for 30 min.The second sample was prepared by ablating the silver target in thepreviously produced sample that was aged for 2 weeks. The abla-tion durations for the last three samples were 3 h. All the sampleswere more than 7 months old and they were stable without usingany dispersants or surfactants, as shown in Fig. 11. Para-metthanuwat et al. [120] prepared silver nanofluids using an ul-trasonic bath at 43 kHz for 3 h. It was found that the stability was upto 48 h. Hajian et al. [121] produced Silver in DIewater nanofluid bya chemical method which consists of reduction of Ag ions. Thenanofluids were put in an ultrasonic bath for about 15 min beforeinjection into the heat pipe. Tamjid and Guenther [122] used acolloid of silver nanoparticles produced by the Sputtering onRunning Liquid technique (VERL) at volumetric solids concentra-tion of 4.37%. The colloid was stirred and agitated thoroughly for5 min by an ultrasonic agitator in continuous mode to ensureuniform dispersion of the nanoparticles in diethylene glycol.Asirvatham et al. [123] mixed silver nanoparticles with deionizedwater under ultrasonic vibration with power density of 750 W atfrequency of 20 kHz for 12 h. The measured pH values were 7.4, 7.1and 6.8 for volume concentrations of 0.3%, 0.6% and 0.9%, respec-tively. They reported that the nanofluids were essentially uniform,but not without some agglomeration of the particles. Sharma et al.[124] synthesized silver nanofluids using silver nitrate (precursor),ethylene glycol (reducing agent), and poly (acrylamide-co-acryl-icacid) (dispersion stabilizer). They found that the size of nano-particles and dispersion stability are controlled by theconcentration of PAAeco-AA and the reaction conditions. Hari et al.[125] produced spherical and rod silver nanoparticles. The silvernanorods were prepared by seed mediated chemical synthesis andstabilized with CTAB surfactant micelle, while the spherical nano-particles were stabilized with trisodium citrate. It was found thatthe nanofluids were stable for a maximum period of one week. Paulet al. [126,127] synthesized nano-gold and silver dispersed waterbased nanofluids by wet chemical bottom up approach. It wasrevealed that uniform distribution, chemical nature (metallic) orpurity of the gold nanoparticles and color of the nanofluid

Fig. 12. Effect of ultrasonic time on the effective thermal conductivity of nanofluid(299.08 K) [132].

Z. Haddad et al. / International Journal of Thermal Sciences 76 (2014) 168e189 179

remained unchanged without sedimentation or agglomerationeven after 48 h. Kim et al. [128] dispersed Au-powders in water by6-h ultrasonic-wave irradiation. Also, they produced gold nano-particles suspended in water by pulsed laser ablation in liquids.They reported that in spite of 6 h ultrasonic-wave irradiation,almost all of the Au-powder was precipitated inwater. The AueNPswere still suspended in water after 1 month without any surfac-tants. Lo et al. [129] prepared silver nanofluid by the submerged arcnanoparticles synthesis system (SANSS). They observed that thenanoparticles were well dispersed in deionized after using ultra-sonic vibration for 15 min. Kang et al. [130] added silver nano-particles produced by a catalytic chemical vapor depositionmethod, using an ultrasonic homogenizer.

It should be noted that gold and silver-citrate or Auethiolatenanofluids can be kept stable for a period of several months [118].Moreover, Agedeionized water nanofluids prepared using multi-pulse laser ablation in liquid approach were stable for severalmonths without using dispersant [119].

2.2.2. Copper-nanofluidsIntrinsically high resistance to corrosion makes copper as an

ideal metal for heat exchangers of all kinds, including solar waterheating systems. Nano copper can be used in the manufacture ofprinting inks allowing conductive patterns that have to passthrough a thin film, creating a path of electrical conductivity for usein scientific experiments, and also to create printable circuits forelectronic applications. Also inwater, nano copper has been used atleast since 1931, as a fungicide in the cultivation of vines and fruittrees, and as an algaecide in swimming pool water treatment.

Xuan and Li [131] prepared transformer oileCu suspensions andwatereCu suspensions. The first suspension was stabilized using22 wt.% of oleic acid as dispersant (Several percentages of oleic acidwas tested). Then, the suspension was vibrated for 10 h in an ul-trasonic vibrator. It was found that the stabilization of the sus-pension can last about one week in the stationary state withoutsedimentation. The second suspension was stabilized using 9 wt.%laurate salt (Several percentages of laurate salt was tested), andthen vibrated in an ultrasonic vibrator. It was observed that thesuspension can be stable more than 30 h with some clusters. Yanget al. [132] used an aqueous solution of cetyltrimethyl ammoniumchloride (CTAC)/sodium salicylate (NaSl) to prepare copper nano-fluids. NaSl was added to the solution with the same weight con-centration of that of CTAC, and distilled water was used as solvent.The CTAC/NaSl solution was stirred about 8 h. Then the

nanoparticles were added into the base fluid by introducing anintensive sonication of ultrasonic disruptor. Furthermore, an opti-mum time of sonicationwas determined by measuring the thermalconductivity of 0.6 vol.% Cu nanoparticles suspended in 50 ppmCTAC/NaSl aqueous solution for different ultrasonic time, as shownin Fig. 12. It was observed that when the ultrasonic time is over 3 h,the thermal conductivity of Cu nanofluids tends to be a constantvalue, meaning that 3-h ultrasonic time is enough to disperse theCu nanofluid. Also, it was reported that the prepared nanofluids byintroducing 3-h intensive sonication show a stable property. Penget al. [133] dispersed copper nanoparticles, produced by hydrogendirect current arc plasma evaporation method in a refrigerant R113.They used three types of surfactants which are miscible with R113;Sodium Dodecyl Sulfate (SDS), Cetyltrimethyl Ammonium Bromide(CTAB) and Sorbitan Monooleate (Span-80). The mixture of Cu-R113 with surfactants was vibrated for 1 h. It was found that themixture can be kept stable for 24 h, and the duration of theexperiment for each sample of Cu-R113 nanofluid with surfactantwas less than 4 h which is shorter than 24 h. Kole and Dey [134]prepared surfactant free copperewater nanofluids by addition ofan appropriate amount of Cu nanoparticles in distilled water. Thedispersion was done by intense ultrasonication for 10 h at 200 Wand subsequent homogenization for another 10 h using a magneticstirrer. The suspension stability of Cu nanoparticles in the preparednanofluids was tested for more than15 days without any visibletrace of sedimentation, but with the presence of clusters with anaverage diameter between around 122 and 164 nm. Li et al.[135,136] mixed copper nanoparticles and water with (SDBS orCTAB) surfactant. The pH was controlled using HCl and NaOH. Itwas shown that the average particle sizes obtained in the absenceof SDBS surfactant was 6770 nm and in the presence of SDBS sur-factant was 207 nm, indicating better stability of CueH2O sus-pensionwhen using SDBS surfactant. In addition, pH ¼ 8.5e9.5 canbe selected as an operating pH for the suspensions with SDBSsurfactant. It was observed that the stabilization of the suspensionwith CTAB dispersant can last about 1 week in the stationary statewithout sedimentation, while the suspension without dispersantexhibits weaker dispersion and quickly occurs aggregation.Kathiravan et al. [137] prepared copper nanoparticles by the sput-tering method. Then the nanoparticles were dispersed inwater andwater with 9.0% SDS anionic surfactant using an ultrasonic bath forabout 10 h. It was found that the nanoparticles were dispersed inwater evenly even after 10 h of ultrasonic vibration with someagglomerates. Robertis et al. [138] prepared copper nanofluids us-ing a one-step technique. They used ethylene glycol as base fluid,copper (II) nitrate hydrate as copper source; sodium hypophosphitemonohydrate as reducing agent and polyvinyl pyrrolidone (PVP) asstabilizer. The nanofluidswere synthesized using amicrowave ovenwithout further dispersion. The particles settlement was about28.5% in 50 days. They claimed that smallest particles tend toremain suspended for longer periods. However, larger particles candrag down these small particles and therefore reduce the stabilityperiod of the suspension. Riehl and Santos [139] stated that anagitation of copper nanofluids using an ultrasonic bath for 1.5 h wasenough to achieve a homogeneous solution, as no sedimentationwas observed after 4 h of observation. Senthilkumar et al. [140]used an ultrasonic homogenizer for 6 h. Li et al. [141] vibratedthe suspension for 4 h in an ultrasonic vibrator, and they found thatthe sample suspensions were kept stable in the stationary statewith little apparent sediment during the whole experiment pro-cess. Lu et al. [142] dispersed copper nanoparticles using an ultra-sonic box with a working frequency of 25e40 kHz for 10 h.

All the mentioned copper preparation methods showed thatcopper nanofluids cannot be stable more than one month. Thepreparation of nanofluids containing different concentrations,

Table 2Summary of nanofluids preparation.

Authors Particlematerial

Base fluid Parameters Characterization Surfactant/pH control Nanofluid stability

Hu et al. [5] AlNApproximatelyspherical

Ethanol f ¼ 0.5e4 vol.%D ¼ 20 nm

TEM Castor oil Stable for more than 2weeks.

Choi et al. [6] AlNSphericalAl2O3

Rod-shapeSpherical

Transformer oil f � 4 vol.%D ¼ 50 nmD ¼ 2nm � 20e200 nmD ¼ 13 nm

e Oleic acid Sedimentation wasvery clear for non-filtered Al2O3 nanofluidafter one month

Yu et al. [7] AlN EGPG

f ¼ 1.0e10 vol.%D ¼ 50 nm

SEMMalvern nanosizer

e e

Wozniak et al. [8] AlN PPG 425PPG 2000

f ¼ 10e25 vol.%D ¼ 0.800e1.80 mm

SEMZetasizer nano-ZS

e AlNePPG 2000suspensiondemonstrated slightlysedimentation

Yu et al. [9] ZnO EG f ¼ 0.2e5 vol.%D ¼ 10e20 nm

SEMMalvern nano-sizer

e e

Moosavi et al. [10] ZnOApproximatelyspherical

EGG

f � 3 vol.%e

XRDTEMSEMMalvern nano-sizer

Ammonium citrate Stable for severalmonths

Raykar and Singh [11] ZnONon-spherical

Deionizedwater

f ¼ 0.075%, 0.25% and0.5 wt.%D w 80 nm

SEMDLS

acac Stable over 9 months to1 year

Kole and Dey [12] ZnONearlyspherical

EG f ¼ 0.5e3.75 vol.%D < 50 nm

TEMMalvern nano-sizerDLS

e Stable for 30 dayswithout any trace ofvisible sedimentation

Chung et al. [13] ZnOOff-white &white

Deionizedwater

f ¼ 0.4e2 vol.%D ¼ 40e100 nm &D ¼ 20 nm

TEMPCS

Ammoniumpolymethacrylate

e

Suganthi and Rajan[14]

ZnOSpherical

Water f ¼ 0.25e2 vol.%D ¼ 30e45 nm

SEMZetasizerXRD

SHMP Stability was confirmedthrough visualobservation

Zafarani-Moattar andMajdan-Cegincara[15]

ZnO PEG f � 8 wt.%D ¼ 20 nm

DLSTEMUVeVis spectroscopy

e Stable at least for140 min

Saleh et al. [16] ZnO PEG F ¼ 0.025e0.5 vol.%D ¼ 18 or 23 nm

XRDUVeVis spectroscopy

e e

Lee et al. [17] ZnOSpherical andrectangular

EG F ¼ 0.5e5.5 vol.%D < 100 nm

XRDTEM

e e

Kayhani et al. [18] TiO2

SphericalDIewater f ¼ 0.1e2 vol.%

D ¼ 15 nmSEM Hexamethyldisilazane Stable for several days

without any visiblesedimentation.

He et al. [19] TiO2

SphericalDIewater f ¼ 1.0%, 2.5% and

4.9 wt.%D ¼ 20 nm

SEMMalvern nano-sizer

PH ¼ 11 Stable for months

Murshed et al. [20] TiO2

Sphericalrod-shape

Deionizedwater

f � 5 vol.%D ¼ 15 nmD ¼ 10 nm � 40 nm

TEMParticle size analyzer

Oleic acid & CTAB No sedimentation wasobserved after 24 h.

Duangthongsuk andWongwises [21]

TiO2

SphericalWater f ¼ 0.2e2 vol.%

D ¼ 21 nmTEM CTAB A little agglomeration

was observed 3 h aftersonication

Kim et al. [22] TiO2 (nearlyspherical)Al2O3

(spherical)ZnO(elongated)

WaterEG

D ¼ 10, 34, 70 nmD ¼ 38 nmD ¼ 10, 30, 60 nm

TEM SDS No trial to stabilize thefluid was successful forthe 10 nm ZnO/EGnanofluid

Abbasian Arani andAmani [23]

TiO2

ApproximatelySpherical

DIewater f ¼ 0.2%e2.0 vol.%D ¼ 30 nm

TEM CTAB Suspensions werestable for several hours(days)

Utomo et al. [24] TiO2

Al2O3

Spherical

DIewater f ¼ 0.1%e9 vol.%D ¼ 20e30 nmf ¼ 0.1%e15 vol.%D ¼ 50e60 nm

TEM Octyl silaneammonium polyacrylate

e

Longo and Zilio [25] TiO2

Al2O3

Spherical

DIewater f ¼ 1%, 2%, 4% and6 vol.%D ¼ 30e50 � 10 nmf ¼ 1%, 2% and 4 vol.%D ¼ 30 � 10 nm

Malvern nano-sizer e Stability for more thanone month

Chen et al. [26] TiO2

Rod-likeEGeTNT f ¼ 0.1%e1.8 vol.%

D ¼ w10 nmL ¼ w100 nm

Malvern nano-sizerSEM

e Stability over theperiod of two months

Z. Haddad et al. / International Journal of Thermal Sciences 76 (2014) 168e189180

Table 2 (continued )

Authors Particlematerial

Base fluid Parameters Characterization Surfactant/pH control Nanofluid stability

Mo et al. [27] Rutile TiO2

(rod-shape)Anatase TiO2

(spherical)

Deionizedwater

F ¼ 0.05, 0.3, 0.7 wt.%D ¼ 15 nmD ¼ 20 nm � 50 nm

TEMSpectrophotometer

SDS Nanofluids kept stablefor 286 h

Bobbo et al. [28] TiO2

SWCNHSpherical

Deionizedwater

F ¼ 0.15, 013, 1 vol.%D ¼ 20e30 nmD ¼ 60 nm

SEMDLSMalvern nano-sizer

SDSPEG

SWCNH-nanofluidswere very stable evenafter several days.However, TiO2-nanofluids were lessstable

Hojjat et al. [29] TiO2

CuOg-Al2O3

0.5 wt.% CMCsolution indeionizedwater

F ¼ 0.1e4.0 vol.%D ¼ 10 nmD ¼ 30e50 nmD ¼ 25 nm

e e No sedimentation afterseveral daysPreparation of stableAl2O3 nanofluids wasmore difficult forf > 1.5%

Fedele et al. [30] TiO2 Bidistilledwater

f ¼ 1%, 10%, 20% and35 wt.%D ¼ 30 nm

DLSMalvern nano-sizerDLS

Acetic acid Stable suspension for35 days usingsonication

Tajik et al. [31] TiO2

a-Al2O3

DIewater f ¼ 0.005, 0.15 and0.2 vol.%D ¼ 27e43 nmD ¼ 30e40 nm

SEM e e

Chakraborty et al. [32] TiO2

CylindricalAl2O3

Spherical

DIewater F ¼ 0.1e2 vol.%D ¼ 25e70 nmD ¼ 20 nm � 40 nmD ¼ 20e50 nm

SEM e Stable for the durationof experiment

Mitra et al. [33] TiO2

Random shapeMWCNT

Water F ¼ 0.1 wt.%D ¼ 20e70 nmF ¼ 0.01 wt.%D ¼ 20e70 nm � 100e500 nm

e e e

Sajadi and Kazemi [34] TiO2 Deionizedwater

f ¼ 0.05e0.25 vol.%D ¼ 30 nm

e e e

Suriyawong andWongwises [35]

TiO2

Circular shapeWater F ¼ 0.00005e0.01 vol.%

D ¼ 21 nmTEM e e

Timofeeva et al. [40] SiO2 TH66 f1.2%e7 vol.%D ¼ 15 nm

SEMDLS

BZCCTABBAC

Nanofluids appearedstable without anyvisual phase separationfor at least a week.

Yang and Liu [41] SiO2 Water F ¼ 0.5e2.5 wt.%D ¼ 30 nm

SEM Trimethyoxysilane Functionalizednanofluid can still keepdispersing well for 12months even at 10 wt.%.Traditional nanofluid:sedimentation occurredafter several days

Anoop et al. [42] SiO2 Deionizedwater

f ¼ 0.2%, 0.5% and1 wt.%D w 20 nm

e pH ¼ 4.5 Nanofluids exhibitedgood stability over time

Qu and Wu [43] SiO2

Al2O3

Spherical

Pure water f ¼ 0.1e0.6 wt.%D ¼ 30 nmf ¼ 0.1e1.2 wt.%D ¼ 56 nm

TEM pH ¼ 9.7pH ¼ 4.9

Alumina nanoparticleswere better dispersed

Fazeli et al. [44] SiO2

SphericalDIewater f ¼ 3.5e5 vol.%

D ¼ 18 nme e Stable for a period of

72 hwithout any visiblesettlement.

Pang et al. [45] SiO2

Al2O3

Puremethanol

f ¼ 0.005e0.5 vol.%D ¼ 10e20 nmD ¼ 40e50 nm

SEMDLS

e The dispersion stabilityof the methanol-basedAl2O3 nanofluids wasstated to be good

Bolukbasi and Ciloglu[46]

SiO2 Deionizedwater

f ¼ 0.05e0.1 vol.%D ¼ 34 nm

TEMXRD

e Stable during theperiod of experiment

Darzi et al. [47] SiO2 DIewater f ¼ 0.5 and 0.1%D ¼ 30 nm

e e e

Kulkarni et al. [48] SiO2

Al2O3

CuO

60:40 EG/W f � 6 vol.%D ¼ 50 nmD ¼ 45 nmD ¼ 30 nm

DLS e e

Hwang et al. [49] SiO2

MWCNTCuO

DIewater f � 6 vol.%D ¼ 7 nmD ¼ 10e50mm � 10e30 nmD ¼ 35.4 nm

TEM or SEM e e

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Z. Haddad et al. / International Journal of Thermal Sciences 76 (2014) 168e189 181

Table 2 (continued )

Authors Particlematerial

Base fluid Parameters Characterization Surfactant/pH control Nanofluid stability

Rohini Priya et al. [50] CuONonspherical

Water F � 0.016 vol.%Length to thicknessratio w10

SEMXRD

Tiron Stability was confirmedthrough visualobservation

Suresh et al. [51] CuO Water F ¼ 0.1%, 0.2% and0.3 vol.%D ¼ 15.7 nm

XRD e e

Lee et al. [52] CuOSpherical

DIewater F ¼ 0.001 vol.%D ¼ 15 nmD ¼ 55 nm

TEMXRD

e Dispersion stability ofCuO/DIW nanofluidfabricated by one-stepmethod is much betterthan that prepared bytwo-step method

Yang and Liu [53] CuO Deionizedwater

F ¼ 0.1%e1.5 wt.%D ¼ 50 nm

TEMDLS

e e

Chang et al. [54] CuO Water F ¼ 0.01%e0.4 vol.%D ¼ 20e300 nm

TEMDLSXRD

NaHMP When the CuO contentwas higher than0.40 vol.%, thesuspension was veryunstable and the CuOnanoparticles tended tosettle within severalminutes

Harikrishnan andKalaiselvam [55]

CuO Oleic acid F ¼ 0.5%e2.0 wt.%D ¼ 1e80 nm

TEMXRDMalvern zeta sizer

e Based on the zetaresult, CuOnanoparticles werestably dispersed anduniformly distributedin base fluid

Saeedinia et al. [56] CuO Oil F ¼ 0.2e2 wt.%D ¼ 50 nm

SEMXRD

e The completesedimentation occurredafter a week

Selvakumar and Suresh[57]

CuO DIewater F ¼ 0.1% and 0.2 vol.%D ¼ 27e37 nm

e e Nanofluids were stableafter a week exceptvery littlesedimentation.

Kannadasan et al. [58] CuO DIewater F ¼ 0.1% and 0.2 vol.%D ¼ 10e15 nm

XRD e Very little settlement ofnanoparticles evenafter 25 days

Byrne et al. [59] CuO DIewater F ¼ 0.005%, 0.01% and0.1 vol.%D ¼ 30e50 nm

SEMDLS

CTAB Suspension was muchmore stable when usingsurfactant

Liu et al. [60] CuO Deionizedwater

F ¼ 0.5e2 vol.%D ¼ 50 nm

e pH ¼ 7 e

Fotukian and Esfahany[61]

CuO DIewater F � 3 vol.%D ¼ 30e50 nm

e e No precipitation wasobserved after 5 h

Liu et al. [62] CuO Water F ¼ 0.5% and 1.0 wt.%D ¼ 30e50 nm

TEM e The stability anduniformity ofnanoparticlesuspensions were poorafter several days

Zeinali Heris [63] CuO 40% water60% EG

F ¼ 0.1%, 0.2% and0.5 wt.%D ¼ 40 nm

e e No precipitation ofnanoparticles wasobserved after 22 h.

Sonawane et al. [66] Al2O3 ATF F ¼ 0.1e1 vol.%D ¼ 50 nm

TEM Oleic acidTween-20 LR

The solution was foundto be stable even after24 h

Suresh et al. [67] Al2O3

Nearlyspherical

DIewater F ¼ 0.3, 0.4 and0.5 vol.%D ¼ 40.3 nm

XRDTEM

e The nanofluid was verystable for several weekswithout visuallyobservablesedimentation

Beck et al. [68] Al2O3 EG F ¼ 1, 3 and 4 wt.%D ¼ 20 nm

e e Dispersions wereremained uniform forthe duration of theexperiments

Sharma et al. [69] Al2O3 DIewater F < 3 vol.%D ¼ 47 nm

e SDBS Nanofluid wasobserved to be stablefor over a week

Gharagozloo andGoodson [71]

Al2O3 Deionizedwater

F ¼ 1, 3 and 5 vol.%D ¼ 40.2 nm(0.05% vol.)

DLS Nitric acid Nanofluid was stablewith only minorsettling after a week

Jung et al. [72] Al2O3 DIewater F ¼ 10�5e10�1 vol.%D ¼ 45 nm

DLS PVA The aggregatedparticles were stablysuspended for morethan 1 month

Z. Haddad et al. / International Journal of Thermal Sciences 76 (2014) 168e189182

Table 2 (continued )

Authors Particlematerial

Base fluid Parameters Characterization Surfactant/pH control Nanofluid stability

Soltani et al. [73] g-Al2O3

Nearlyspherical

CMC F ¼ 0.8e1.4 wt.%D ¼ 20e30 nm

e e e

Ho et al. [74] Al2O3 Ultra-pureMilli-Q water

F ¼ 0.1e4 vol.%D ¼ 33 nm

e pH Suspensions werestable for at least twoweeks

Yousefi et al. [75] Al2O3

SphericalDouble DIewater

F ¼ 0.2 and 0.4 wt.%D ¼ 15 nm

e Triton X-100 The nanofluid stabilitywas about three daysafter sonication

Raveshi et al. [76] Al2O3

SphericalWEG50 F ¼ 0.05e1 vol.%

D ¼ 20e30 nmTEM SDBS No obvious

sedimentation beforestarting the experiment

Hung et al. [77] Al2O3 DIewater F ¼ 0.5, 1 and 3 wt.%D ¼ 20 nm

UV/VIS spectrometerXRDTEM

Chitosan The difference betweeninitial and finalconcentrations (after 2weeks) of Al2O3/waternanofluid was less than5%

Heyhat et al. [78] g-Al2O3

SphericalDIewater F ¼ 0.1e2 vol.%

D ¼ 40 nmSEM e Nanofluids are

physically stableSingh et al. [79] Al2O3 Deionized

waterEG

F¼ 0.25, 0.5 and 1 vol.%D ¼ 45 and 150 nm

TEM pH e

Pandey and Nema [80] Al2O3 DIewater F ¼ 2, 3 and 4 vol.%D ¼ 40e50 nm

e e No sedimentation wasobserved in theproduced nanofluidsafter 24 h

Chandrasekar et al. [81] Al2O3 DIewater F ¼ 0.33e5 vol.%D ¼ 43 nm

SEM e e

Qu et al. [82] Al2O3

SphericalPure water F ¼ 0.1e1.2 wt.%

D ¼ 56 nmSEM pH Nanoparticles could be

stably suspended inwater at least for 3 days

Anoop et al. [83] Al2O3 Water F ¼ 1e6 wt.%D ¼ 45 and 150 nm

TEM pH The suspensions werestable for several weeks

Lee et al. [84] Al2O3 DIewater F ¼ 0.01e0.3 vol.%D ¼ 30 � 5 nm

TEM e Good dispersion withlittle aggregates

Kim et al. [85] Al2O3

SphericalFibrous

Deionizedwater

F ¼ 5.5 vol.%D ¼ 80 nm (spherical)

TEMXRD

e Aggregation of aluminananofibers has a badinfluence on the long-term dispersionstability

Hegde et al. [86] Al2O3

SphericalDIewater F ¼ 0.1e0.5 g/l

D ¼ 80 nmTEM e No agglomeration

formed 2 h aftersonication

Esmaeilzadeh et al. [87] g- Al2O3 DIewater F ¼ 0.5 and 1.0 vol.%D ¼ 15 nm

TEMXRD

e No sedimentationthroughout the testingperiod

Ali et al. [88] Al2O3 DIewater F ¼ 0.21, 0.51 and0.75 wt.%D ¼ 10 nm

SEM e No precipitation wasobserved two days afterthe ultrasonic vibration

Jacob et al. [89] Al2O3 Deionizedwater

F¼ 0.25, 0.5 and 1 vol.%D ¼ 50 nm

e pH e

Teng et al. [90] Al2O3 Water F ¼ 0.5, 1 and 3 wt.%D ¼ 20e30 nm

TEM Chitosan e

Chen et al. [91] CNTs DIewaterEG

f ¼ 0.2%e1 vol.%D� H¼ 15 nm � 30 mm

TEM e TCNTs suspensionsremained stable formany months with novisible precipitation

Su et al. [92] CNTs Ammonia f ¼ 12.05%e25 vol.%e

TEM e

Meng et al. [93] CNTs Glycol f ¼ 0.5%e4 wt.% TEMSpectrophotometer

e Nanofluids couldremain stable for morethan two monthswithout sedimentation

Liu et al. [94] CNTs Deionizedwater

f ¼ 0.5%e4 wt.%D � H ¼ 15 nm � 5e15 mm

TEM e e

Wang et al. [95] CNTs DIewater f ¼ 0.052%e1.27 vol.%D � H ¼ 20e30 nm � 5e30 mm

TEM e e

Meibodi et al. [96] CNTs DIewater f ¼ 1e5 vol.%ID/OD ¼ 0.8e1.1 nm/1e4 nm

TEM e e

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Z. Haddad et al. / International Journal of Thermal Sciences 76 (2014) 168e189 183

Table 2 (continued )

Authors Particlematerial

Base fluid Parameters Characterization Surfactant/pH control Nanofluid stability

Liu et al. [97] CNTs EGEngine oil

f ¼ 0.2e1.0 vol.%f ¼ 1e2.0 vol.%

TEMSEMXRD

NHS The sedimentation ofthe suspensions wasnot visible

Babu and PrasannaKumar [98]

CNTs DIewater f ¼ 0.25e1.0 wt.%D ¼ 9e15 nm

TEMSEMXRD

e e

Kumaresan and Velraj[99]

CNTs Deionizedwater-EG

f ¼ 0.25e1.0 vol.%D�H¼ 30e50 nm� 10e20 mm

SEM SDBS Nanofluid was stablefor more than 3 months

Lamas et al. [100] CNTs DIewatereEG f ¼ 0.25e1.5 vol.%A ¼ 333 and 667

e e e

Yousefi et al. [101] CNTs Double DIewater

f ¼ 0.2 wt.%D ¼ 10e30 nm

TEM Triton X-100 Nanofluid was stable upto 10 days

Indhuja et al. [102] CNTs Water f ¼ 0.14e0.24 vol.%D � H ¼ 10 nm � 5e15 mm

SEMZetasizer

Gum arabic e

Garg et al. [103] CNTs DIewater f ¼ 1 wt.%D � H ¼ 10e20 nm � 0.5e40 mm

TEM Gum arabic Nanofluid stable forover 1 month with novisible sedimentationor settling

Ashtiani et al. [104] CNTs HT-B oil f ¼ 0.1e0.4 vol.%D ¼ 10e30 nm

SEM e e

Phuoc et al. [106] CNTs Deionizedwater

f ¼ 0.1e0.4 vol.%D ¼ 5e10 nm � 10e30 mm

e Chitosan Nanofluid was stablefor months

Ding et al. [107] CNTs DIewater f ¼ 0e1.0 vol.%D ¼ 5e10 nm � 10e30 mm

SEMTEM

pH Nanofluids made in thisway were very stablefor months

Li et al. [108] Fe3O4

Fef ¼ 1.0e5.0 vol.%D ¼ 20 nmD ¼ 26 nm

e Sodiumdodecylbenzenesulfonate

The suspension can lastabout from severalhours to one week inthe stationary sate

Sundar et al. [109] Fe3O4 DIewater f ¼ 0e0.6 vol.%D ¼ 36 nm

e pH ¼ 3 e

Sundar et al. [110] Fe3O4

CubicalEGewater f ¼ 0e1.0 vol.%

D ¼ 5e70 nmXRDSEMTEM

e No particlesedimentation wasobserved up to 80 days

Asadzadeh et al. [111] Fe3O4 EG f ¼ 0.015e0.1 vol.%D ¼ 50 nm

e e No sedimentation wasobserved in the 12 hsubsequent to thenanofluid preparation

Abareshi et al. [112] Fe3O4 DIewater f ¼ 0.025e3.0 vol.%e

XRDTEMFTIRVSM

Tetramethyl ammoniumhydroxide

Nanofluids have gooddispersion and stability.

Yu et al. [114] Fe3O4 Kerosene f ¼ 0e1.0 vol.%D ¼ 15 nm

SPMXRDDLS

Oleic acid Nanofluids were stable

Phuoc and Massoudi[115]

Fe2O3 Deionizedwater

f ¼ 1e4 vol.%D ¼ 20e40 nm

e PVP Nanofluids were stablefor about two weeks

Guo et al. [116] G-Fe2O3 EGedeionizedwater

f ¼ 0.005e0.02 vol.%D ¼ 20 nm

TEM e e

Sheikhbahai et al. [117] Fe3O4 EGedeionizedwater

f ¼ 0.01e0.1 vol.%D < 50 nm

SEM e No sedimentation wasobserved after 8 h

Patel et al. [118] AuAg

WaterToluene

F ¼ 0.00013e0.005D ¼ 10e20 nmD ¼ 3e4 nm

TEM Thiolate The samples werestable over a period ofseveral months

Phuoc et al. [119] Ag Deionizedwater

F ¼ 0.01 vol.%D ¼ 20e30 nm

TEM e The samples werestable for severalmonths

Parametthanuwat et al.[120]

Ag DIewater F ¼ 0.5% w/vD < 100 nm

e e The stability is up to48 h

Hajian et al. [121] Ag DIewater F ¼ 50, 200 and600 wt.%e

TEM e e

Tamjid and Guenther[122]

Ag DEG F ¼ 0.11e4.37 vol.%D w 40 nm

TEM e e

Asirvatham et al. [123] AgNearlyspherical

Deionizedwater

F ¼ 0.3%, 0.6% and0.9 vol.%D w 80 nm

SEM e e

Sharma et al. [124] Ag EG F ¼ 1000e10,000 wt.% TEMEDXXRDUVevis spectroscopyZeta potential analyzer

PAAeco-AA e

Z. Haddad et al. / International Journal of Thermal Sciences 76 (2014) 168e189184

Table 2 (continued )

Authors Particlematerial

Base fluid Parameters Characterization Surfactant/pH control Nanofluid stability

Hari et al. [125] Ag Deionizedwater

e SEMTEMUVevis spectroscopy

CTAB Nanofluids were stablefor a maximum periodof one week

Paul et al. [126] AuSpherical

Water F ¼ 0.6 � 10�4 to2.6 � 10�4 vol.%D ¼ 21 nm

XRDSEMTEM

e No sedimentation oragglomeration evenafter 48 h.

Paul et al. [127] AgSpherical

Water F ¼ 10�5 to 2.6 � 10�3 MD ¼ 55e95 nm

XRDSEMTEMDLS

e e

Kim et al. [128] AuSpherical

Water F ¼ 0.6 � 10�4 to2.6 � 10�4 vol.%D ¼ 45e130 nm(commercial)D ¼ 7.1e12.1 nm (18 h-PLAL)

XRD?????SEMTEMZeta potential analyzer

e AueNPs were stillsuspended in waterafter 1 month

Lo et al. [129] AgSpherical

Deionizedwater

F ¼ 0.5 vol.%D ¼ 6e25 nm

TEMXRDSEM

e e

Kang et al. [130] Ag Pure water F ¼ 1e100 mg/lD ¼ 35 nm

TEM e e

Xuan and Q. Li [131] Cu Transformer oilWater

f ¼ 2 and 5 vol.%f ¼ 5 vol.%D w 50 nm

TEM Oleic acidLaurate salt

Transformer oil-Cususpension can lastabout one week in thestationary stateWatereCu suspensioncan be stable more than30 h

Yang et al. [132] CuSpherical

CTAC/NaSl f ¼ 0.05e2.5 vol.%D ¼ 50 nm

SEM e e

Peng et al. [133] Cu R113 f¼ 0.1, 0.5 and 1.0 wt.%D ¼ 20 nm

TEMSpectrophotometer

SDS, CTAB and Span-80 Nanofluids were keptstable for 24 h

Kole and T.K. Dey [134] Cu DIewater f ¼ 0.0005e0.5 wt.%D ¼ 40 nm

TEMMalvern ZS Nano Sanalyser

e No visible trace ofsedimentation for >15days

Li et al. [135,136] CuSpherical ornearly spherical

Pure water f ¼ 0.0005e0.5 wt.%D ¼ 25 nm

TEMMalvern ZS Nano Sanalyser

SDBS, CTAB Suspension with CTABdispersant can lastabout 1 week withoutsedimentation

Kathiravan et al. [137] Cu DIewater f ¼ 0.25, 0.5 and1.0 wt.%D ¼ 10 nm

TEMXRDAFM

SDS Nanoparticles weredispersed in waterevenly even after 10 h

Robertis et al. [138] CuNearlyspherical

EG e

D ¼ 50 nmTEMXRD

PVP The particlessettlement was about28.5% in 50 days

Z. Haddad et al. / International Journal of Thermal Sciences 76 (2014) 168e189 185

metallic and nonmetallic material and sizes of nanoparticlesdispersed in different base fluids are summarized in Table 2.

It should be noted that is difficult to compare the stability ofnanofluids reported by different researchers. This is due to thefact that, the stability was found to be dependent on pH (i.e., zetapotential), sonication time, volume fraction, nanoparticle sizeand type, base fluids, surfactants and nanofluid productionmethods.

3. Zeta potential

The stability of the dispersed particles is influenced by theirsurface charge or zeta potential. The significance of zeta potential

Table 3Isoelectric point in water and density of different nanoparticles.

Nanoparticles CNTs SiO2 AlN Al2O3 TiO2

IEP 7.5 2 e 9.2 6.5Density (g/cm3) 2.1 2.2 3.26 3S.9 4.2

(negative or positive) is that its value can be related to the stabilityof colloidal dispersions. It is well known that suspensions with zetapotential above 30 mV are physically stable and above 60 mV showexcellent stability while a suspension below 20 mV has limitedstability, and below 5 mV undergoes pronounced aggregation[143]. The density value and isoelectric point (IEP) of differentnanoparticles are presented in Table 3.

There are few studies that discuss the change of zeta potentialwith pH values. Fig. 13 shows the zeta potential of various types ofnanoparticles dispersed in water without any surfactant. Kim et al.[128] found that any AueNPs suspended in water, have an almostsame zeta potential value of about�30mV regardless of pH (6.8e4)and vol.%, though no surfactants were used. It was reported that the

Fe3O4 ZnO CuO Cu Ag Au

e e 9.5 e e 35.24 5.6 6.315 8.94 10.5 19.32

Fig. 13. Zeta potential of various nanoparticles dispersed in water as a function of pH[144].

Z. Haddad et al. / International Journal of Thermal Sciences 76 (2014) 168e189186

independence of zeta potential value on pH and volume fractionmay be due to the fact that the size of AueNPs was almost un-changed; the zeta potential value of the AueNPs/water nanofluidsprepared by18-h PLAL was �21.2 � 1.10 mV even after 3 months.

TiO2, Al2O3 and SiO2 nanofluids show a maximum zeta potentialof 46 mV, 50 mV at lower pH values and 49 mV at higher pH values,respectively. Pastoriza-Gallego et al. [145] stated that Al2O3ewa-terenanofluids with pH values above 8 would clearly producesample flocculation. Mondragon et al. [146] showed that particlevolume fraction also affects the zeta potential of silicaewaternanofluids. Moreover, it was observed that an increase of solidmassload from 0.10 to 0.20 w/w leads to a decrease in the zeta potentialand surface charge.

CNTs and CuO nanofluids show a maximum zeta potentialaround 25.5 mV, and this indicates that the force of electrostaticrepulsion between nanoparticles is not enough to overcome theattraction force between particles. For CuO nanofluids, resultsshowed that when NaHMP was present in the CuO-suspension,the zeta potential was lower than �40 mV for every pH. Also,the nanofluid was found to be stable without adding additive atpH value higher than 12 [54]. For CNT nanofluids, it was reportedthat addition of gemini surfactant at pH of about 7 increases thezeta potential to 50 mV, making CNTs disperse well in base fluid[147].

4. Conclusion

The present paper presents an inclusive review on the nano-fluids preparation methods. Although many preparation methodswere proposed, it is still a challenge to make a nanofluid homo-geneous and long-term stable with negligible agglomeration, andwithout affecting the thermophysical properties.

Three methods have been used to prepare nanofluids: 1) Soni-cation: it was shown that sonication time has an effect on the zetapotential and nanoparticles sizes. Therefore, an optimum time ofsonication should be taken into account. In addition, some nano-fluids were sonicated just to be kept stable for the duration of theexperiment, which limits the commercialization of nanofluids. 2)pH control: the value of zeta potential is generally obtained byadjusting the pH value according to just one small volume fractionvalue even though the particle volume fraction affects the zeta

potential, i.e., pH value. 3) Surfactants: a critical miscell concen-tration should be respected to avoid speedly sedimentation ofnanoparticles.

Some researchers prepared their nanofluids without using sur-factants or adjusting pH since they aim at changing the thermo-physical properties of nanofluid. For such case, only TiO2 in EGeTNT that showed a maximum stability of over the period of twomonths. Therefore, it is unavoidable to prevent particle sedimen-tation without using dispersants or pH control. However, an opti-mum amount of surfactant and pH value can be found to keep thephysical properties constant.

The most attractive property of nanofluids is the enhancementof the thermal conductivity. Therefore, further research in-vestigations are needed to comprehensively understand the sta-bility of nanofluids before evolving new energy efficient heattransfer fluids specific to applications.

Acknowledgments

Authors would like to thank to reviewers due to their appro-priate and constructive suggestions as well as their proposed cor-rections, which have been utilized in improving the quality of thepaper.

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