Chemical Physics Letters 554 (2012) 236–242

Contents lists available at SciVerse ScienceDirect

Chemical Physics Letters

journal homepage: www.elsevier .com/locate /cplet t

Viscosity of low volume concentrations of magnetic Fe3O4 nanoparticlesdispersed in ethylene glycol and water mixture

L. Syam Sundar a,⇑, E. Venkata Ramana b, M.K. Singh a, A.C.M. De Sousa a

a Department of Mechanical Engineering, University of Averio, 3810-193 Aveiro, Portugalb I3N, Department of Physics, University of Averio, 3810-193 Aveiro, Portugal

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 August 2012In final form 15 October 2012Available online 24 October 2012

0009-2614/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.cplett.2012.10.042

⇑ Corresponding author.E-mail address: sslingala@rediffmail.com (L. Syam

This Letter reveals an experimental investigation of rheological properties of Fe3O4 nanoparticles dis-persed in 60:40%, 40:60% and 20:80% (by weight) ethylene glycol and water mixture. Magnetic nanopar-ticles were synthesized by chemical coprecipitation method. The experiments were carried out in theparticle volume concentration range from 0.0% to 1.0% and temperature range from 0 �C to 50 �C. Viscos-ity of nanofluid increases with increase of volume concentration and decreases with increase of temper-ature. The results indicate that the 60:40% EG/W based nanofluid is 2.94 times more viscous compared tothe other base fluids. Einstein model was under predicting the experimental viscosity for all the basefluids.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Single phase fluids like water, ethylene glycol, engine oil andtransformer oil are widely used as coolant to prevent overheatingin electronic devices, maintain desired conditions by removingheat in transportation vehicles and in process industries. However,these single phase fluids have poor thermal properties. In the lastdecade, nanofluids have the great interest due to their enhancedthermal properties. Choi et al. [1] observed 160% thermal conduc-tivity enhancement with carbon nanotubes dispersed in engine oil.Sundar and Sharma [2] obtained thermal conductivity enhance-ment of 6.52% with Al2O3 nanofluid, 24.6% with CuO nanofluid at0.8% volume concentration compared to water. Parekh and Lee[3] observed 30% of thermal conductivity enhancement with9.9 nm size of Fe3O4 nanofluid at 4.7% volume concentration inthe temperatures range of 25–65 �C. Philip et al. [4] obtained300% thermal conductivity enhancement for 6.3% volume fractionof 6.7 nm size of Fe3O4 nanoparticles dispersed in kerosene. Yuet al. [5] investigated kerosene based Fe3O4 nanofluid with Oleicacid as surfactant and found 34% enhancement in thermal conduc-tivity for 1.0% volume fraction with an average particle size of155 nm in the temperature range from 10 �C to 60 �C.

Research works related to convective heat transfer coefficientand friction factor of these nanofluids in a tube by Pak and Cho[6] for Al2O3 and TiO2 nanofluid, Xuan and Li [7] for Cu/water nano-fluid under turbulent flow conditions. Most of the earlier researchworks related for the estimation of convective heat transfer andfriction factor of different nanoparticles dispersed in water under

ll rights reserved.

Sundar).

laminar and turbulent flow conditions. These nanofluids have agreat interest for heating industrial and residential buildings inthe cold regions of the world. Due to the long winter climate con-ditions, different percentage of ethylene glycol or propylene glycolmixed with water is used as a heat transfer fluids [8], because ofthe low freezing point of water. Such fluids are commonly usedin heat exchangers, automobiles and industrial coolants in cold re-gions. These fluids do not freeze even if the operating temperaturereaches to �40 �C. Under low temperatures ethylene glycol mix-ture has better properties compared to propylene glycol mixture[9]. In this Letter, an experimental analysis was performed by mag-netic Fe3O4 nanofluid in order to explore the thermophysical prop-erties. It is very essential to understand the properties forsuccessful application of magnetic nanofluids in cold region. SyamSundar et al. [10] estimated the convective heat transfer and fric-tion factor of water based Fe3O4 nanofluid in a tube. For viscosity,they considered Brickman model. Therefore, magnetic nanoparti-cles dispersed in ethylene glycol/water mixture under volume con-centrations range from 0% to 1.0% were tested in the temperaturerange from 0 �C to 50 �C. The advantage for considering magneticnanoparticles for the present investigation is separation of theseparticles from the base fluid (water or ethylene glycol) is possible,even if the particles are fully dispersed in base fluid, and this is notpossible for nonmagnetic nanoparticles (Al2O3, CuO and TiO2).

Viscosity is one of the important properties for the estimation ofconvective heat transfer coefficient, Prandtl number and Reynoldsnumber of nanofluid. With regard to the viscosity, Masuda et al.[11] have first time measured the viscosity of water based nano-fluid at a temperature range from 273 K to 340 K. Research worksrelated to viscosity of Al2O3 nanoparticles dispersed in water byLee et al. [12], Tseng and Wu [13], TiO2 nanoparticles by Tseng

Figure 1. XRD pattern of the synthesized magnetic particles.

L. Syam Sundar et al. / Chemical Physics Letters 554 (2012) 236–242 237

and Lin [14], nickel powder dispersed in terpineol by Tseng andChen [15], SWCNT and TiO2 nanoparticles by Bobbo et al. [16]and CuO nanoparticles by Kwak and Kim [17]. All the researchersconsidered water as a base fluid and conducted viscosity experi-ments at higher temperatures.

Some researchers have been considered ethylene glycol/watermixture and propylene glycol/water mixture as a base fluid forthe preparation of nanofluid. Namburu et al. [18] first time pre-sented the viscosity data by dispersing CuO nanoparticles into60:40% of EG/W mixture and conducted the experiments in thetemperature range from �35 �C to 50 �C. Prasher et al. [19] consid-ered pure propylene glycol as a base fluid for the estimation of vis-cosity of Al2O3 nanofluid. Naik and Sundar [20] considered 70:30%PG/W mixtures as a base fluid for the preparation of CuO nanofluidand also found better thermal conductivity and viscosity propertiescompared to base fluid. Murshed et al. [21] considered TiO2 andAl2O3 nanoparticles in de-ionized water and pure ethylene glycol.

Most of the research works related to viscosity of different kindof nonmagnetic type nanoparticles dispersed in water, ethyleneglycol and propylene glycol. The viscosity of magnetic Fe3O4 nano-fluid data is not available in the literature. For every application ofthe fluid system, viscosity is the crucial and important parameter.In this regard, the present work focus on the measurement of vis-cosity of magnetic Fe3O4 nanoparticles dispersed in various con-centrations of ethylene glycol/water mixture. The magneticnanoparticles used for the preparation of nanofluid are synthesizedby chemical coprecipitation method. The saturation magnetic fieldof the synthesized nanoparticles were also estimated. Obtainedexperimental viscosity data is compared with the various theoret-ical models and developed equations. Based on the experimentaldata, generalized correlation was proposed.

2. Experimental

2.1. Materials and nanoparticle synthesis

Magnetic nanoparticles were synthesized by chemical coprecip-itation of FeCl3 6H2O, FeCl2 4H2O, sodium hydroxide (NaOH). Thechemicals were purchased from Sigma–Aldrich Chemicals, USA.All the chemicals were reagent grade and used without furtherpurification.

Magnetic nanoparticles were synthesized by FeCl3 6H2O andFeCl2 4H2O precursor salts in the molar ratio of 1.5:1, 2:1 and2.5:1 were dissolved in 20 ml of distilled water. After adding therequired quantity of iron salts in the distilled water, the watersolution was converted to orange colour. Then adding an aqueousNaOH solution and maintain the pH 12. Stir the solution vigorouslyup to 45 min and then observe the solution becomes black colour.It indicates that the reaction was done and observes the formationof magnetic particles. The precipitate was washed several timeswith distilled water and acetone and dried at 80 �C for 24 h.

2.2. XRD, SEM and TEM analysis

Synthesized magnetic Fe3O4 nanoparticles from coprecipitationmethod were analyzed by X-ray diffraction, Siemens D-500, 45 kV,40 mA. The size and shape of the nanoparticles were determinedby Hitachi H-9000 TEM and Hitachi SU-70 SEM. Meanwhile, se-lected area electron diffraction (SAED) was performed to identifythe crystallinity of the particles. Figure 1 shows the XRD patternof the sample, which is quite identical to pure magnetite and alsono characteristic peaks of impurities were observed. There are aseries of characteristic peaks: D = 2.9644(220), 2.52808(311),2.0961(400), 1.7115(422), 1.6136(511), 1.4822(440) and1.2786(533). From the XRD patterns, over the 2h range from 10

to 90� at rate of 2.5�/min, using Cu–Ka radiation with a wavelength of k ¼ 0:15418 nm. The average core size of the particlescan be evaluated from Scherrer equation.

D ¼ 0:94kBð2hÞ cos h

ð1Þ

Where D is equivalent to the average core diameter of the par-ticles, k is the wavelength of the incident X-ray, B(2h) denotes thefull width in radian subtended by the half maximum intensitywidth of the powder peak and h is the corresponds to the angleat that maximum peak. For the maximum peak in the XRD pattern(2h) is observed as 35.502, and B(2h) is 1.3359�. For the wavelength (k) being 0.15148 nm, D is obtained as 11.42 nm estimatedfrom Eq. (1). TEM, SEM and SAED patterns were shown in Fig-ure 2a–c respectively and it is also observed that the sample hascubic crystal shape.

2.3. Effect of Fe3+/Fe2+ ratio on magnetization

In order to separate the magnetic nanoparticles from the basefluid, it is important to understand the magnetic effect of the par-ticles. Magnetization field on the molar ratios of Fe3+ and Fe2+

(1.5:1, 2:1 and 2.5:1) was studied by using vibrating sample mag-netometer, Cryogenic Limited, UK. The M–H hysteresis loops ofFe3O4 derived under different magnetic field strength are shownin Figure 3a and the magnetic effect of the particle while dispersedin base fluid is shown in Figure 3b. It demonstrates that all thethree molar ratios of Fe3+:Fe2+ like 1.5:1, 2:1 and 2.5:1 are havingdifferent magnetic field intensities. It can be observed that, the par-ticles with molar ratio (Fe3+:Fe2+) of 1.5:1 possess the maximumsaturation magnetization of 66.5 emu/g, which is much higherthan the (about 58.91 emu/g) synthesized particles under the sameconditions [22,23]. The saturation magnetization of molar ratio(Fe3+:Fe2+) of 1.5:1 (66.5 emu/g) are higher than that of 2:1(48.3 emu/g) and 2.5:1 (33.7 emu/g). A Zetasizer Nano ZS (Mal-vern) was used to analyse the average dimension of the synthe-sized nanoparticles with three molar ratios in solution. TheZetasizer works measuring the Brownian motion of the particlesin the sample by means of the Dynamic Light Scattering (DLS)and then calculating the size of the particles. It is observed thatthe molar ratio of 1.5:1, the particles size distribution is more than70 nm and the particles are settle very speedily in base fluid. This isreflected in the magnetic nature where the curves show a coerciv-ity of 62 (Oe), which is comparable to the earlier report of particle

Figure 2. (a) TEM image (b) selected area electron diffraction (c) SEM image.

Figure 3. (a) Magnetization curves for different Fe3+:Fe2+ ratios (b) Particlesattracted by an external magnet.

238 L. Syam Sundar et al. / Chemical Physics Letters 554 (2012) 236–242

size 55 nm by Verges et al. [24]. Similarly, the molar ratio of 2.5:1,the particle size distribution is less than 8 nm, the dispersion ofthese particles are good in base fluid, but the magnetization effectof the particles are low. The molar ration of 2:1, the particle sizedistribution is 11.42 nm and the uniform dispersion of the particlesin the base fluid was observed and also having excellent magneti-zation. So, the stoichiometric ratio for the preparation of magnetiteparticle, the molar ratio of (Fe3+:Fe2+) is 2:1 is derived based on themagnetization effect and also dispersion of the particles in the basefluid.

2.4. Magnetic nanofluid preparation

Magnetic nanofluid was prepared by adding required quantityof Fe3O4 nanoparticles in 50 g of base fluid. In this analysis basefluid constitutes a mixture of ethylene glycol and water. Three dif-ferent concentrations of base fluid like 60:40%, 40:60% and 20:80%of EG/W mixture was considered for the preparation magneticnanofluid. The solution was kept in ultrasonic bath up to 2 h. Mag-netic stirring is not possible, because the nanoparticles are havingmagnetic properties and those are stick to magnetic beet. No sur-factant was used for the preparation of nanofluid. There is no par-ticle sedimentation was observed up to 80 days. So, this period issufficient to complete the viscosity measurements for different

L. Syam Sundar et al. / Chemical Physics Letters 554 (2012) 236–242 239

volume concentrations of nanofluid. The same procedure isadopted for the preparation of different volume concentrations of40:60% EG/W and 20:80% EG/W based nanofluid. The quantity ofFe3O4 nanoparticles required for given volume concentration wasestimated from Eq. (2).

u� 100 ¼

WFe3O4qFe3O4

� �WFe3O4qFe3O4

� �þ Wbase fluid

qbase fluid

h i ð2Þ

Density of magnetite ðqFe3O4Þ ¼ 5810 kg/m3, weight of base fluid

(Wb) = 50 g, density of base fluid obtained from ASHRAE [9] hand-book, for 60:40% EG/W (qBase fluid) = 1081.35 kg/m3, for 40:60% EG/W = 1055.39 kg/m3 and 20:80% EG/W = 1026.02 kg/m3 and volumeconcentration range from (u) = 0–1.0%.

The stability of the nanoparticles in the base fluid was mea-sured in terms of zeta potential. The zeta potential for 60:40%,40:60% and 20:80% EG/W based nanofluid at 1.0% volume concen-tration at various pH was measured by using Malvern ZS Nano Sanalyzer, UK. The measurements was run at a voltage of V = 10 V,temperature of T = 25 �C with switch time of t = 50 s. Each experi-ment was repeated five times and the mean value was recorded.The measurement of the zeta potential has become an importantevaluation for understanding the dispersion behavior of nanoparti-cles in a liquid medium [25]. The zeta potential of 1.0% volumeconcentration of 60:40% EG/W is 54 mV, 40:60% EG/W is 45 mVand 20:80% EG/W is 49 mV were observed at the solution pH is5. All the measured nanofluids show a Zeta potential higher than30 mV.

Figure 4. Viscosity of 60:40% EG/W based nanofluid with effect of temperature.

2.5. Viscosity measurements of magnetic nanofluid

The viscosity of different volume concentrations of Fe3O4 nano-fluid was measured in the range from 0 �C to 50 �C with an AR-1000 rheometer (TA Instruments), UK, and it consists of plate-conegeometry. In the present analysis 40 mm diameter with 4� conewas used. Temperature was measured with PT-100 thermo resis-tance inside the Peltier plate of the rheometer with an accuracyof 0.1 �C. The temperature of the plate is controlled with Julabotemperature controller bath, Germany. The important thing inthe measurements is sample loading. Some trials with water,quantity of about 23 ml was considered for the analysis. Make surethat the sample was not having any air bubbles inside and sampleswere loaded with pipette. Before starting the measurements withnanofluid, the rheometer was calibrated with known viscosity of60:40%, 40:60% and 20:80% EG/W mixture at each and every tem-perature and compared with the ASHRAE [9] handbook. After that,nanofluid with different volume concentrations was inserted intothe equipment for the estimation of viscosity. All the experimentswere performed at constant temperature and at different shearrate from 21.63 s�1 to 2740 s�1 and temperature range from 0 �Cto 50 �C. Each experiment was performed three times at constantshear rate and the average value is considered as final value. Initialmeasurements were done with the know viscosity of base fluid(60:40% EG/W) at different temperatures and the obtained valuesare compared with the values of ASHRAE [9] handbook. It is evi-dent that a maximum of 2.3% and 4.5% were obtained betweenexperimental and hand book data with temperatures at 0 �C to50 �C. So, this indicates that the instrument can be comfortablyused to measure the viscosity of nanofluid.

The equation for viscosity of nanofluid is given by

s ¼ l _c ð3Þ

where s is shear stress, l is viscosity and c is shear strain.

3. Results and discussion

3.1. Experimental viscosity

The next step was to determine whether the fluid displays non-Newtonian properties after the addition of Fe3O4 nanoparticles.From the measurements, it is observed that the shear stress forall the volume concentrations of nanofluid increases linearly withincrease of shear strain. Since ethylene glycol water mixture exhib-its Newtonian behavior, it dominates the rheological property andthe whole mixture behaves like a Newtonian fluid with low con-centrations of Fe3O4 nanoparticles.

After we performed the base case experiments, confirming thatthe obtained readings were correct and that the fluid was Newto-nian, viscosity measurements of fluid samples with different vol-ume concentrations were carried out with varying temperaturesbetween 0 �C and 50 �C. The similar trend of Newtonian behaviourhave been observed by Namburu et al. [18] by dispersing CuOnanoparticles in 60:40% EG/W mixture at 6.12% volume concentra-tion in the temperature range from �35 �C to 50 �C and Kulkarniet al. [26] by dispersing CuO nanoparticles at 15% volume concen-tration in the temperature range from 5 �C to 50 �C. Kole and Dey[27] also observed the Newtonian behaviour by consideringAl2O3 nanoparticles into car engine coolant oil.

The measured viscosity of nanofluid at different concentrationsand different base fluids like 60:40% EG/W is shown in Figure 4,40:60% EG/W is shown in Figure 5 and 20:80% EG/W is shown inFigure 6 under the temperature range from 0 �C to 50 �C. It indi-cates that, viscosity increases with increase of volume concentra-tions and decreases with increase of temperatures. For all thebase fluids, the enhancement in viscosity is more at a temperatureof 50 �C compared to temperature of 0 �C. The viscosity enhance-ment is more for 60:40% EG/W based nanofluid compared to40:60% and 20:80% EG/W based nanofluid under same percentageof volume concentration. At a temperature of 50 �C, the viscosity of1.0% volume concentration of 60:40% EG/W nanofluid is 2.94 times,40:60% EG/W nanofluid is 1.61 times and 20:80% EG/W nanofluidis 1.42 times more compared to the same base fluids. At a temper-ature of 0 �C, the viscosity of 1.0% volume concentration of 60:40%EG/W nanofluid is 2.13 times, 40:60% EG/W nanofluid is 1.92 timesand 20:80% EG/W nanofluid is 1.4 times more compared to thesame base fluids. The similar trend of viscosity enhancement withvolume concentration for 60:40% EG/W based CuO nanofluid have

Figure 5. Viscosity of 40:60% EG/W based nanofluid with effect of temperature.

Figure 6. Viscosity of 20:80% EG/W based nanofluid with effect of temperature.

Figure 7. Viscosity of 60:40%, 40:60% and 20:80% EG/W based nanofluid with effectof volume concentration.

240 L. Syam Sundar et al. / Chemical Physics Letters 554 (2012) 236–242

been observed by Namburu et al. [18]. The viscosity of 60:40%,40:60% and 20:80% EG/W based nanofluid with effect of volumeconcentration under 0 �C and 50 �C is shown in Figure 7. It isclearly indicating that viscosity increases with increase of particlevolume concentration in the base fluid. This is caused due to shearresistance offered by the particles onto the fluid layer. With largerthe particle concentration in the base fluid, larger the quantity is ofparticles are required.

3.2. Theoretical models

There exist very few established theoretical models that may beused to predict the effective viscosity of nanofluids and most ofsuch models are derived from well known Einstein model [28].As nanofluid is a two-phase fluid, one may expect that it wouldhave common features with solid–liquid mixtures. However, thequestion regarding the applicability of these classical models foruse in nanofluids still remains doubtful. Some of the widely usedmodels for nanofluids are mentioned below.

Einstein model [28] can be used for very low volume concentra-tion u < 0.02%, which is given below:

lnf ¼ lbf ð1þ 2:5uÞ ð4Þ

where lnf is the viscosity of the nanofluid, lbf is the viscosity of thebase fluid.

Brickman model [29] is the extension of Einstein model, whichcan be used for moderate volume concentrations.

lnf ¼ lbf1

ð1�uÞ2:5

!ð5Þ

Batchelor model [30] can be considered with nanoparticleBrownian motion and their interaction.

lnf ¼ lbf ð1þ 2:5uþ 6:5u2Þ ð6Þ

These entire equations based on the assumptions that the vis-cosity of the nanofluid is only a function of the base fluid viscosityand the particle concentration and that the nanoparticles can bemodelled as rigid spherical particles. All these equations are pre-dicted more or less same values under same volume concentrationand temperature.

The above models [28–30] predict the viscosity at low volumeconcentrations and for higher volume concentrations most of theauthors considering the Krieger–Dougherty equation [31]. TheKrieger–Dougherty model has a form:

lnf

lbf¼ 1� ua

um

� ��½g�um

ð7Þ

where um is maximum concentration ua is effective volume con-centration of aggregates and [g] is the intrinsic viscosity, whichfor monodisperse systems has a typical value of 2.5. Chen et al.[32] modified the Krieger–Dougherty equation by consideringua ¼ uðaa

a3�DÞ.

lnf

lbf¼ 1� u

um

aa

a

� �1:2� ��½g�um

ð8Þ

where, aa and a are the radii of aggregates and primary nanoparti-cles, respectively. The term ‘D’ is defined as fractal index, which fornanoparticles has a typical value of 1.8 [19,32]. A simple expressionwas proposed by Kitano et al. [33] involving um was also used topredict the viscosity of two phase mixture:

Figure 9. Comparison between present data and developed correlation of Eq. (12).

L. Syam Sundar et al. / Chemical Physics Letters 554 (2012) 236–242 241

lnf

lbf¼ 1� u

um

� ��2

ð9Þ

In order to apply the Eqs. (7)–(9) um should be calculated. In thepresent analysis the maximum volume concentration was consid-ered is 1.0%. This volume concentration is restricting the validityrange of the models, um was calculated based on Liu [34], on allthe experimental data, being 5.25% for Fe3O4 nanofluids.

Viscosity correlations for 60:40% EG/W mixture based CuOnanofluid was proposed by Namburu et al. [18].

Logðlnf Þ ¼ Ae�BTð60 : 40% EG=W based CuO nanofluidÞ ð10Þ

0 < u < 6:12%

A ¼ 165:56� 29:643ðuÞ þ 1:8375ðuÞ2

B ¼ 0:0186� 0:001ðuÞ þ 4� 10�6ðuÞ2

The present experimental viscosity of different volume concentra-tions of Fe3O4 nanofluid is compared with Einstein model [28]. Itis evident that the Einstein model not accurately predicting the vis-cosity for the volume concentration more than u = 0.02%. This mod-el works under very low volume concentrations. The relativeviscosity between present experimental data of 60:40% EG/W basedFe3O4 nanofluid is shown in Figure 8 in comparison with Namburuet al. [18] data of 60:40% EG/W based CuO nanofluid. Under samevolume concentration of 1.0% the relative viscosity of Fe3O4 nano-fluid is more compared to the relative viscosity of CuO nanofluid.The enhancement is caused because of the various particles withvarious particle size and various preparation methods employed.But the same trend of relative viscosity increases with percentageof volume concentration is observed in the present analysis.

The applicability of theoretical models to nanofluids is a stillunsolved problem. Here, a simple equation, with similar form toEq. (4), was proposed by considering 288 data points for 40:60%EG/W and 20:80% EG/W based Fe3O4 nanofluid with an averagedeviation of 6.1% and standard deviation of 8.2%.

lnf ¼ lbf ð1þuÞ0:68 ð11Þ

Similarly for 60:40% EG/W based Fe3O4 nanofluid by considering144 data points correlation is proposed with an average deviationof 8% and standard deviation of 9.1%.

Figure 8. Present relative viscosity is in comparison with data of Namburu et al.[18].

lnf ¼ lbf ð1þuÞ1:205 ð12Þ

The present experimental viscosity for 60:40% EG/W based Fe3O4

nanofluid is shown in Figure 9 along with the developed correlationEq. (12) for validation purpose. The proposed equations are helpfulto estimate the viscosity of Fe3O4 nanofluid at specific temperatureand volume concentration within the given range without conduct-ing any experiments.

4. Conclusion

Ultrafine, uniform, nearly cubical, and high purity Fe3O4 nano-particles were prepared by controlled chemical coprecipitationmethod from the aqueous solution of ferrous chloride/ferric chlo-ride and sodium hydroxide. The results show that Fe3O4 nanopar-ticles can be produced with an average size ranging from 5 nm to70 nm by changing the operational parameters. Excellent particledispersion in the base fluid was obtained at pH 5 for all the particlevolume concentration.

The enhancement in viscosity for 1.0% volume concentration of60:40% EG/W nanofluid is 2.94 times, 40:60% EG/W nanofluid is1.61 times and 20:80% EG/W nanofluid is 1.42 times comparedto the same base fluids at a temperature of 50 �C. The enhancementin viscosity for 1.0% volume concentration of 60:40% EG/W nano-fluid is 2.13 times, 40:60% EG/W nanofluid is 1.92 times and20:80% EG/W nanofluid is 1.4 times compared to the same basefluids at a temperature of 0 �C. The magnetic nanofluids can com-fortably use as heat transfer fluids. The unique advantage of thesefluids is having magnetic response even if the nanoparticles arefully dispersed in the base fluid.

Acknowledgments

The authors would like to thank the Portuguese Foundation ofCinecia e Technologia, through a grant funded by Ministry of Sci-ence and Technology. One of the authors (L.S.S.) would like tothank FCT for his Post-Doctoral research grant (SFRH/BPD/79104/2011).

References

[1] S.U.S. Choi, Z.G. Zhang, W. Yu, F.E. Lockwood, E.A. Grulke, Appl. Phys. Lett. 79(14) (2001) 2252.

[2] L.S. Sundar, K.V. Sharma, Int. J. Nanopart. 1 (1) (2008) 66.

242 L. Syam Sundar et al. / Chemical Physics Letters 554 (2012) 236–242

[3] K. Parekh, H.S. Lee, J. Appl. Phys. 107 (2010) 09A310.[4] J. Philip, P.D. Shima, B. Raj, Appl. Phys. Lett. 91 (2007) 203108.[5] W. Yu, H. Xie, L. Chen, Y. Li, Colloids Surf. A: Physicochem. Eng. Aspects 355

(2010) 109.[6] B.C. Pak, Y. Cho, J. Exp. Heat Transfer 11 (1998) 151.[7] Y. Xuan, Q. Li, J. Heat Transfer 125 (2003) 151.[8] F.C. McQuiston, J.D. Parker, J.D. Spitler, Heating Ventilating and Air-

Conditioning, John Wiley & Sons Inc., New York, 2000.[9] ASHRAE Handbook 1985 Fundamentals, American Society of Heating,

Refrigerating and Air-Conditioning Engineers Inc., Atlanta, 1985.[10] L. Syam Sundar, M.T. Naik, K.V. Sharma, M.K. Singh, T.Ch. Siva Reddy, Exp.

Thermal Fluid Sci. 37 (2012) 65.[11] H. Masuda, A. Ebata, K. Teramae, N. Hishinuma, Netsu Bussei 4 (4) (1993) 227

(in Japanese).[12] J.H. Lee, K.S. Hwang, S.P. Jang, B.H. Lee, J.H. Kim, S.U.S. Choi, C.J. Choi, Int. J. Heat

Mass Transf. 51 (2008) 2651.[13] W.J. Tseng, C.H. Wu, Acta Mater. 50 (2002) 3757.[14] W.J. Tseng, K.C. Lin, Mater. Sci. Eng., A 355 (2003) 186.[15] W.J. Tseng, C.N. Chen, Mater. Sci. Eng., A 347 (2003) 145.[16] S. Bobbo, L. Fedele, A. Benetti, L. Colla, M. Fabrizio, C. Pagura, S. Barison, Exp.

Thermal Fluid Sci. 36 (2012) 65.[17] K. Kwak, C. Kim, Korea–Aust. Rheol. J. 17 (2) (2005) 35.

[18] P.K. Namburu, D.P. Kulkarni, D. Misra, D.K. Das, Exp. Thermal Fluid Sci. 32(2007) 397.

[19] R. Prasher, D. Song, J. Wang, P. Phelan, Appl. Phys. Lett. 89 (2006) 133108.[20] M.T. Naik, L.S. Sundar, World Acad. Sci. Eng. Technol. 59 (2011) 440.[21] S.M.S. Murshed, K.C. Leong, C. Yang, Int. J. Therm. Sci. 47 (2008) 560.[22] R.Y. Hong, T.T. Pan, H.Z. Li, J. Magn. Magn. Mater. 303 (2006) 60.[23] R.Y. Hong, T.T. Pan, Y.P. Han, H.Z. Li, J. Ding, S.J. Han, J. Magn. Magn. Mater. 310

(2007) 37.[24] M.A. Verges, R. Costo, A.G. Roca, J.F. Marco, G.F. Goya, C.J. Serna, M.P. Morales, J.

Phys. D: Appl. Phys. 41 (2008) 134003.[25] L. Garcell, M.P. Morales, M.A. Verges, P. Tartaj, C.J. Serna, J. Colloid Interface Sci.

205 (1998) 470.[26] D.P. Kulkarni, D.K. Das, G.A. Chukwu, J. Nanosci. Nanotechnol. 6 (2006) 1150.[27] M. Kole, T.K. Dey, Exp. Thermal Fluid Sci. 34 (2010) 677.[28] A. Einstein, Investigations on the Theory of the Brownian Movement, Dover

Publications, Inc., New York, 1956.[29] H.C. Brinkman, J. Chem. Phys. 20 (1952) 571.[30] G.K. Batchelor, J. Fluid Mech. 83 (1977) 97.[31] I.M. Krieger, T.-J. Dougherty, Trans. Soc. Rheol. 3 (1959) 137.[32] H. Chen, Y. Ding, Y. He, C. Tan, Chem. Phys. Lett. 444 (2007) 333.[33] T. Kitano, T. Kataoka, T. Shirota, Rheol. Acta 20 (1981) 207.[34] D.M. Liu, J. Mater. Sci. 35 (2000) 5503.