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CHARACTERIZATION OF TEMPERATURE DEPENDENCE OF INTERFACIAL TENSION AND VISCOSITY OF NANOFLUID

S. M. Sohel Murshed and Nam-Trung Nguyen

School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Republic of Singapore

Proceedings of MNHT2008 Micro/Nanoscale Heat Transfer International Conference

January 6-9, 2008, Tainan, Taiwan

MNHT2008-52171

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ABSTRACT

Investigations on temperature dependence of surface tension, interfacial tension and viscosity a nanofluid are reported in this paper. Experimental results show that nanofluid having TiO2 nanoparticles (15 nm) in deionized water exhibit substantially smaller surface tension and oil-based interfacial tension than those of the base fluid (i.e. deionized water). These surface and interfacial tensions of this nanofluid were found to decrease almost linearly with increasing temperature. The Brownian motion of nanoparticles in base fluid was identified as a possible mechanism for reduced surface and interfacial tensions of nanofluid. The measured effective viscosity of nanofluid was found to be insignificantly higher than that of base fluid and it also decreases with increasing fluid temperature. Keywords: Nanofluids, surface tension, interfacial tension, viscosity, temperature

1. INTRODUCTION Nanofluid, a new and innovative class of composite fluid is

engineered by dispersing nanometer-sized particles in conventional fluids such as water. With increased interest in microfluidic systems, interfacial phenomena are receiving more attention from researchers worldwide. Particularly, interfacial tension and viscosity of liquid play an important role in continuous flow- and droplet-based microfluidics. For instance, in droplet-based microfluidics, the size of droplet can be controlled with the interfacial tension and dynamic viscosity as the droplet diameter is directly related to the interfacial tension and the viscosity of the liquid. Besides microfluidics, many industrial applications such as spreading of droplet, surface acting agent and many chemical engineering applications rely on the effect of surface tension. The temperature dependence of interfacial tension and viscosity of are also crucial in thermally mediated droplet formation and droplet breakup [1]. In the field of nanofluids, only Wasan and Nikolov [2] studied

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the spreading of nanofluids on solid surface and found that ordering of nanoparticles near the liquid/solid contact line can improve the spreading of nanofluids. To the best of our knowledge, no investigation was reported on the characterization of surface and interfacial tension of nanofluids.

Although viscosity influences the interfacial phenomena and flow characteristics, very few studies [3-5] have been performed on the effective viscosity of nanofluids as a function of volumetric loading of nanoparticle. The primary results from these studies indicate that the viscosity of a nanofluid increases with increasing nanoparticle volume fraction. On top of very few experimental studies, no theoretical model is available for the prediction of effective viscosity of nanofluids as a function of temperature and particle volume fraction. Therefore, it is worthwhile to study temperature-dependent viscosity of nanofluids for their potential applications especially in microfluidic systems.

For the first time, focusing on the potential applications of nanofluids in droplet-based microfluidics, the temperature dependence of surface tension, interfacial tension in mineral oil, and viscosity a nanofluid are characterized in this study.

2. THEORETICAL

The surface tension of a liquid and interfacial tension between two immiscible liquids can easily and precisely be measured but cannot directly be determined due to the lack of well-established theoretical model. Therefore, experimentally fitted correlations are widely used to estimate the surface and interfacial tensions. Moreover, no theoretical study has been reported on these interfacial phenomenon and viscosity of nanofluids. In this study, two empirical expressions are used to estimate the surface and interfacial tensions of sample fluids.

The surface tension (N/m) of water, which is most commonly used in microfluidics can be estimated from [6]: 1

lg (0.076 0.00017 )T −γ = − ⋅Κ (1)

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Do

where T is the temperature of the system and lgγ (N/m) is the surface tension of liquid in gaseous phase. This experimentally fitted expression is only related with temperature. However, the surface tension of a liquid also strongly depends on the presence of contaminants (e.g. dirt) or dispersion agents such as surfactants.

Based on the surface tension of each phase, Girifalco and Good [7] developed a model to estimate the interfacial tension, which is given as: 2ab a b a bγ = γ + γ − Φ γ γ (2)

where aγ and bγ are the surface tensions of phase a and b, respectively and Φ is a constant which is equal to the ratio of energies of adhesion and cohesion for two phases. The values of Φ for a great number of liquids are given by Girifalco and Good [7].

The effective viscosity ( effη ) of dilute suspensions is commonly determined by Einstein’s [8] viscosity model i.e. (1 2.5 )eff fη = η + φ , which is independent of fluid temperature and dependent on particle volume fraction ( φ ) and base fluid viscosity ( fη ). It is however, noted that Einstein’s classical model was found to severely underpredict the observed viscosity of nanofluids at room temperature [3-5]. Therefore, the measured temperature-dependent viscosity data are not compared with the predictions by the classical models.

3. EXPERIMENTAL 3.1 Sample preparation

Nanofluid was prepared by dispersing 0.1 volume % of

spherical shaped titanium dioxide (TiO2) nanoparticles of 15 nm diameter in deionized water (DIW). To ensure proper dispersion of nanoparticles the sample nanofluid was homogenized by using an ultrasonic dismembrator (Fisher Scientific Model 500) and a magnetic stirrer. The mineral oil (Sigma 5904) was used as other liquid for interfacial tension measurement.

3.1 Measurements

A Surface Tensiometer (DCAT 21) was used to measure

the surface tension of sample fluids at different temperatures ranging from 25-55 ºC. The interfacial tension of the sample fluids was measured with a flexible video system (FTA 200, First Ten Angstroms). Since this system (FTA 200) does not have any facility to measure the interfacial tension as a function of temperature, the mineral oil (Sigma 5904) was heated in a small container made of Pyrex glass to obtain the different temperatures.

Viscosities of deionized water and sample nanofluid were measured with a Lowshear (LS 40) Rheometer at different temperatures. The experimental facilities were calibrated by measuring the properties of known fluids.

All measurements were performed at atmospheric pressure and at the temperature range of 25-55 ºC.

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4. RESULTS AND DISCUSSION 4.1 Temperature dependence of surface and interfacial tensions

Figure 1 depicts the comparison of measured temperature-

dependent surface tensions of sample fluids (i.e. deionized water and nanofluids) and the predictions by Eq. (1) as well as data obtained from the literature for water [9]. It can be seen that dispersion of such a small volume fraction (0.1 volume %) of TiO2 nanoparticles considerably reduces the surface tension of deionized water and it was found to decrease almost linearly with increasing temperature (Fig.1). It is not clear about the observed tend of surface tension of deionized water which differs with the literature data [9] at temperature beyond 25 ºC. One possible reason could be due to using deionized water in this study and normal water in the literature data.

Temperature (oC)20 25 30 35 40 45 50 55 60

Surf

ace

tens

ion

(mN

/m)

50

60

70

80

90Predicted SFT of water [from Eq. (1)]SFT of water [9]Measured SFT of DI Water Measured SFT of nanofluid

Fig. 1 Comparisons of measured surface tensions of DI

water, nanofluid and data obtained from literature

Figure 2 shows that nanofluid exhibits significantly smaller interfacial tension in mineral oil compared to that of base fluid (DIW). The measured results are in good agreement with the calculated results from the classical Girifalco and Good model given by Eq. (2). It is noted that the measured value of surface tension of each phase (DIW, nanofluid and oil) was used to estimate the interfacial tension by this model. Unlike to deionized water, the measured data for interfacial surface tension of nanofluid show nearly linear trend of decreasing with increasing temperature (Fig. 2). Since the values of Φ for DIW/mineral oil and nanofluid/mineral oil based systems are not available in the literature, they are chosen to be 0.54 and 0.67, respectively to fit the experimental results. Nevertheless, these fitted values are within the range of given values for water in similar organic liquids [7]. Interestingly, the present experimental results introduced two new values of Φ to be used in Girifalco and Good model for estimating the interfacial tensions of DIW/oil and nanofluid/oil-based systems.

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Temperature (oC)20 25 30 35 40 45 50 55 60

Inte

rfac

ial t

ensi

on (m

N/m

)

30

35

40

45

50

55

60DIW/Oil DIW/Oil [from Eq. (2)]Nanofluid/Oil Nanofluid/Oil [from Eq. (2)]

Fig. 2 Temperature dependence of interfacial tension of DIW/mineral oil and nanofluid /mineral oil

The results presented in Figs.1 and 2 clearly demonstrate

that the surface and interfacial tensions of this nanofluid are significantly smaller than those of the base fluid. The reason is that nanoparticles in the nanofluids can easily experience Brownian motion and interact with the liquid molecules as well as the nanoparticles themselves resulting to a reduced cohesive energy at the interface. Moreover, an elevated temperature intensifies the Brownian motion and it is known that the lower the cohesive energy the smaller the surface or interfacial tension. It can be inferred that addition of very small amount of nanoparticles into the liquid can change its surface and interfacial tensions. 4.2 Temperature dependence of viscosity

Experimental results on temperature-dependent viscosity of deionized water, nanofluid and standard data of water [10] as well as results from the literature [11] are shown in Fig. 3.

Temperature (oC)20 25 30 35 40 45 50 55 60

Vis

cosi

ty (m

Pa.s)

0.5

0.6

0.7

0.8

0.9

1.0Standard data of water [10]Water [11]Measured for DI WaterMeasured for nanofluid

Fig. 3 Comparisons of measured viscosity of DI water and

nanofluid with data obtained from literature

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As can be seen from Fig. 3, the measured effective viscosities of sample fluids are found to decrease significantly with increasing fluid temperature. However, compared to base fluid, no significant change in viscosity of nanofluid was observed. The slightly higher viscosity of nanofluid could be due to particle clustering, interactions and interparticle potential such as van der Waals force. More studies are needed on the temperature dependence of viscosity of different types of nanofluids.

5. CONCLUSIONS

Nanofluid was found to exhibit considerably smaller surface tension and oil-based interfacial tension than those of the base fluid itself. Experimental results also demonstrate that surface and interfacial tension of nanofluid decrease with increasing temperature. The Brownian motion of nanoparticles in base fluid was identified as a mechanism for reduced surface and interfacial tensions of nanofluid. Compared to base fluid (DIW) no significant change in viscosity of nanofluid was found in this study. Since no model is available to estimate the interfacial tension or viscosity of nanofluids, there is a need to develop models for these properties by taking into account the effects of temperature and volumetric loading of nanoparticles. It is also imperative to conduct more studies in order to elucidate the mechanisms for temperature-dependent interfacial properties and viscosity of nanofluids.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the support from the

Agency of Science, Technology and Research (A*STAR), Singapore (grant number SERC 052 101 0108 “Droplet-based micro/nanofluidics”).

REFERENCES

[1] Ting, T.H., Yap, Y.F., Nguyen, N.T., Wong, T. N., Chai, J.C., and Yobas, L., 2006, “Thermally mediated breakup of drops in microchannels”, Applied Physics Letters, vol. 89, pp. 234101-1 – 234101-3.

[2] Wasan, D.T. and Nikolov, A.D., 2003 “Spreading of nanofluids on solids”, Nature, vol. 423, pp.156-159.

[3] Pak, B.C. and Cho, Y.I., 1998, “Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles”, Experimental Heat Transfer, vol. 11, pp. 151-170.

[4] Prasher, R.S., Song, D, Wang, J., and Phelan, P.E., 2006, “Measurements of nanofluid viscosity and its implications for thermal applications”, Applied Physics Letters, vol. 89, pp. 133108-1 – 133108-3.

[5] Murshed, S.M.S., Leong, K.C. and Yang C., 2007, “Investigations of thermal conductivity and viscosity of nanofluids”, International Journal of Thermal Sciences (in press).

[6] Nguyen, N.T. and Wereley, S.T., 2002, “Fundamentals and Applications of Microfluidics”, 1st edn, Artech House, London.

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[7] Girifalco, L.A. and Good, R.J., 1957, “The theory for the estimation of surface and interfacial energies. I. derivation and application to interfacial tension”, Journal of Physical Chemistry, vol. 61, pp. 904-909.

[8] Einstein, A., 1956, “Investigations on the Theory of the Brownian Movement”, Dover Publications, Inc. New York.

[9] Frohn, A. and Roth, N., 2000, “Dynamics of Droplets”, Springer, Berlin.

[10] Lide, D.R., ed., 2007, “CRC Handbook of Chemistry and Physics”, (87th Edition), Taylor and Francis, Boca Raton, Florida.

[11] Kampmeyer, P.M., 1952, “The temperature dependence of viscosity of water and mercury”, Journal of Applied Physics, vol. 23, pp. 99-102.

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