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Effect of particle concentration, temperature and surfactant on surface tension of nanofluids
International Communications in Heat and Mass Transfer 49 (2013) 110–114
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International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
Effect of particle concentration, temperature and surfactant on surface tension of nanofluids☆ S.S. Khaleduzzaman, I.M. Mahbubul ⁎, I.M. Shahrul, R. Saidur Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e
i n f o
Available online 5 November 2013 Keywords: Nanofluids Surface tension Volume fraction Temperature Surfactant
a b s t r a c t Heat transfer performance with nanofluids depends on the thermo physical properties of the suspension. Surface tension is an important property for heat transfer calculation. In this paper, various parameters that effect on the surface tension of nanofluids such as nanofluid preparation method, effect of volume fraction, temperature, and surfactants on nanofluids have been studied. Additionally, precise assessments on the theoretical correlations related to the surface tension of nanofluids have also been included. Based on the existing experimental results, surface tension augments respectively with volume fraction intensification. Surface tension of nanofluids decreases accordingly with the increase of temperature and surfactant concentration. Nevertheless, there have been some contradictory results on the effect of volume fraction and surfactant on surface tension of nanofluids. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Nanofluids are the solid–liquid mixture of nanoparticles into base fluid [1]. Nanofluids are prepared by dispersing nanometer-sized particles, generally less than 100 nm, in a base fluid such as water, ethylene glycol, propylene glycol, oil and other conventional heat transfer fluids. Addition of high thermal conductivity nanoparticles (e.g., copper, aluminum, silver, CNT) to the base fluid increases the thermal conductivity of such mixtures, thus enhancing their overall heat transfer capability. During the last decade, abundant experimental as well as numerical studies have been done to explore the advantages of nanofluids under a wide variety of conditions [2]. Most of these studies are related with the heat transfer performance [3–5], thermal conductivity [6,7], and viscosity of nanofluids [8–10]. Few review papers [11,12] have emphasized only on thermal conductivity and certain efforts have also been made in the review of viscosity of nanofluid [13]. Surface tension is also an important property when analyzing the performance of a thermal system. The competency of manipulating surface tension of a liquid has a wide range of useful applications, including enhanced boiling heat transfer, oil recovery efficiency, and the capability of cleaning oil spills [14,15]. Studies on surface tension of nanofluids are limited in the literature [16]. Surface tension of a liquid can be reduced by adding either surface active agents or nanoparticles [17]. Murshed et al. [18] and Kumar and Milanova [19] both showed that surface tension of carbon nanotube-based nanofluids was higher than that of water base fluid. Moosavi et al. [20] demonstrated that the surface tension of base fluid (ethylene glycol) increased by a little over 7% ☆ Communicated by Dr. W.J. Minkowycz ⁎ Corresponding author. E-mail address: [email protected] (I.M. Mahbubul). 0735-1933/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.icheatmasstransfer.2013.10.010
with the addition of 3.0 vol.% of ZnO nanoparticles. The opposite trend as decrement of surface tension for nanofluid has also been reported in literature. Murshed et al. [15] measured the surface tension of TiO2/ water nanofluid using a surface tensiometer. They found that the addition of TiO2 nanoparticles to water reduced the surface tension of the resulting nanofluid at room temperature. Vafaei et al. [21] measured the surface tension of Bi2Te3/water nanofluids and found that the surface tension decreased with the increase of particle concentration until it reached a minimum level and then increased with the increase of particle concentration [22]. In addition, the authors reported that, the nanofluid with 10.4 nm nanoparticle has a bigger value of surface tension than the nanofluid having 2.5 nm size particles at similar mass concentration [21]. Material burnout and critical heat transfer flux of the nanofluids surface tension were also affected by surfactants [19,22]. Kumar and Milanova [19] and Murshed et al. [18] showed that the addition of NaBDS (sodium dodecyl benzene sulfonate) surfactant reduces the surface tension of DI water and DI water based nanofluids with the addition of carbon nanotubes. However, Chen et al. [14] showed that the addition of PVP (Poly vinyl pyrrolidone) surfactants does not affect the surface tension of the nanofluid. They reported that, the addition of silver nanoparticles reduced the surface tension rather than the PVP surfactant in their experiment. Available literature confirms that, control of gas–liquid surface tension of nanofluids depends on concentration of nanoparticle as well as temperature and surfactants [21]. Based on the literature survey it is found that, most of the researches done mainly considering the effect of volume concentration on surface tension and some about effects of surfactants, yet along with some contradictions. No authors have discussed the effect of volume fraction, surfactants, and temperature over surface tension of nanofluid all together in one paper as a wide range of study. Only, Tanvir and Qiao [22]
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Nomenclature CNT Carbon nanotube D Diameter (m) DI Deionized Water DIW Distilled Water EG Ethylene glycol g Acceleration of gravity (m/s2) k Calibration factor MWCNTs Multi-walled carbon nanotubes NaBDS Sodium dodecyl benzenesulfonate Q Flow rate (ml/h) T Temperature (K) Ro Radius of curvature at origin (m) rd Radius of contact line (m) r Outer radius of the capillary (m) SWNTs Single-walled carbon nanotubes T Time (s) V Volume (m3) vol. Volume fraction W Water wt. Weight percentage of concentration
111
of nanofluids. In the subsequent sections, measurement methods used by the researchers, experimental results concerning influence of volume fraction, temperature, and surfactants on surface tension, as well as theoretical models and correlations indicating surface tension in terms of volume concentration, temperature and particle diameter have been described consecutively. 2. Measurement methods Researchers have used different methods to measure the surface tension of nanofluids. Most authors have determined the surface tension of nanofluid by Pendant drop method [14,23]. A Rame-Hart Model 500 standard goniometer was used for real-time surface tension measurements [22]. The Pendant drop method was adapted to determine the surface tension of the suspended droplet. This method utilizes the Young–Laplace equation to determine the surface tension of the droplet based on the shape of the droplet [24]. Bubble pressure method [19,25] is also used for surface tension determination. Some authors used some other methods for surface tension measurement, like Teflon FEP strip [26], Wilhelmy method [27], Sigma 703 tensiometer [28], and Ring method [20]. Table 1 shows a list of experimental processes used by the researchers. 3. Experimental results and discussion
Greek symbols Φ Constant θ Contact angle (°) ρ Density (kg/m3) σ Surface tension (mN/m) σg ln Effective gas–liquid–nanoparticle surface tension (N/m) ζ Surface deformation
Subscripts L Liquid s Solid v Vapor
discussed the effects of diameter, concentration and surfactants over surface tension of nanofluids. Therefore, the need for a comprehensive criticism of all important parameters to provide researchers with sufficient information on surface tension of nanofluids is clearly felt. The objective of this paper is to provide adequate information about the effect of volume fraction, temperature, and surfactant on the surface tension
3.1. Effect of volume fraction The effects of volumetric concentration of nanofluids on the surface tension have been plotted in Fig. 1. The results indicate that surface tension depends on nanoparticle volume concentration, and it almost increases with the increase of volumetric fraction. From Fig. 1, it is found that, the result from the studies of Kim et al. [28] and Godson [25] showed that surface tension of nanofluid linearly increases with the increase of volume concentration of nanofluid. Furthermore, Moosavi et al. [20] found that the surface tension enhancement ratio is linearly increased with the increase of nanoparticle volume concentration for ZnO/EG nanofluids with the concentration of 0 to 3 vol.%. However, the results of Tanvir and Qiao [22] show the irregular trends as for the Al2O3/DIW nanofluid, the surface tension initially increased and then decreased and again began to increase and for MWCNTs/DIW nanofluid, surface tension initially decreased with the increase of particle concentration and then increased. Tanvir and Qiao [22] showed another trend of the effect of particle concentration over the surface tension of nanofluids among different nanoparticles with ethalon as base fluid. Fig. 2 shows surface tension of ethanol-based nanofluids (including Al, B, Al2O3, and MWCNTs nanoparticles). In the case of ethanol, surface tension increased up to
Table 1 Different experimental process used in the literatures about surface tension of nanofluids. Base fluid
Particle name (diameter in nm)
Particle fraction (vol./wt.) %
Experimental process
Ref.
DIW DIW Ethanol n-decane W DIW DIW DIW W W Air-W W EG Gas–Liquid W
Silver (b100) Al2O3 (25), MWCNTs (8–15) Al2O3 (25), Al (18), B (80) Al2O3 (25), Al (18), B (80) SWNTs Laponite (25–30) Silver (10–30) Fe2O3 (10–30) Al2O3 (47) CuO (30) Silica (6–460) Al2O3 (110–210), Zirconia (110–250), SiO2 (20–40) ZnO (67.17) Bi2Te3 (2.5 and 10.4 nm) TiO2 (15)
0.3–1.2 0.1–10 0.1–10 0.1–10
Bubble pressure method Pendant drop method Pendant drop method Pendant drop method Bubble pressure method Pendant droplet method Pendant droplet method Pendant droplet method Teflon FEP strip Pendant drop method Wilhelmy method Sigma 703 tensiometer Ring method Sessile drop method Tensiometer
[25] [22] [22] [22] [19] [14] [14] [14] [26] [23] [27] [28] [20] [21] [15]
0.1– 2 0.04 – 0.05 0.05– 0.1 0.5– 4 2–8 0.0001– 0.1 0.0001– 0.1 1–3 4.5–14.5 0.1
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Fig. 3. Surface tension decreases with the increase of nanoparticle concentration.
Fig. 1. Surface tension of nanofluids increases with the increase of volume fraction.
0.3 wt.% with the augmentation of particle concentration of nanofluids. The results clearly show that at high particle concentrations, surface tension increased accordingly with particle concentration in all cases. However, at low particle concentrations surface tension trend was different according to the base fluid, particle type. Nevertheless, in a nutshell, it could be concluded that surface tension of nanofluids increases accordingly with the increase of nanoparticle concentration. Some contradictory results were also found regarding the effect of the volume fractions of nanofluids on surface tension. Fig. 3 shows the opposite trend as surface tension of nanofluids is decreasing with the increase of particle volume concentration. From Fig. 3, it is found that, the result from Jeong et al. [26] indicated that the surface tension of Al2O3/water nanofluid rapidly decreased up to 1 vol.% concentration, and afterwards decreased slightly. They found that surface tension decreased almost 20% for 4 vol.% concentration of nanoparticles. Similarly, Pantzali et al. [23] discovered that the surface tension of CuO/water nanofluids steadily decreased up to 2 vol.% of particle concentration, and remained unchanged for 3 vol.% to 4 vol.%. Furthermore, Chen et al. [14] found that the surface tension of laponite and distilled water constantly decreases with the increase of volume concentration. Okubo [27] investigated the surface tension of aqueous suspension of DIB76 (sphere d = 109 nm) and DIC27 (sphere d = 91 nm) and found that, surface tension of these fluids also decreased with the increase of particle concentration. Table 2 presents the surface tension increasing and decreasing rate of nanofluids with particle volume fraction.
3.2. Effect of temperature Although there are some contradictions that have been observed for the effect of the particle volume fractions on surface tension of nanofluids, however, the researchers agreed upon the fact that surface tension of nanofluids decreases with the increase of temperature. The relationship between the surface tension of nanofluid as a function of temperature has been plotted in Fig. 4. The results indicate that, surface tension of nanofluids decreases with the increase of temperature. Godson [25] found 10.29% and 8.55% decrement of surface tension for water and 1.2 vol.% concentration of silver/water nanofluids, respectively for the temperature range of 50 °C to 80 °C. Moreover, Murshed et al. [15] observed that, surface tension of TiO2/water and TiO2/oil nanofluids significantly decreases with the increase of temperature. Similarly, Goncalves et al. [29] found the same decreasing trend of surface tension with the increase of temperature for ethanol. 3.3. Effect of adding surfactant Surfactants are used to stabilize nanofluids and influence the surface tension of nanofluids. The effect of surfactant on the surface tension of nanofluid has been shown in Fig. 5. From Fig. 5, surface tension decreases clearly with the increase of concentration of surfactant. Kumar and Milanova [19] studied the effect of surfactant on surface tension of nanofluids for SWNTs/Water nanofluid with NaBDS surfactant and found that in the case of water and surfactants, surface tension initially decreased gradually 0.62 vol.% and became constant for 0.62 vol.% to 1.30 vol.%. However, when the SWNTs was added to DI water with surfactant, the surface tension was the same for low volume fraction (0.05 vol.% to 0.2 vol.%). After this range, it gradually decreased from 0.25 vol.% to 1.3 vol.% as most of the surfactant molecule was mixed with DI water. Similarly, in case of n-decane, 0.1 wt.% of Al/surfactant (Sorbitan Oleate) mixture showed that with the increase of the volume fractions of surfactant, the resulting surface tension of nanofluid would reduce shown in Fig. 6. As the long-chain surfactant molecules attached to the solid particle, they form a layer between the particle and the Table 2 The change in surface tension of nanofluids in terms of nanoparticle concentration.
Fig. 2. Surface tension of nanofluids increases accordingly with the increase of nanoparticle concentration [22].
Base fluid
Particle name (diameter in nm)
Particle fraction (vol./wt.) %
Surface tension increase (+) or decrease (−) %
Ref.
DIW W EG W DIW DIW W
Silver (b 100) CuO (30) ZnO (67.17) Al2O3 (110–210) Laponite (25–30) MWCNTs (8–15) Al2O3 (47)
0.3–1.2 vol. 2–8 vol. 1–3 vol. 0.001–0.1 vol. 0.1–2 vol. 0.1–10 wt. 0.5–4 vol.
+1.4 to +12.0 −29.167 to −47.22 +1.015 to +1.07 +0.27 to +4.17 −1.03 to −44.33 −0.14 to +7.36 −9.46 to −20.22
[25] [23] [20] [28] [14] [22] [26]
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113
Fig. 4. Surface tension variation as a function of temperature. Fig. 6. Surface tension variation with surfactant concentration [22].
surrounding fluid molecules. Such layers increase the potential between particles and impart a repulsive force between them. This in turn causes a reduction in surface tension [22]. 4. Theoretical studies The surface tension and interfacial tension among immiscible liquids need to be calculated accurately. For this reason, surface tension and interfacial tension are investigated and approximated through correlations. There are some established correlations available about the surface tension of mixtures. Murshed et al. [15] developed a model to determine the surface tension of two phase fluid based on the theory of Girifalco [30] which can be expressed as: pffiffiffiffiffiffiffiffiffiffiffi γ ab ¼ γ a þ γb −2Φ γ a γb
ð1Þ
where γa and γb are the surface tensions of phases a and b, respectively, and Φ is a constant, where the percentage of adhesion and cohesion energies is the same for both phases. The authors solved the Laplace– Young equation by using known boundary conditions and droplet parameters to find out the nanofluid surface tension. Scientifically, the Laplace–Young equation represents the equilibrium involving the surface tension force and the external forces exerted on the liquid droplet,
like gravity. Laplace–Young equation uses the separated interface in the fluids to describe the equilibrium state. This equation indicates that the pressure difference across the interface is equivalent to the product of the curvature multiplied by the gas–liquid surface tension [31]. Additionally, in order to determine the nanofluid effective surface tension of the gas–liquid, different droplet factors and conditions are used for the solution of Laplace–Young equation [21,31]. The Laplace– Young equation can be written as: d2 z dz 2 ρgz dr dr2 þ h i h dz2 32 dz2 i12 ¼ Rο þ σ g ln 1 þ dr r 1 þ dr
where σg ln is the effective gas–liquid nanofluid surface tension (Nm−1), ρ is the liquid density (kg m−3); Ro is the radius of curvature at origin (m); g is the acceleration of gravity (m s−2); R is the radius of the contact line (m); and z is the location point. In these calculations, the density of the nanofluid was assumed to be the same as the base liquid since even for the maximum particle concentration the density of the nanofluid solution was only ∼0.3% larger than the density of water. Based on the proof result, surface tensions of gas–liquid vary from ± 33% yields modify in the point of the peak between +3.6% and −5.4%. These changes were bigger than the experimental uncertainty of the top point (∼1%). Chen et al. [14] observed that the wetting ability of a liquid manifests itself through the equilibrium contact angle (h), which in turn can be related to surface tension through the Young's equation: σ vs −σ ls ¼ σ vl cosθ
Fig. 5. Surface tension of nanofluids decreases with the increase of surfactant concentration [19].
ð2Þ
ð3Þ
where, σ is the surface tension, subscripts v, s, and l stand for vapor, solid, and liquid, respectively. Due to the uniqueness of surface temporal and morphological characteristics, it is hard to find the values of σvs, and σls. For instance, there may be impurity, and changeable coarseness, etc. on the surface [32]. According to Eq. (3), if the value of θ decreases (i.e. augmented wettability and absolute micro layer), σvl will be reduced. For applications in boiling heat transfer, σvl needs to be specified to be able to obtain the surface tension. Value of surface tension is needed to find out the various things like as to learn about manners of magnetic liquids in plane layers. As recitation, the manners of magnetic stripe systems in the structure of the smectic correlation, curvature and compression flexibility constants depend on the magnetic ground and so on. These elastic modules can be determined from macroscopic experiments, and it is necessary to identify for an evaluation with the theoretical predictions. The
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customized Laplace law in order to take into the explanation the magnetic phenomena [33] is: p1 −p2 ¼
2 2 σ μ0 1 ∂ ζ ″ − →:→ with ¼ − 2 ¼ −ζ n M Rc Rc 2 ∂x
ð4Þ
where, σ is the surface tension between the fluids. Auge et al. [34] experimentally observed that it was possible to replace well-known flow rate Q by preset volume of a stalagmometer. In addition, probably droplet growing time T can be determined by the calibrated droplet chamber. Finally, the fluids surface tension (σ) can be find out from Eq. (5).
T¼
Vdrop 2πrkσ ¼ ρgQ Q
ð5Þ
whereas ρ, V, g, and r mean the liquid density, volume of the droplet, acceleration due to gravity and outer radius of the capillary, respectively. The calibration factor k (0 b k b 1) is influenced especially by the constriction radius of the droplet before falling down. Auge et al. [34] normalize the surface tension equation effect with flow rate of non-ionic surfactants added with base fluid as water has shown in Eq. (6). σ ¼ 47:08097 þ 0:03661Q þ 7:84281E
−5
Q
2
ð6Þ
However, no theoretical study has been reported on the interfacial phenomena and temperature-dependent surface tension of nanofluids [15]. 5. Conclusion Throughout this study, it could be summarized that volume fraction, temperature, and additive surfactant have significant effects over surface tension of nanofluids. The results indicate that surface tension of nanofluids increases with the increase of nanoparticle concentration. Besides this, there are some contradictory results indicating that, surface tension of nanofluids decreases with the increase of nanoparticle concentration. No existing model or correlation is capable of precise prediction of the surface tension enhancement or detraction with respect to volume fractions. In terms of temperature effect, researchers agree that surface tension of nanofluids decreases with the increase of temperature. Furthermore, surface tension of nanofluids decreases with the increase of surfactant concentration. However, in one case, an opposite result was found as surface tension of nanofluid increased accordingly with the addition of surfactant. More studies are required to confirm the effect of these parameters as there are ambiguities within the available results. Acknowledgment The authors would like to acknowledge the Ministry of Higher Education Malaysia (MoHE) and the University of Malaya for the financial support. This research was carried out under the UM MoHE High Impact Research Grant (HIRG) scheme (Project No. UM.C/HIR/MOHE/ENG/40). References [1] S.U.S. Choi, Developments and applications of non-Newtonian flows, ASME FED 231 (66) (1995) 99–103. [2] R.S. Vajjha, D.K. Das, A review and analysis on influence of temperature and concentration of nanofluids on thermophysical properties, heat transfer and pumping power, Int. J. Heat Mass Transf. 55 (15–16) (2012) 4063–4078. [3] M.R. Sohel, R. Saidur, M.F.M. Sabri, M. Kamalisarvestani, M.M. Elias, A. Ijam, Investigating the heat transfer performance and thermophysical properties of nanofluids in a circular micro-channel, Int. Commun. Heat Mass Transfer 42 (2013) 75–81.
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Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
Effect of particle concentration, temperature and surfactant on surface tension of nanofluids☆ S.S. Khaleduzzaman, I.M. Mahbubul ⁎, I.M. Shahrul, R. Saidur Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e
i n f o
Available online 5 November 2013 Keywords: Nanofluids Surface tension Volume fraction Temperature Surfactant
a b s t r a c t Heat transfer performance with nanofluids depends on the thermo physical properties of the suspension. Surface tension is an important property for heat transfer calculation. In this paper, various parameters that effect on the surface tension of nanofluids such as nanofluid preparation method, effect of volume fraction, temperature, and surfactants on nanofluids have been studied. Additionally, precise assessments on the theoretical correlations related to the surface tension of nanofluids have also been included. Based on the existing experimental results, surface tension augments respectively with volume fraction intensification. Surface tension of nanofluids decreases accordingly with the increase of temperature and surfactant concentration. Nevertheless, there have been some contradictory results on the effect of volume fraction and surfactant on surface tension of nanofluids. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Nanofluids are the solid–liquid mixture of nanoparticles into base fluid [1]. Nanofluids are prepared by dispersing nanometer-sized particles, generally less than 100 nm, in a base fluid such as water, ethylene glycol, propylene glycol, oil and other conventional heat transfer fluids. Addition of high thermal conductivity nanoparticles (e.g., copper, aluminum, silver, CNT) to the base fluid increases the thermal conductivity of such mixtures, thus enhancing their overall heat transfer capability. During the last decade, abundant experimental as well as numerical studies have been done to explore the advantages of nanofluids under a wide variety of conditions [2]. Most of these studies are related with the heat transfer performance [3–5], thermal conductivity [6,7], and viscosity of nanofluids [8–10]. Few review papers [11,12] have emphasized only on thermal conductivity and certain efforts have also been made in the review of viscosity of nanofluid [13]. Surface tension is also an important property when analyzing the performance of a thermal system. The competency of manipulating surface tension of a liquid has a wide range of useful applications, including enhanced boiling heat transfer, oil recovery efficiency, and the capability of cleaning oil spills [14,15]. Studies on surface tension of nanofluids are limited in the literature [16]. Surface tension of a liquid can be reduced by adding either surface active agents or nanoparticles [17]. Murshed et al. [18] and Kumar and Milanova [19] both showed that surface tension of carbon nanotube-based nanofluids was higher than that of water base fluid. Moosavi et al. [20] demonstrated that the surface tension of base fluid (ethylene glycol) increased by a little over 7% ☆ Communicated by Dr. W.J. Minkowycz ⁎ Corresponding author. E-mail address: [email protected] (I.M. Mahbubul). 0735-1933/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.icheatmasstransfer.2013.10.010
with the addition of 3.0 vol.% of ZnO nanoparticles. The opposite trend as decrement of surface tension for nanofluid has also been reported in literature. Murshed et al. [15] measured the surface tension of TiO2/ water nanofluid using a surface tensiometer. They found that the addition of TiO2 nanoparticles to water reduced the surface tension of the resulting nanofluid at room temperature. Vafaei et al. [21] measured the surface tension of Bi2Te3/water nanofluids and found that the surface tension decreased with the increase of particle concentration until it reached a minimum level and then increased with the increase of particle concentration [22]. In addition, the authors reported that, the nanofluid with 10.4 nm nanoparticle has a bigger value of surface tension than the nanofluid having 2.5 nm size particles at similar mass concentration [21]. Material burnout and critical heat transfer flux of the nanofluids surface tension were also affected by surfactants [19,22]. Kumar and Milanova [19] and Murshed et al. [18] showed that the addition of NaBDS (sodium dodecyl benzene sulfonate) surfactant reduces the surface tension of DI water and DI water based nanofluids with the addition of carbon nanotubes. However, Chen et al. [14] showed that the addition of PVP (Poly vinyl pyrrolidone) surfactants does not affect the surface tension of the nanofluid. They reported that, the addition of silver nanoparticles reduced the surface tension rather than the PVP surfactant in their experiment. Available literature confirms that, control of gas–liquid surface tension of nanofluids depends on concentration of nanoparticle as well as temperature and surfactants [21]. Based on the literature survey it is found that, most of the researches done mainly considering the effect of volume concentration on surface tension and some about effects of surfactants, yet along with some contradictions. No authors have discussed the effect of volume fraction, surfactants, and temperature over surface tension of nanofluid all together in one paper as a wide range of study. Only, Tanvir and Qiao [22]
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Nomenclature CNT Carbon nanotube D Diameter (m) DI Deionized Water DIW Distilled Water EG Ethylene glycol g Acceleration of gravity (m/s2) k Calibration factor MWCNTs Multi-walled carbon nanotubes NaBDS Sodium dodecyl benzenesulfonate Q Flow rate (ml/h) T Temperature (K) Ro Radius of curvature at origin (m) rd Radius of contact line (m) r Outer radius of the capillary (m) SWNTs Single-walled carbon nanotubes T Time (s) V Volume (m3) vol. Volume fraction W Water wt. Weight percentage of concentration
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of nanofluids. In the subsequent sections, measurement methods used by the researchers, experimental results concerning influence of volume fraction, temperature, and surfactants on surface tension, as well as theoretical models and correlations indicating surface tension in terms of volume concentration, temperature and particle diameter have been described consecutively. 2. Measurement methods Researchers have used different methods to measure the surface tension of nanofluids. Most authors have determined the surface tension of nanofluid by Pendant drop method [14,23]. A Rame-Hart Model 500 standard goniometer was used for real-time surface tension measurements [22]. The Pendant drop method was adapted to determine the surface tension of the suspended droplet. This method utilizes the Young–Laplace equation to determine the surface tension of the droplet based on the shape of the droplet [24]. Bubble pressure method [19,25] is also used for surface tension determination. Some authors used some other methods for surface tension measurement, like Teflon FEP strip [26], Wilhelmy method [27], Sigma 703 tensiometer [28], and Ring method [20]. Table 1 shows a list of experimental processes used by the researchers. 3. Experimental results and discussion
Greek symbols Φ Constant θ Contact angle (°) ρ Density (kg/m3) σ Surface tension (mN/m) σg ln Effective gas–liquid–nanoparticle surface tension (N/m) ζ Surface deformation
Subscripts L Liquid s Solid v Vapor
discussed the effects of diameter, concentration and surfactants over surface tension of nanofluids. Therefore, the need for a comprehensive criticism of all important parameters to provide researchers with sufficient information on surface tension of nanofluids is clearly felt. The objective of this paper is to provide adequate information about the effect of volume fraction, temperature, and surfactant on the surface tension
3.1. Effect of volume fraction The effects of volumetric concentration of nanofluids on the surface tension have been plotted in Fig. 1. The results indicate that surface tension depends on nanoparticle volume concentration, and it almost increases with the increase of volumetric fraction. From Fig. 1, it is found that, the result from the studies of Kim et al. [28] and Godson [25] showed that surface tension of nanofluid linearly increases with the increase of volume concentration of nanofluid. Furthermore, Moosavi et al. [20] found that the surface tension enhancement ratio is linearly increased with the increase of nanoparticle volume concentration for ZnO/EG nanofluids with the concentration of 0 to 3 vol.%. However, the results of Tanvir and Qiao [22] show the irregular trends as for the Al2O3/DIW nanofluid, the surface tension initially increased and then decreased and again began to increase and for MWCNTs/DIW nanofluid, surface tension initially decreased with the increase of particle concentration and then increased. Tanvir and Qiao [22] showed another trend of the effect of particle concentration over the surface tension of nanofluids among different nanoparticles with ethalon as base fluid. Fig. 2 shows surface tension of ethanol-based nanofluids (including Al, B, Al2O3, and MWCNTs nanoparticles). In the case of ethanol, surface tension increased up to
Table 1 Different experimental process used in the literatures about surface tension of nanofluids. Base fluid
Particle name (diameter in nm)
Particle fraction (vol./wt.) %
Experimental process
Ref.
DIW DIW Ethanol n-decane W DIW DIW DIW W W Air-W W EG Gas–Liquid W
Silver (b100) Al2O3 (25), MWCNTs (8–15) Al2O3 (25), Al (18), B (80) Al2O3 (25), Al (18), B (80) SWNTs Laponite (25–30) Silver (10–30) Fe2O3 (10–30) Al2O3 (47) CuO (30) Silica (6–460) Al2O3 (110–210), Zirconia (110–250), SiO2 (20–40) ZnO (67.17) Bi2Te3 (2.5 and 10.4 nm) TiO2 (15)
0.3–1.2 0.1–10 0.1–10 0.1–10
Bubble pressure method Pendant drop method Pendant drop method Pendant drop method Bubble pressure method Pendant droplet method Pendant droplet method Pendant droplet method Teflon FEP strip Pendant drop method Wilhelmy method Sigma 703 tensiometer Ring method Sessile drop method Tensiometer
[25] [22] [22] [22] [19] [14] [14] [14] [26] [23] [27] [28] [20] [21] [15]
0.1– 2 0.04 – 0.05 0.05– 0.1 0.5– 4 2–8 0.0001– 0.1 0.0001– 0.1 1–3 4.5–14.5 0.1
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Fig. 3. Surface tension decreases with the increase of nanoparticle concentration.
Fig. 1. Surface tension of nanofluids increases with the increase of volume fraction.
0.3 wt.% with the augmentation of particle concentration of nanofluids. The results clearly show that at high particle concentrations, surface tension increased accordingly with particle concentration in all cases. However, at low particle concentrations surface tension trend was different according to the base fluid, particle type. Nevertheless, in a nutshell, it could be concluded that surface tension of nanofluids increases accordingly with the increase of nanoparticle concentration. Some contradictory results were also found regarding the effect of the volume fractions of nanofluids on surface tension. Fig. 3 shows the opposite trend as surface tension of nanofluids is decreasing with the increase of particle volume concentration. From Fig. 3, it is found that, the result from Jeong et al. [26] indicated that the surface tension of Al2O3/water nanofluid rapidly decreased up to 1 vol.% concentration, and afterwards decreased slightly. They found that surface tension decreased almost 20% for 4 vol.% concentration of nanoparticles. Similarly, Pantzali et al. [23] discovered that the surface tension of CuO/water nanofluids steadily decreased up to 2 vol.% of particle concentration, and remained unchanged for 3 vol.% to 4 vol.%. Furthermore, Chen et al. [14] found that the surface tension of laponite and distilled water constantly decreases with the increase of volume concentration. Okubo [27] investigated the surface tension of aqueous suspension of DIB76 (sphere d = 109 nm) and DIC27 (sphere d = 91 nm) and found that, surface tension of these fluids also decreased with the increase of particle concentration. Table 2 presents the surface tension increasing and decreasing rate of nanofluids with particle volume fraction.
3.2. Effect of temperature Although there are some contradictions that have been observed for the effect of the particle volume fractions on surface tension of nanofluids, however, the researchers agreed upon the fact that surface tension of nanofluids decreases with the increase of temperature. The relationship between the surface tension of nanofluid as a function of temperature has been plotted in Fig. 4. The results indicate that, surface tension of nanofluids decreases with the increase of temperature. Godson [25] found 10.29% and 8.55% decrement of surface tension for water and 1.2 vol.% concentration of silver/water nanofluids, respectively for the temperature range of 50 °C to 80 °C. Moreover, Murshed et al. [15] observed that, surface tension of TiO2/water and TiO2/oil nanofluids significantly decreases with the increase of temperature. Similarly, Goncalves et al. [29] found the same decreasing trend of surface tension with the increase of temperature for ethanol. 3.3. Effect of adding surfactant Surfactants are used to stabilize nanofluids and influence the surface tension of nanofluids. The effect of surfactant on the surface tension of nanofluid has been shown in Fig. 5. From Fig. 5, surface tension decreases clearly with the increase of concentration of surfactant. Kumar and Milanova [19] studied the effect of surfactant on surface tension of nanofluids for SWNTs/Water nanofluid with NaBDS surfactant and found that in the case of water and surfactants, surface tension initially decreased gradually 0.62 vol.% and became constant for 0.62 vol.% to 1.30 vol.%. However, when the SWNTs was added to DI water with surfactant, the surface tension was the same for low volume fraction (0.05 vol.% to 0.2 vol.%). After this range, it gradually decreased from 0.25 vol.% to 1.3 vol.% as most of the surfactant molecule was mixed with DI water. Similarly, in case of n-decane, 0.1 wt.% of Al/surfactant (Sorbitan Oleate) mixture showed that with the increase of the volume fractions of surfactant, the resulting surface tension of nanofluid would reduce shown in Fig. 6. As the long-chain surfactant molecules attached to the solid particle, they form a layer between the particle and the Table 2 The change in surface tension of nanofluids in terms of nanoparticle concentration.
Fig. 2. Surface tension of nanofluids increases accordingly with the increase of nanoparticle concentration [22].
Base fluid
Particle name (diameter in nm)
Particle fraction (vol./wt.) %
Surface tension increase (+) or decrease (−) %
Ref.
DIW W EG W DIW DIW W
Silver (b 100) CuO (30) ZnO (67.17) Al2O3 (110–210) Laponite (25–30) MWCNTs (8–15) Al2O3 (47)
0.3–1.2 vol. 2–8 vol. 1–3 vol. 0.001–0.1 vol. 0.1–2 vol. 0.1–10 wt. 0.5–4 vol.
+1.4 to +12.0 −29.167 to −47.22 +1.015 to +1.07 +0.27 to +4.17 −1.03 to −44.33 −0.14 to +7.36 −9.46 to −20.22
[25] [23] [20] [28] [14] [22] [26]
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Fig. 4. Surface tension variation as a function of temperature. Fig. 6. Surface tension variation with surfactant concentration [22].
surrounding fluid molecules. Such layers increase the potential between particles and impart a repulsive force between them. This in turn causes a reduction in surface tension [22]. 4. Theoretical studies The surface tension and interfacial tension among immiscible liquids need to be calculated accurately. For this reason, surface tension and interfacial tension are investigated and approximated through correlations. There are some established correlations available about the surface tension of mixtures. Murshed et al. [15] developed a model to determine the surface tension of two phase fluid based on the theory of Girifalco [30] which can be expressed as: pffiffiffiffiffiffiffiffiffiffiffi γ ab ¼ γ a þ γb −2Φ γ a γb
ð1Þ
where γa and γb are the surface tensions of phases a and b, respectively, and Φ is a constant, where the percentage of adhesion and cohesion energies is the same for both phases. The authors solved the Laplace– Young equation by using known boundary conditions and droplet parameters to find out the nanofluid surface tension. Scientifically, the Laplace–Young equation represents the equilibrium involving the surface tension force and the external forces exerted on the liquid droplet,
like gravity. Laplace–Young equation uses the separated interface in the fluids to describe the equilibrium state. This equation indicates that the pressure difference across the interface is equivalent to the product of the curvature multiplied by the gas–liquid surface tension [31]. Additionally, in order to determine the nanofluid effective surface tension of the gas–liquid, different droplet factors and conditions are used for the solution of Laplace–Young equation [21,31]. The Laplace– Young equation can be written as: d2 z dz 2 ρgz dr dr2 þ h i h dz2 32 dz2 i12 ¼ Rο þ σ g ln 1 þ dr r 1 þ dr
where σg ln is the effective gas–liquid nanofluid surface tension (Nm−1), ρ is the liquid density (kg m−3); Ro is the radius of curvature at origin (m); g is the acceleration of gravity (m s−2); R is the radius of the contact line (m); and z is the location point. In these calculations, the density of the nanofluid was assumed to be the same as the base liquid since even for the maximum particle concentration the density of the nanofluid solution was only ∼0.3% larger than the density of water. Based on the proof result, surface tensions of gas–liquid vary from ± 33% yields modify in the point of the peak between +3.6% and −5.4%. These changes were bigger than the experimental uncertainty of the top point (∼1%). Chen et al. [14] observed that the wetting ability of a liquid manifests itself through the equilibrium contact angle (h), which in turn can be related to surface tension through the Young's equation: σ vs −σ ls ¼ σ vl cosθ
Fig. 5. Surface tension of nanofluids decreases with the increase of surfactant concentration [19].
ð2Þ
ð3Þ
where, σ is the surface tension, subscripts v, s, and l stand for vapor, solid, and liquid, respectively. Due to the uniqueness of surface temporal and morphological characteristics, it is hard to find the values of σvs, and σls. For instance, there may be impurity, and changeable coarseness, etc. on the surface [32]. According to Eq. (3), if the value of θ decreases (i.e. augmented wettability and absolute micro layer), σvl will be reduced. For applications in boiling heat transfer, σvl needs to be specified to be able to obtain the surface tension. Value of surface tension is needed to find out the various things like as to learn about manners of magnetic liquids in plane layers. As recitation, the manners of magnetic stripe systems in the structure of the smectic correlation, curvature and compression flexibility constants depend on the magnetic ground and so on. These elastic modules can be determined from macroscopic experiments, and it is necessary to identify for an evaluation with the theoretical predictions. The
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customized Laplace law in order to take into the explanation the magnetic phenomena [33] is: p1 −p2 ¼
2 2 σ μ0 1 ∂ ζ ″ − →:→ with ¼ − 2 ¼ −ζ n M Rc Rc 2 ∂x
ð4Þ
where, σ is the surface tension between the fluids. Auge et al. [34] experimentally observed that it was possible to replace well-known flow rate Q by preset volume of a stalagmometer. In addition, probably droplet growing time T can be determined by the calibrated droplet chamber. Finally, the fluids surface tension (σ) can be find out from Eq. (5).
T¼
Vdrop 2πrkσ ¼ ρgQ Q
ð5Þ
whereas ρ, V, g, and r mean the liquid density, volume of the droplet, acceleration due to gravity and outer radius of the capillary, respectively. The calibration factor k (0 b k b 1) is influenced especially by the constriction radius of the droplet before falling down. Auge et al. [34] normalize the surface tension equation effect with flow rate of non-ionic surfactants added with base fluid as water has shown in Eq. (6). σ ¼ 47:08097 þ 0:03661Q þ 7:84281E
−5
Q
2
ð6Þ
However, no theoretical study has been reported on the interfacial phenomena and temperature-dependent surface tension of nanofluids [15]. 5. Conclusion Throughout this study, it could be summarized that volume fraction, temperature, and additive surfactant have significant effects over surface tension of nanofluids. The results indicate that surface tension of nanofluids increases with the increase of nanoparticle concentration. Besides this, there are some contradictory results indicating that, surface tension of nanofluids decreases with the increase of nanoparticle concentration. No existing model or correlation is capable of precise prediction of the surface tension enhancement or detraction with respect to volume fractions. In terms of temperature effect, researchers agree that surface tension of nanofluids decreases with the increase of temperature. Furthermore, surface tension of nanofluids decreases with the increase of surfactant concentration. However, in one case, an opposite result was found as surface tension of nanofluid increased accordingly with the addition of surfactant. More studies are required to confirm the effect of these parameters as there are ambiguities within the available results. Acknowledgment The authors would like to acknowledge the Ministry of Higher Education Malaysia (MoHE) and the University of Malaya for the financial support. This research was carried out under the UM MoHE High Impact Research Grant (HIRG) scheme (Project No. UM.C/HIR/MOHE/ENG/40). References [1] S.U.S. Choi, Developments and applications of non-Newtonian flows, ASME FED 231 (66) (1995) 99–103. [2] R.S. Vajjha, D.K. Das, A review and analysis on influence of temperature and concentration of nanofluids on thermophysical properties, heat transfer and pumping power, Int. J. Heat Mass Transf. 55 (15–16) (2012) 4063–4078. [3] M.R. Sohel, R. Saidur, M.F.M. Sabri, M. Kamalisarvestani, M.M. Elias, A. Ijam, Investigating the heat transfer performance and thermophysical properties of nanofluids in a circular micro-channel, Int. Commun. Heat Mass Transfer 42 (2013) 75–81.
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