- Email: [email protected]
Preparation and rheological behavior of ethylene glycol-based TiO2 nanofluids
Colloids and Surfaces A: Physicochem. Eng. Aspects 509 (2016) 86–90
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Preparation and rheological behavior of ethylene glycol-based TiO2 nanofluids Ruiwen Shu ∗ , Ying Gan, Haoyuan Lv, Dexin Tan School of Chemical Engineering, Anhui University of Science and Technology, Huainan 232001, PR China
h i g h l i g h t s
g r a p h i c a l
• The stable ethylene glycol-based TiO2
EG/TiO2 nanofluids were prepared and the effects of particle concentration and temperature on their rheological behavior were explored.
(EG/TiO2 ) nanofluids were prepared by dispersing TiO2 nanoparticles into the base fluid of EG via ultrasonication. • A Newtonian-pseudoplastic transition with a critical exponent of 3.2 was firstly observed in EG/TiO2 nanofluids by shear rheology. • EG/TiO2 nanofluids showed remarkably enhanced viscosity compared with the base fluid of EG, and the nanofluids showed a characteristic shear thinning behavior particularly at particle concentration in excess of ∼5 wt%. • Temperature had a remarkable influence on the nonlinear rheological behavior of the concentrated EG/TiO2 nanofluids.
a r t i c l e
i n f o
Article history: Received 25 May 2016 Received in revised form 29 August 2016 Accepted 31 August 2016 Available online 31 August 2016 Keywords: TiO2 Ethylene glycol Nanofluids Rheological behavior
a b s t r a c t
a b s t r a c t In this work, the ethylene glycol-based TiO2 (EG/TiO2 ) nanofluids were prepared by dispersing TiO2 nanoparticles into the base fluid of EG via ultrasonication. The rheological behavior of EG/TiO2 nanofluids over 25–45 ◦ C and particle weight concentration of 0–35 wt% were investigated. It was found that the steady state shear viscosity of the nanofluids was remarkably enhanced compared with the base fluid of EG. Moreover, the nanofluids showed a characteristic shear thinning behavior particularly at particle concentrations in excess of ∼5 wt%. Significantly, a Newtonian-pseudoplastic transition with a critical exponent of 3.2 was firstly observed as particle concentration increased, suggesting that the nanofluids had entered into the semi-diluted regime and some TiO2 nanoparticles were assembled into agglomerations. In addition, the results of shear viscosity curves and thixotropic loops at different temperature demonstrated that temperature had a remarkable influence on the nonlinear rheological behavior of the concentrated nanofluids. Therefore, the present study could shed light on the processing and application of TiO2 -related nanofluids and nanocomposites. © 2016 Elsevier B.V. All rights reserved.
1. Introduction ∗ Corresponding author. E-mail address: [email protected] (R. Shu). http://dx.doi.org/10.1016/j.colsurfa.2016.08.091 0927-7757/© 2016 Elsevier B.V. All rights reserved.
Nanofluids, which are suspensions of nanosized particles commonly in a Newtonian base fluid like water, ethylene glycol,
R. Shu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 509 (2016) 86–90
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Fig. 1. XRD pattern (a) and TEM image (b) of TiO2 particles.
propylene glycol and engine oil [1–3]. The nanofluids are proposed as the next generation heat transfer media due to the fact that their thermal transport capacities are significantly higher than those of the base liquids [2]. Nanofluids have attracted great interest owing to their potential benefits for numerous applications, such as microelectronics, energy supply and transportation [4]. Most of the studies have focused on the heat transfer behavior of the nanofluids [5]. However, only a few studies have involved in their rheological behavior compared to those on the thermal properties [5]. It is well-known that the rheological characterization can provide much valuable information on the processing of materials. Thus, the rheological behavior is critical in ensuring that the nanofluids function as expected at the operational conditions for practical application [3,6]. Therefore, it is necessary to explore the rheological properties of the nanofluids. Numerous studies have been devoted to the nanofluids containing titanium dioxide (TiO2 ) due to its high stability, low cost and environment-friendly nature [7]. Because of the higher thermal conductivity and stability of the nanofluids than those of the traditional heat transfer fluids, it is found that TiO2 -related nanofluids can be also used as heat transfer fluids instead of conventional working fluids include water, ethylene glycol and oil [8,9]. Rheological properties of the TiO2 -related nanofluids has been extensively investigated in terms of shear flow curve and dynamic frequency sweep curve [3,5,10–15]. However, to the best of our knowledge, there are few reports on the Newtonian-pseudoplastic transition and temperature-dependent thixotropy of the ethylene glycolbased TiO2 (EG/TiO2 ) nanofluids so far. In the present study, we have prepared a series of stable ethylene glycol-based nanofluids containing TiO2 nanoparticles with a particle weight concentration (w) of 0–35 wt% by ultrasonication. Moreover, the effects of the particle concentration and temperature on the rheological properties (steady state shear viscosity, yield stress and thixotropy) were investigated.
2.2. Preparation of EG/TiO2 nanofluids The TiO2 powders were fully dispersed in the base fluid of EG by ultrasonication for 1 h and magnetically stirred for another 1 h. The weight concentration of TiO2 in EG/TiO2 nanofluids was changed from 0 to 35 wt%. No dispersant was used in the suspensions.
2.3. Characterizations X-ray diffraction (XRD) measurements were performed on a LabX XRD-6000 (Shimadzu, Japan) with a scan rate of 2◦ /min in the range of 2 = 10–60◦ , using Cu-Ka radiation ( = 0.154 nm) at room temperature. The morphological analysis of the TiO2 powders was performed with a field emission transmission electron microscopy (FETEM, JEM-2100F, Japan).
2.4. Rheological measurements Rheological measurements were carried out at 25–45 ◦ C with a stress-controlled rheometer AR-G2 (TA, USA) using a cone and plate geometry with diameter of 25 mm and cone angle of 2◦ . In the steady state shear measurements, the viscosity () was monitored . as a function of shear rate ( ) ranging from 0.001 to 1000 s−1 . The shear rate was increased continuously within the 60 s integration time. Thixotropic behavior were measured as follows: flow curves . (shear stress () vs. shear rate ( )) are measured with an up and down shear rate sweep, i.e. by increasing or decreasing the shear rate logarithmically with a waiting time of 5 s for each data point. All samples were allowed to equilibrate for 15 mins after loading into the rheometer prior to each measurement.
3. Results and discussion 3.1. Structure and morphology analysis 2. Experimental section 2.1. Materials TiO2 powders were purchased from Shanghai jiang Hu titanium dioxide chemical Co. Ltd., and were used after dried at 60 ◦ C in vacuum for 24 h. Ethylene glycol was analytical grade and used as received without further purification, which was purchased from Sinopharm Chemical Reagent Co. Ltd.
The XRD pattern of TiO2 particles are presented in Fig. 1(a). It is clearly showed that the characteristic diffraction peaks appear at 25.6◦ , 38.2◦ , 48.4◦ , 54.2◦ , 55.3◦ , 63.0◦ , 68.8◦ and 75.2◦ , corresponding to (101), (004), (200), (105), (211), (204), (116) and (220) crystal plane of anatase-TiO2 (JCPDS 21-1272) [16]. The micromorphology of TiO2 powders was observed by TEM, as shown in Fig. 1(b). It can be found that the TiO2 exhibits irregularly flake-like shape and has a lateral dimension in the range of 40–200 nm.
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Fig. 2. Steady state shear viscosity curves of the EG/TiO2 nanofluids for different particle concentrations at 25 ◦ C.
Fig. 3. Zero shear viscosity (0 ) of the EG/TiO2 nanofluids as a function of particle concentration (w) at 25 ◦ C. The horizontal and vertical dashed lines indicate the viscosity of EG ( ≈ 0.012 Pa s) and critical concentration (wc = 5 wt%), respectively. The red dashed line denotes a logarithmic slope of 3.2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.2. Newtonian-pseudoplastic transition The effect of concentration of TiO2 nanoparticles on the flow behavior of EG/TiO2 nanofluids was investigated by steady state shear rheology. Fig. 2 shows the shear viscosity curves of EG/TiO2 nanofluids with different particle concentrations at 25 ◦ C. For w ≤ 5 wt%, the viscosity increases slightly compared with the base fluid of EG (w = 0 wt%). However, as w furtherly increases, the EG/TiO2 nanofluids change from Newtonian fluids to pseudoplastic fluids. For 7 wt% ≤ w ≤ 25 wt%, the whole shear viscosity curves can be clearly classified into three regions, which were separated by red dashed lines in Fig. 2: the first Newton region (I), the shear thinning region (II), and the second Newton region (III). The typical shear thinning behavior at the middle shear rate region can be attributed to the TiO2 nanoparticles in EG nanofluids align under the influence of shear flow field [5]. However, as w > 25 wt%, only the shear thinning region (II) can be observed. In order to determine the critical concentration (wc ) at which the Newtonian-pseudoplastic transition occurs, the zero shear viscosity (0 ) at 25 ◦ C is plotted against w, as depicted in Fig. 3. The 0 is defined as the viscosity at the low shear Newton plateau region. At the critical transition point, the 0 increases dramatically. The 0 changes little as w ≤ 5 wt%, while starts to increase dramatically beyond 5 wt% with 0 –w3.2 . Therefore, a significant Newtonian-pseudoplastic transition in the nanofluids is found with the increase of particle concentration. A power exponent of 3.2 (0 –w3.2 ) is observed from Fig. 3, which indicates the nanofluids
.
Fig. 4. Shear stress () versus shear rate ( ) of the EG/TiO2 nanofluids for different particle concentrations at 25 ◦ C.
Fig. 5. Yield stress (y ) versus particle concentration (w) of EG/TiO2 nanofluids at 25 ◦ C.
have entered into the semi-diluted regime and some TiO2 nanoparticles are assembled into agglomerations. The power exponent could be related to the fractal dimension (df ) of the agglomerations of flake-like TiO2 . Similar results have been reported in a semidilute solution of stiff poly(␥-benzyl lglutamate) by Larson et al. [17] and silicone oil dispersion of graphene oxide/polyaniline (GO/PANI) in our previous work [18]. In addition, the viscosity of the EG/TiO2 nanofluids (0 = 3.945 Pa s) is remarkably larger than that of the base fluid of EG ( ≈ 0.012 Pa s), i.e. 329 times by filling with only 8.46 w/v% TiO2 nanoparticles if adopts the calculation method introduced by Yapici et al. [3]. Therefore, the flake-like TiO2 is an effective filler to enhance the viscosity of the base fluid of EG. Fig. 4 shows the shear stress () of the nanofluids as a func. tion of shear rate ( ) with different particle concentrations at 25 ◦ C. . . For w ≤ 5 wt%, a linear relationship between and ( ∼ 1 ) is observed, indicating a Newtonian flow behavior. For w > 5 wt%, the . yield stress ( y ) can be obtained by fitting the ∼ curves with . Herschel-Bulkley equation ( = y + n ) [19,20]. The y is plotted against w at 25 ◦ C, as shown in Fig. 5. It can be found that the y enlarges with the increase of the w. Therefore, the concentrated EG/TiO2 nanofluids are typical yield stress fluids. 3.3. Temperature-dependent nonlinear rheological behavior In order to explore the effect of temperature on the nonlinear rheological behavior of EG/TiO2 nanofluids, we performed steady state shear and thixotropy tests at different temperature (T = 25, 35 and 45 ◦ C). The EG is used as a control test. Fig. 6 shows the
R. Shu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 509 (2016) 86–90
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Fig. 6. Shear rate dependency of viscosity as a function of temperature for the base fluid of EG (a) and 35 wt% EG/TiO2 nanofluids (b).
increased, suggesting that the nanofluids had entered into the semidiluted regime and some TiO2 nanoparticles were assembled into agglomerations. In addition, the results of thixotropic loops and shear viscosity demonstrated that the temperature had a significant influence on the nonlinear rheological behavior of the concentrated nanofluids. It is believed that the study of rheological behavior of EG/TiO2 nanofluids could be beneficial to understand the processing and application of TiO2 -related nanofluids and nanocomposites. Acknowledgements
Fig. 7. Thixotropic behavior of 35 wt% EG/TiO2 nanofluids at different temperature.
steady state viscosity curves of the base fluid of EG and 35 wt% EG/TiO2 nanofluids at different temperature. From Fig. 6(a), it can be found that the base fluid of EG is a kind of typical Newtonian fluid and the viscosity significantly decreases with the increasing of temperature due to the thermal motion of liquid molecules aggravates. However, the concentrated EG/TiO2 nanofluids exhibit significant shear thinning behavior at all tested temperature. We also notice that the temperature has small effect on the viscosity of the nanofluids compared with the base fluid of EG, i.e. the thermal stability of the nanofluids enhances. Thixotropy is a very important rheological property because it can reflect the time-dependent evolution of structure in particle filled suspensions [21]. As we know, the thixotropy can be characterized by the appearance of hysteresis loops in up and down shear rate sweep curves. Fig. 7 depicts the thixotropic loops of 35 wt% EG/TiO2 nanofluids for different temperature. It is obviously found that the nanofluids present distinct thixotropy at all tested temperature. As the temperature elevates, the area of thixotropic loops slightly increases, suggesting the thixotropy enhances. The results of Figs. 6 and 7 demonstrate that temperature has a significant influence on the nonlinear flow behavior of EG/TiO2 nanofluids. 4. Conclusions The effects of the particle concentration and temperature on the rheological behavior of EG/TiO2 nanofluids were investigated. The steady state shear results showed that the viscosity of the nanofluids remarkably enhanced (329 times with a filled volume fraction of 8.46 w/v%) compared with the base fluid of EG, and the concentrated nanofluids exhibited characteristic shear thinning behavior. Significantly, a Newtonian-pseudoplastic transition with a critical exponent of 3.2 was firstly observed as particle concentration
This work was financially supported by the National Natural Science Foundation of China (Grant No. 51507003), the National Training Program of Innovation and Entrepreneurship for Undergraduates (Grant No. 201510361069), the Doctor’s Start-up Research Foundation of Anhui University of Science and Technology (Grant No. ZY537). References [1] I.M. Mahbubul, R. Saidur, M.A. Amalina, Latest developments on the viscosity of nanofluids, Int. J. Heat Mass Transfer 55 (2012) 874–885. [2] W. Yu, H. Xie, L. Chen, Y. Li, Investigation of thermal conductivity and viscosity of ethylene glycol based ZnO nanofluid, Thermochim. Acta 491 (2009) 92–96. [3] K. Yapici, N.K. Cakmak, N. Ilhan, Y. Uludag, Rheological characterization of polyethylene glycol based TiO2 nanofluids, Korea Aust. Rheol. J. 26 (2014) 355–363. [4] H. Xie, J. Wang, T. Xi, Y. Liu, F. Ai, Q. Wu, Thermal conductivity enhancement of suspensions containing nanosized alumina particles, J. Appl. Phys. 91 (2002) 4568–4572. [5] H. Chen, Y. Ding, A. Lapkin, X. Fan, Rheological behaviour of ethylene glycol-titanate nanotube nanofluids, J. Nanopart. Res. 11 (2009) 1513–1520. [6] K.V. Wong, O. De Leon, Applications of nanofluids: current and future, Adv. Mech. Eng. 2 (2010) 519659. [7] A. Ghadimi, I.H. Metselaar, The influence of surfactant and ultrasonic processing on improvement of stability, thermal conductivity and viscosity of titania nanofluid, Exp. Therm. Fluid Sci. 51 (2013) 1–9. [8] M. Vakili, A. Mohebbi, H. Hashemipour, Experimental study on convective heat transfer of TiO2 nanofluids, Heat Mass Transfer 49 (2013) 1159–1165. [9] W. Duangthongsuk, S. Wongwises, Heat transfer enhancement and pressure drop characteristics of TiO2 –water nanofluid in a double-tube counter flow heat exchanger, Int. J. Heat Mass Transfer 52 (2009) 2059–2067. ˜ [10] D. Cabaleiro, M.J. Pastoriza-Gallego, C. Gracia-Fernández, M.M. Pineiro, L. Lugo, Rheological and volumetric properties of TiO2 -ethylene glycol nanofluids, Nanoscale Res. Lett. 8 (2013) 1–13. [11] S. Bobbo, L. Fedele, A. Benetti, L. Colla, M. Fabrizio, C. Pagura, S. Barison, Viscosity of water based SWCNH and TiO2 nanofluids, Exp. Therm. Fluid Sci. 36 (2012) 65–71. [12] V. Penkavova, J. Tihon, O. Wein, Stability and rheology of dilute TiO2 -water nanofluids, Nanoscale Res. Lett. 6 (2011) 1–7. [13] Y. He, Y. Jin, H. Chen, Y. Ding, D. Cang, H. Lu, Heat transfer and flow behaviour of aqueous suspensions of TiO2 nanoparticles (nanofluids) flowing upward through a vertical pipe, Int. J. Heat Mass Transfer 50 (2007) 2272–2281. [14] H. Chen, Y. Ding, Y. He, C. Tan, Rheological behaviour of ethylene glycol based titania nanofluids, Chem. Phys. Lett. 444 (2007) 333–337. [15] W.J. Tseng, K.-C. Lin, Rheology and colloidal structure of aqueous TiO2 nanoparticle suspensions, Mater. Sci. Eng. A Struct. 355 (2003) 186–192.
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[16] H.G. Yang, H.C. Zeng, Preparation of hollow anatase TiO2 nanospheres via Ostwald ripening, J. Phys. Chem. B 108 (2004) 3492–3495. [17] D. Mead, R. Larson, Rheooptical study of isotropic solutions of stiff polymers, Macromolecules 23 (1990) 2524–2533. [18] Q. Yin, R. Shu, H. Xing, D. Tan, Y. Gan, G. Xu, Rheological behavior and electrical properties of graphene oxide/polyaniline nanocomposites, Nano 11 (2016) 1650020. [19] N. Koumakis, A. Pamvouxoglou, A. Poulos, G. Petekidis, Direct comparison of the rheology of model hard and soft particle glasses, Soft Matter 8 (2012) 4271–4284.
[20] R. Shu, Q. Yin, H. Xing, D. Tan, Y. Gan, G. Xu, Colloidal and rheological behavior of aqueous graphene oxide dispersions in the presence of poly (ethylene glycol), Coll. Surf. A 488 (2016) 154–161. [21] K. Vasu, R. Krishnaswamy, S. Sampath, A. Sood, Yield stress, thixotropy and shear banding in a dilute aqueous suspension of few layer graphene oxide platelets, Soft Matter 9 (2013) 5874–5882.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Preparation and rheological behavior of ethylene glycol-based TiO2 nanofluids Ruiwen Shu ∗ , Ying Gan, Haoyuan Lv, Dexin Tan School of Chemical Engineering, Anhui University of Science and Technology, Huainan 232001, PR China
h i g h l i g h t s
g r a p h i c a l
• The stable ethylene glycol-based TiO2
EG/TiO2 nanofluids were prepared and the effects of particle concentration and temperature on their rheological behavior were explored.
(EG/TiO2 ) nanofluids were prepared by dispersing TiO2 nanoparticles into the base fluid of EG via ultrasonication. • A Newtonian-pseudoplastic transition with a critical exponent of 3.2 was firstly observed in EG/TiO2 nanofluids by shear rheology. • EG/TiO2 nanofluids showed remarkably enhanced viscosity compared with the base fluid of EG, and the nanofluids showed a characteristic shear thinning behavior particularly at particle concentration in excess of ∼5 wt%. • Temperature had a remarkable influence on the nonlinear rheological behavior of the concentrated EG/TiO2 nanofluids.
a r t i c l e
i n f o
Article history: Received 25 May 2016 Received in revised form 29 August 2016 Accepted 31 August 2016 Available online 31 August 2016 Keywords: TiO2 Ethylene glycol Nanofluids Rheological behavior
a b s t r a c t
a b s t r a c t In this work, the ethylene glycol-based TiO2 (EG/TiO2 ) nanofluids were prepared by dispersing TiO2 nanoparticles into the base fluid of EG via ultrasonication. The rheological behavior of EG/TiO2 nanofluids over 25–45 ◦ C and particle weight concentration of 0–35 wt% were investigated. It was found that the steady state shear viscosity of the nanofluids was remarkably enhanced compared with the base fluid of EG. Moreover, the nanofluids showed a characteristic shear thinning behavior particularly at particle concentrations in excess of ∼5 wt%. Significantly, a Newtonian-pseudoplastic transition with a critical exponent of 3.2 was firstly observed as particle concentration increased, suggesting that the nanofluids had entered into the semi-diluted regime and some TiO2 nanoparticles were assembled into agglomerations. In addition, the results of shear viscosity curves and thixotropic loops at different temperature demonstrated that temperature had a remarkable influence on the nonlinear rheological behavior of the concentrated nanofluids. Therefore, the present study could shed light on the processing and application of TiO2 -related nanofluids and nanocomposites. © 2016 Elsevier B.V. All rights reserved.
1. Introduction ∗ Corresponding author. E-mail address: [email protected] (R. Shu). http://dx.doi.org/10.1016/j.colsurfa.2016.08.091 0927-7757/© 2016 Elsevier B.V. All rights reserved.
Nanofluids, which are suspensions of nanosized particles commonly in a Newtonian base fluid like water, ethylene glycol,
R. Shu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 509 (2016) 86–90
87
Fig. 1. XRD pattern (a) and TEM image (b) of TiO2 particles.
propylene glycol and engine oil [1–3]. The nanofluids are proposed as the next generation heat transfer media due to the fact that their thermal transport capacities are significantly higher than those of the base liquids [2]. Nanofluids have attracted great interest owing to their potential benefits for numerous applications, such as microelectronics, energy supply and transportation [4]. Most of the studies have focused on the heat transfer behavior of the nanofluids [5]. However, only a few studies have involved in their rheological behavior compared to those on the thermal properties [5]. It is well-known that the rheological characterization can provide much valuable information on the processing of materials. Thus, the rheological behavior is critical in ensuring that the nanofluids function as expected at the operational conditions for practical application [3,6]. Therefore, it is necessary to explore the rheological properties of the nanofluids. Numerous studies have been devoted to the nanofluids containing titanium dioxide (TiO2 ) due to its high stability, low cost and environment-friendly nature [7]. Because of the higher thermal conductivity and stability of the nanofluids than those of the traditional heat transfer fluids, it is found that TiO2 -related nanofluids can be also used as heat transfer fluids instead of conventional working fluids include water, ethylene glycol and oil [8,9]. Rheological properties of the TiO2 -related nanofluids has been extensively investigated in terms of shear flow curve and dynamic frequency sweep curve [3,5,10–15]. However, to the best of our knowledge, there are few reports on the Newtonian-pseudoplastic transition and temperature-dependent thixotropy of the ethylene glycolbased TiO2 (EG/TiO2 ) nanofluids so far. In the present study, we have prepared a series of stable ethylene glycol-based nanofluids containing TiO2 nanoparticles with a particle weight concentration (w) of 0–35 wt% by ultrasonication. Moreover, the effects of the particle concentration and temperature on the rheological properties (steady state shear viscosity, yield stress and thixotropy) were investigated.
2.2. Preparation of EG/TiO2 nanofluids The TiO2 powders were fully dispersed in the base fluid of EG by ultrasonication for 1 h and magnetically stirred for another 1 h. The weight concentration of TiO2 in EG/TiO2 nanofluids was changed from 0 to 35 wt%. No dispersant was used in the suspensions.
2.3. Characterizations X-ray diffraction (XRD) measurements were performed on a LabX XRD-6000 (Shimadzu, Japan) with a scan rate of 2◦ /min in the range of 2 = 10–60◦ , using Cu-Ka radiation ( = 0.154 nm) at room temperature. The morphological analysis of the TiO2 powders was performed with a field emission transmission electron microscopy (FETEM, JEM-2100F, Japan).
2.4. Rheological measurements Rheological measurements were carried out at 25–45 ◦ C with a stress-controlled rheometer AR-G2 (TA, USA) using a cone and plate geometry with diameter of 25 mm and cone angle of 2◦ . In the steady state shear measurements, the viscosity () was monitored . as a function of shear rate ( ) ranging from 0.001 to 1000 s−1 . The shear rate was increased continuously within the 60 s integration time. Thixotropic behavior were measured as follows: flow curves . (shear stress () vs. shear rate ( )) are measured with an up and down shear rate sweep, i.e. by increasing or decreasing the shear rate logarithmically with a waiting time of 5 s for each data point. All samples were allowed to equilibrate for 15 mins after loading into the rheometer prior to each measurement.
3. Results and discussion 3.1. Structure and morphology analysis 2. Experimental section 2.1. Materials TiO2 powders were purchased from Shanghai jiang Hu titanium dioxide chemical Co. Ltd., and were used after dried at 60 ◦ C in vacuum for 24 h. Ethylene glycol was analytical grade and used as received without further purification, which was purchased from Sinopharm Chemical Reagent Co. Ltd.
The XRD pattern of TiO2 particles are presented in Fig. 1(a). It is clearly showed that the characteristic diffraction peaks appear at 25.6◦ , 38.2◦ , 48.4◦ , 54.2◦ , 55.3◦ , 63.0◦ , 68.8◦ and 75.2◦ , corresponding to (101), (004), (200), (105), (211), (204), (116) and (220) crystal plane of anatase-TiO2 (JCPDS 21-1272) [16]. The micromorphology of TiO2 powders was observed by TEM, as shown in Fig. 1(b). It can be found that the TiO2 exhibits irregularly flake-like shape and has a lateral dimension in the range of 40–200 nm.
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Fig. 2. Steady state shear viscosity curves of the EG/TiO2 nanofluids for different particle concentrations at 25 ◦ C.
Fig. 3. Zero shear viscosity (0 ) of the EG/TiO2 nanofluids as a function of particle concentration (w) at 25 ◦ C. The horizontal and vertical dashed lines indicate the viscosity of EG ( ≈ 0.012 Pa s) and critical concentration (wc = 5 wt%), respectively. The red dashed line denotes a logarithmic slope of 3.2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.2. Newtonian-pseudoplastic transition The effect of concentration of TiO2 nanoparticles on the flow behavior of EG/TiO2 nanofluids was investigated by steady state shear rheology. Fig. 2 shows the shear viscosity curves of EG/TiO2 nanofluids with different particle concentrations at 25 ◦ C. For w ≤ 5 wt%, the viscosity increases slightly compared with the base fluid of EG (w = 0 wt%). However, as w furtherly increases, the EG/TiO2 nanofluids change from Newtonian fluids to pseudoplastic fluids. For 7 wt% ≤ w ≤ 25 wt%, the whole shear viscosity curves can be clearly classified into three regions, which were separated by red dashed lines in Fig. 2: the first Newton region (I), the shear thinning region (II), and the second Newton region (III). The typical shear thinning behavior at the middle shear rate region can be attributed to the TiO2 nanoparticles in EG nanofluids align under the influence of shear flow field [5]. However, as w > 25 wt%, only the shear thinning region (II) can be observed. In order to determine the critical concentration (wc ) at which the Newtonian-pseudoplastic transition occurs, the zero shear viscosity (0 ) at 25 ◦ C is plotted against w, as depicted in Fig. 3. The 0 is defined as the viscosity at the low shear Newton plateau region. At the critical transition point, the 0 increases dramatically. The 0 changes little as w ≤ 5 wt%, while starts to increase dramatically beyond 5 wt% with 0 –w3.2 . Therefore, a significant Newtonian-pseudoplastic transition in the nanofluids is found with the increase of particle concentration. A power exponent of 3.2 (0 –w3.2 ) is observed from Fig. 3, which indicates the nanofluids
.
Fig. 4. Shear stress () versus shear rate ( ) of the EG/TiO2 nanofluids for different particle concentrations at 25 ◦ C.
Fig. 5. Yield stress (y ) versus particle concentration (w) of EG/TiO2 nanofluids at 25 ◦ C.
have entered into the semi-diluted regime and some TiO2 nanoparticles are assembled into agglomerations. The power exponent could be related to the fractal dimension (df ) of the agglomerations of flake-like TiO2 . Similar results have been reported in a semidilute solution of stiff poly(␥-benzyl lglutamate) by Larson et al. [17] and silicone oil dispersion of graphene oxide/polyaniline (GO/PANI) in our previous work [18]. In addition, the viscosity of the EG/TiO2 nanofluids (0 = 3.945 Pa s) is remarkably larger than that of the base fluid of EG ( ≈ 0.012 Pa s), i.e. 329 times by filling with only 8.46 w/v% TiO2 nanoparticles if adopts the calculation method introduced by Yapici et al. [3]. Therefore, the flake-like TiO2 is an effective filler to enhance the viscosity of the base fluid of EG. Fig. 4 shows the shear stress () of the nanofluids as a func. tion of shear rate ( ) with different particle concentrations at 25 ◦ C. . . For w ≤ 5 wt%, a linear relationship between and ( ∼ 1 ) is observed, indicating a Newtonian flow behavior. For w > 5 wt%, the . yield stress ( y ) can be obtained by fitting the ∼ curves with . Herschel-Bulkley equation ( = y + n ) [19,20]. The y is plotted against w at 25 ◦ C, as shown in Fig. 5. It can be found that the y enlarges with the increase of the w. Therefore, the concentrated EG/TiO2 nanofluids are typical yield stress fluids. 3.3. Temperature-dependent nonlinear rheological behavior In order to explore the effect of temperature on the nonlinear rheological behavior of EG/TiO2 nanofluids, we performed steady state shear and thixotropy tests at different temperature (T = 25, 35 and 45 ◦ C). The EG is used as a control test. Fig. 6 shows the
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Fig. 6. Shear rate dependency of viscosity as a function of temperature for the base fluid of EG (a) and 35 wt% EG/TiO2 nanofluids (b).
increased, suggesting that the nanofluids had entered into the semidiluted regime and some TiO2 nanoparticles were assembled into agglomerations. In addition, the results of thixotropic loops and shear viscosity demonstrated that the temperature had a significant influence on the nonlinear rheological behavior of the concentrated nanofluids. It is believed that the study of rheological behavior of EG/TiO2 nanofluids could be beneficial to understand the processing and application of TiO2 -related nanofluids and nanocomposites. Acknowledgements
Fig. 7. Thixotropic behavior of 35 wt% EG/TiO2 nanofluids at different temperature.
steady state viscosity curves of the base fluid of EG and 35 wt% EG/TiO2 nanofluids at different temperature. From Fig. 6(a), it can be found that the base fluid of EG is a kind of typical Newtonian fluid and the viscosity significantly decreases with the increasing of temperature due to the thermal motion of liquid molecules aggravates. However, the concentrated EG/TiO2 nanofluids exhibit significant shear thinning behavior at all tested temperature. We also notice that the temperature has small effect on the viscosity of the nanofluids compared with the base fluid of EG, i.e. the thermal stability of the nanofluids enhances. Thixotropy is a very important rheological property because it can reflect the time-dependent evolution of structure in particle filled suspensions [21]. As we know, the thixotropy can be characterized by the appearance of hysteresis loops in up and down shear rate sweep curves. Fig. 7 depicts the thixotropic loops of 35 wt% EG/TiO2 nanofluids for different temperature. It is obviously found that the nanofluids present distinct thixotropy at all tested temperature. As the temperature elevates, the area of thixotropic loops slightly increases, suggesting the thixotropy enhances. The results of Figs. 6 and 7 demonstrate that temperature has a significant influence on the nonlinear flow behavior of EG/TiO2 nanofluids. 4. Conclusions The effects of the particle concentration and temperature on the rheological behavior of EG/TiO2 nanofluids were investigated. The steady state shear results showed that the viscosity of the nanofluids remarkably enhanced (329 times with a filled volume fraction of 8.46 w/v%) compared with the base fluid of EG, and the concentrated nanofluids exhibited characteristic shear thinning behavior. Significantly, a Newtonian-pseudoplastic transition with a critical exponent of 3.2 was firstly observed as particle concentration
This work was financially supported by the National Natural Science Foundation of China (Grant No. 51507003), the National Training Program of Innovation and Entrepreneurship for Undergraduates (Grant No. 201510361069), the Doctor’s Start-up Research Foundation of Anhui University of Science and Technology (Grant No. ZY537). References [1] I.M. Mahbubul, R. Saidur, M.A. Amalina, Latest developments on the viscosity of nanofluids, Int. J. Heat Mass Transfer 55 (2012) 874–885. [2] W. Yu, H. Xie, L. Chen, Y. Li, Investigation of thermal conductivity and viscosity of ethylene glycol based ZnO nanofluid, Thermochim. Acta 491 (2009) 92–96. [3] K. Yapici, N.K. Cakmak, N. Ilhan, Y. Uludag, Rheological characterization of polyethylene glycol based TiO2 nanofluids, Korea Aust. Rheol. J. 26 (2014) 355–363. [4] H. Xie, J. Wang, T. Xi, Y. Liu, F. Ai, Q. Wu, Thermal conductivity enhancement of suspensions containing nanosized alumina particles, J. Appl. Phys. 91 (2002) 4568–4572. [5] H. Chen, Y. Ding, A. Lapkin, X. Fan, Rheological behaviour of ethylene glycol-titanate nanotube nanofluids, J. Nanopart. Res. 11 (2009) 1513–1520. [6] K.V. Wong, O. De Leon, Applications of nanofluids: current and future, Adv. Mech. Eng. 2 (2010) 519659. [7] A. Ghadimi, I.H. Metselaar, The influence of surfactant and ultrasonic processing on improvement of stability, thermal conductivity and viscosity of titania nanofluid, Exp. Therm. Fluid Sci. 51 (2013) 1–9. [8] M. Vakili, A. Mohebbi, H. Hashemipour, Experimental study on convective heat transfer of TiO2 nanofluids, Heat Mass Transfer 49 (2013) 1159–1165. [9] W. Duangthongsuk, S. Wongwises, Heat transfer enhancement and pressure drop characteristics of TiO2 –water nanofluid in a double-tube counter flow heat exchanger, Int. J. Heat Mass Transfer 52 (2009) 2059–2067. ˜ [10] D. Cabaleiro, M.J. Pastoriza-Gallego, C. Gracia-Fernández, M.M. Pineiro, L. Lugo, Rheological and volumetric properties of TiO2 -ethylene glycol nanofluids, Nanoscale Res. Lett. 8 (2013) 1–13. [11] S. Bobbo, L. Fedele, A. Benetti, L. Colla, M. Fabrizio, C. Pagura, S. Barison, Viscosity of water based SWCNH and TiO2 nanofluids, Exp. Therm. Fluid Sci. 36 (2012) 65–71. [12] V. Penkavova, J. Tihon, O. Wein, Stability and rheology of dilute TiO2 -water nanofluids, Nanoscale Res. Lett. 6 (2011) 1–7. [13] Y. He, Y. Jin, H. Chen, Y. Ding, D. Cang, H. Lu, Heat transfer and flow behaviour of aqueous suspensions of TiO2 nanoparticles (nanofluids) flowing upward through a vertical pipe, Int. J. Heat Mass Transfer 50 (2007) 2272–2281. [14] H. Chen, Y. Ding, Y. He, C. Tan, Rheological behaviour of ethylene glycol based titania nanofluids, Chem. Phys. Lett. 444 (2007) 333–337. [15] W.J. Tseng, K.-C. Lin, Rheology and colloidal structure of aqueous TiO2 nanoparticle suspensions, Mater. Sci. Eng. A Struct. 355 (2003) 186–192.
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