- Email: [email protected]
Preparation and rheological studies of uncoated and PVA-coated magnetite nanofluid
Journal of Magnetism and Magnetic Materials 324 (2012) 4143–4146
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Preparation and rheological studies of uncoated and PVA-coated magnetite nanofluid M.E. Khosroshahi a,b,n, L. Ghazanfari a a b
Amirkabir University of Technology, Faculty of Biomedical Engineering, Biomaterials Group, Laser and Nanobiophotonics Laboratory, Tehran, Iran Laser Optics and Photonics Research Center, Amirkabir University of Technology, Tehran, Iran
a r t i c l e i n f o
abstract
Article history: Received 27 December 2011 Received in revised form 1 July 2012 Available online 6 August 2012
Experimental studies of rheological behavior of uncoated magnetite nanoparticles (MNPs)U and polyvinyl alcohol (PVA) coated magnetite nanoparticles (MNPs)C were performed. A Co-precipitation technique under N2 gas was used to prevent undesirable critical oxidation of Fe2 þ . The results showed that smaller particles can be synthesized in both cases by decreasing the NaOH concentration which in our case this corresponded to 35 nm and 7 nm using 0.9 M NaOH at 750 rpm for (MNPs)U and (MNPs)C. The stable magnetic fluid contained well-dispersed Fe3O4/PVA nanocomposites which indicated fast magnetic response. The rheological measurement of magnetic fluid indicated an apparent viscosity range (0.1–1.2) pa s at constant shear rate of 20 s 1 with a minimum value in the case of (MNPs)U at 0 T and a maximum value for (MNPs)C at 0.5 T. Also, as the shear rate increased from 20 s 1 to 150 s 1 at constant magnetic field, the apparent viscosity also decreased correspondingly. The waterbased ferrofluid exhibited the non-Newtonian behavior of shear thinning under magnetic field. & 2012 Elsevier B.V. All rights reserved.
Keywords: Fe3O4 PVA Magnetic properties Rheology Non-Newtonian
1. Introduction Magnetic field-responsive materials are specific subsets of smart materials that can adaptively change their physical properties due to external magnetic field. Magnetic liquids or ferrofluids (FFs) are colloidal system of single domain magnetic nanoparticles (NPs) that are dispersed either in aqueous or organic liquids. Usually solid particles in these fluids are about 10 nm in size and a surfactant is used for stabilization. The most important advantage of these fluids over conventional mechanical interfaces is their ability to achieve a wide range of viscosity (several orders of magnitude) in a fraction of millisecond. This has been a source of various technical and clinical applications [1–8]. Based on the mesoscopic physical, tribological, thermal and mechanical properties of superparamagnetic iron oxide nanoparticles (SPION), they offer a variety of applications in different areas such as FFs, color imaging, and magnetic recording [9,10]. Various approaches have been explored for synthesis and characterization of high-quality magnetic iron oxide NPs [11–17]. There are, generally, two methods of producing NPs: i- physical methods such as gas phase deposition [18], electron beam lithography [19], microwave plasma [20] and laser pyrolysis [21,22] and ii- chemical method mainly using ferric and ferrous chloride as base source but
n Corresponding author at: Amirkabir University of Technology, Faculty of Biomedical Engineering, Biomaterials Group, Laser and Nanobiophotonics Laboratory, Tehran, Iran. Tel.: þ98 2164 542 398; fax: þ 98 216 646 8186. E-mail address: [email protected] (M.E. Khosroshahi).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.07.025
under different synthesizing conditions [23–25]. Applying an appropriate amount of polyvinyl alcohol (PVA) on SPIONs prevents their aggregation mostly via steric hindrance mechanism and gives rise to mono-dispersed NPs [26–31]. The measurement of the suspension’s viscosity can be used to characterize the microstructural state of a dispersion. The rheology of a magnetic particle dispersion is very complicated because such dispersions are multi-component systems consisting of magnetic particles and polymers. Moreover, the polymer is not only present in the solvent but is also adsorbed onto the particle surface [32–34]. Rheology is a major subject of investigating the flow and deformation of materials can be monitored by the application of a field, either magnetic or electric. Most studies have been focused on measurement of field-induced effects in FFs under shear flow [35–37], and shear stress versus shear rate [38–40]. Focusing on the change of fluid’s viscous behavior due to the action of an appropriate magnetic field seems to be the most prominent effect and accounts as a challenging topic in FF research. The aim of the research is to investigate the rheological properties of uncoated and PVA-coated Fe3O4 NPs synthesized in an oxidative environment and its possible impact in clinical applications.
2. Experimental 2.1. Materials All of analytic reagents were purchased from the indicated suppliers and used without further purification: Ferric chloride
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M.E. Khosroshahi, L. Ghazanfari / Journal of Magnetism and Magnetic Materials 324 (2012) 4143–4146
hexahydrate (FeCl3 6H2O) (99%), Ferrous chloride tetrahydrate (FeCl2 4H2O) (99%), sodium hydroxide (NaOH, 99%), hydrochloric acid (HCl, 37%), PVA of 30000–40000 g mol 1 nominal molecular weight and 86–89% degree of hydrolysis were purchased from Merck company. Milli-Q water (18.2 MO) was deoxygenated by bubbling N2 gas for 1 h prior to the use. 2.2. Synthesis Stock solutions of 1.28 M FeCl3 6H2O, 0.64 M FeCl2 4H2O and 0.4 M HCl were prepared as a source of iron by dissolving the respective chemicals in milli-Q water and deoxygenated by bubbling N2 gas to avoid possible oxygen contamination during the synthesis under vigorous stirring. In the same way, stock solutions of (0.9–1.5) M of NaOH were prepared as the alkali sources and the synthesized Fe3O4 samples were classified as S1–S4 using different NaOH concentration i.e. 0.9, 1.1, 1.3 and 1.5 M of NaOH, respectively. Aqueous dispersion of magnetite NPs (MNPs) was prepared by alkalinizing an aqueous mixture of ferric and ferrous salts with NaOH at room temperature. 25 ml of iron source was added drop-wise into 250 ml of alkali source under vigorous magnetic stirring (750 rpm) for 30 min at ambient temperature. A complete precipitation of Fe3O4 should be expected between 7.5 and 14 pH, while maintaining a molar ratio of Fe2 þ : Fe3 þ ¼1:2 under a non-oxidizing environment. The precipitated black powder was isolated by applying an external magnetic field, and the supernatant was removed from the precipitate by decantation. The powder was washed and the solution was decanted twice after centrifugation at 5000 rpm for 15 min. Then 0.01 M HCl was added to neutralize the anionic charge on the particle surface. The cationic colloidal particles were separated by centrifugation and peptized by watering. In order to prevent them from possible oxidation in air as well as from agglomeration, Fe3O4 NPs were coated with PVA shell. 4 g of PVA was dissolved in water and added to the magnetic solution, then it was heated at 90 1C for 30 min under magnetic stirring (750 rpm). The samples were classified as S5–S8 where each sample were synthesized using different NaOH concentration i.e. 0.9, 1.1, 1.3 and 1.5 M of NaOH corresponds to S5, S6, S7 and S8, respectively. The obtained magnetic product was collected by magnetic separation. After coating, the surfactant adsorbed physically on the particle surface was removed by washing, centrifugation and peptizing the solution for three times, then they freeze-dried at 60 1C.
Fig. 1. (a) TEM of Fe3O4 NPs (S1–S4) and (b) TEM of PVA-coated Fe3O4 NPs (S5).
2.3. Characterization Transmission electron microscopy (TEM) was performed using a Phillips CM-200-FEG microscope operating at 120 kV. The magnetorheological property of FFs was measured using a Physica MCR 300 rheometer (Anton paar company). The magnetic field intensity was 0.5 T, which was measured using a Hall-effect sensor.
3. Result and discussion 3.1. Structural properties TEM was used to observe the agglomeration state, particle size distribution, and morphology of uncoated and PVA-coated Fe3O4 NPs (Fig. 1). The mean particle size was examined by TEM imaging. As shown in Fig. 1a, the synthesized Fe3O4 powders consist of almost dispersed particles. However, because of the large specific surface area, high surface energy, and magnetization of Fe3O4 NPs, some of the primary NPs were aggregated into secondary particles during the process of drying. The variation of particle size with
Fig. 2. Particle size versus NaOH concentration for uncoated (S1–S4) and PVA coated (S5–S8) Fe3O4 NPs.
NaOH concentration is shown in Fig. 2, where the size of magnetite particles increases with increase in NaOH %. Thus, the lowest value of 7 nm was achieved for S5 at 0.9 M. As it is seen in this figure, growth of magnetite decreases at lower NaOH %. 3.2. Rheological property of magnetic fluid For the successful application of magnetic fluid, it is very important to obtain its rheological property. Assuming a suspension of MNPs under the influence of a shear flow, the particles will
M.E. Khosroshahi, L. Ghazanfari / Journal of Magnetism and Magnetic Materials 324 (2012) 4143–4146
rotate in the flow with their axis of rotation parallel to the vorticity of the flow. If the magnetic moment of the particles is fixed within the particle, then the Brownian relaxation time is shorter than the Neel time and thus the particles are magnetically hard. An anisotropic change of viscosity of the fluid depends on the strength and the direction of the field relative to the flow. The magnetic field can be applied in perpendicular or collinear direction. In the former case the magnetic field will tend to align the magnetic moment with the field direction. Thus, viscous torque exerted by the flow tries to rotate the particle, where as in the second case the magnetic moment will be aligned in the direction of the field. However, since this is identical with the axis of rotation of the particles no field influence will appear on the rotation of the particle. As a result no change of viscosity of the fluid will be observed. For perpendicular alignment of field and vorticity an absolute maximum of the relative change of viscosity is given by Rmax ¼
3 0 f 2
4145
Fig. 3. Variation of viscosity with shear rate at different applied magnetic field for uncoated (S1) and PVA-coated (S5) Fe3O4 NPs.
ð1Þ
where f0 denotes the volume fraction of the particles including the surfactant. Thus, in a suspension of diluted solution, magnetically hard particles with a volume fraction of magnetic material about 7 vol%, mean particle diameter of about 7 nm, the relative change of viscosity in a field cannot exceed about 40%. The viscosity of the fluid should be a function of the ratio (SR) of viscous to magnetic stress SR ¼
g: Z0 m0 M0 H
ð2Þ
Here g˙ denotes the shear rate, Z0 the viscosity of the solvent, m0 is the susceptibility of vacuum, M0 the spontaneous magnetization of the magnetic material, H the magnetic field strength. In diluted solutions, the ratio of the fluid’s viscosity under influence of H to that of the fluid for H¼0 is only a function of the stress parameter SR
Z ¼ Yðg: Z0 =m0 M 0 HÞ ZðH ¼ 0Þ
ð3Þ
The viscous behavior should be at a constant minimum level for large values of the stress parameter. In the intermediate range the viscosity is assumed to depend on shear and field. Albert Einstein proposed a simple equation to calculate the viscosity of particles suspension, Z ¼ (1þ2.5cs)ZL where, cs is the solid content of FFs and ZL is the viscosity of carrier liquid. The Bingham model takes account of the yield stress of fluids, ˙ : when the shear stress is less than the yield stress t0, t ¼ t0 þ Zg there is no fluid motion. But the Bingham model could not predict the shear-thinning/thickening behaviors of some fluids. The Carson model taking account of both the yield stress threshold and the shear-thinning behavior, adopts a relatively simple form, ˙ . The H–B model also takes account of both the Ot ¼Otc þOZc g yield stress threshold and the shear-thinning phenomenon, but takes a relatively complicated form: t ¼ t0 þk[g˙ n (t0/m0)n], and Z ¼kg˙ n 1. For most of the fluids, the H–B equation can be simplified as: t ¼ t0 þkg˙ n. The consistency index (k) and the shear-thinning exponent (n) are influenced by the intensity of applied magnetic field, particle content in FFs, magnetic properties of MNPs and the dosages of surfactants [41]. There are two kinds of viscosity variations for FFs when the shear rate increases: Newtonian and shear-thinning behaviors. Fig. 3 shows that the viscosity of both uncoated and PVA coated Fe3O4 solution increases with decreasing the shear rate for a given magnetic field. However, at constant shear rate the viscosity increases with increase in the magnetic field (Fig. 4) until it reaches a saturation point which is in agreement with the Pop et al. findings [42]. For FF, the viscosity is determined by the viscosity of carrier liquid (water) and the interaction of MNPs. The viscosity of water
Fig. 4. Viscosity versus applied magnetic field at different shear rate for uncoated (S1) and PVA-coated (S5) Fe3O4 NPs.
is not affected by applied magnetic field, while the MNPs were polarized by magnetic field and arranged their orientation along the direction of magnetic field. The viscosity of magnetic fluid shows the tendency to increase because of the magneto-viscous effect of magnetic solution. As it can be seen in Fig. 4, increasing the magnetic field causes an increase in apparent viscosity which is more significant at lower shear rate. It is also known that under an applied magnetic field orderly microstructures are formed. As the shear rate increases, these microstructures are disoriented under the shear stress, thus, the viscosity of high-concentration FFs diminishes rapidly. Shulman et al. [43] suggested that magnetorheological suspensions under applied magnetic field could be described using the Bingham model. Our results showed that the characteristics of FFs gradually deviated from the Bingham model when the intensity of applied magnetic field increased. Figs. 3 and 4 demonstrate the shear-thinning behavior of FFs under different magnetic fields and without magnetic field. It is seen that the viscosity and shear stress of FFs increases gradually as the intensity of applied magnetic field increases. This is due to the fact that the MNPs are arranged to form a chaining structure along the applied magnetic field and hence the attraction among these micro-chains increases with the intensity of applied magnetic field, and the viscosity. This results in an increase of yield stress of FFs compared with the case without magnetic field. The shear-thinning exponent (n) and viscous coefficient (or called consistency index, k) were correlated from the experimental data, while the yield stress was derived theoretically. It was reported by Hong et al. [41] that chaining microstructures are formed in FFs when the intensity of applied magnetic field is strong enough. The H–B model takes account of both the yield stress and the shear-thinning behavior of FFs. Therefore, this model is currently used for describing the rheological properties FFs with or without applied magnetic field. The results corresponding to the relationship between shear stress and shear rate in different magnetic fields are shown in Fig. 5a and b for without and with PVA, respectively when
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M.E. Khosroshahi, L. Ghazanfari / Journal of Magnetism and Magnetic Materials 324 (2012) 4143–4146
Fig. 5. Shear stress versus shear rate at different applied magnetic field for (a) uncoated (S1) and (b) PVA coated (S5) Fe3O4 NPs.
the ratio of solid concentration of magnetite/PVA is 7%. Similar rheological properties were obtained in the case of PEG-coated Fe3O4 NPs as reported by Hong et al. [41]. The PVA not only acted as an outer surfactant shell but also enhanced the viscosity of the carrier fluid which led to a remarkable reduction of sedimentation velocity. The sedimentation velocity of the particles/aggregates in FF (S5) was about 6.0 mm/month obtained from Stokes law v¼
ðr1 r2 Þgd 18Z
2
ð4Þ
where r1 is the density of solid particles (5.18 g/cm3), r2 is density of carrier fluid (0.998 g/ml), g is the gravitational acceleration (9.81 m/s2), d is the diameter of aggregates (31.8 nm for S5), and Z is the viscosity of the carrier fluid (1.00 mPa s). The density of aggregates is much less than that of the magnetite particles. That is to say, the above Stokes equation overpredicted sedimentation rate of aggregates.
4. Conclusions In this research, Fe3O4/PVA nanocomposites with least diameter value of 7 nm was synthesized. A systematic study of the rheological properties of Fe3O4/PVA core/shell was performed. The obtained magnetic fluid was non-Newtonian (shear-thinning) at volume concentration of 7%. The viscosity of magnetic fluid increased with increasing the applied magnetic field, but decreased with increasing of shear rate. A remarkable magnetoviscous effect was seen in FF. The unexpected increase of magnetoviscous effect at low shear rates was related to the casual agglomeration in the absence of shear rate and more interactions aroused from rearrangement of particles after shear rate increase. It is believed that these binary NPs with their magnetic potential can be considered for relevant biomedical and industrial applications. References [1] T. Ohmori, H. Takahashi, H. Mametsuka, E. Suzuki, Physical Chemistry 2 (2000) 3519. [2] L.H. Huo, W. Li, L.H. Lu, H.N. Cui, S.Q. Xi, J. Wang, B. Zhao, Y.C. Shen, Z.H. Lu, Chemistry of Materials 12 (2000) 790. [3] P. Tartaj, M. del Puetro Morales, S. Veintemillas-Verdaguer, T. Gonza´lez˜ o, C.J. Serna, Journal of Physics 36 (2003) 182. Carren [4] X.Q. Xu, C.H. Deng, M.X. Gao, W.J. Yu, P.Y. Yang, X.M. Zhang, Advanced Materials 18 (2006) 3289. [5] A. Tanimoto, S. Eur.Kuribayashi, Journal de Radiologie 58 (2006) 200. [6] H.Y. Park, M.J. Schadt, L.Y. Wang, I.I.S. Lim, P.N. Njoki, S.H. Kim, M.Y. Jang, J. Luo, C.J. Zhong, Langmuir 23 (2007) 9050. [7] C.D. Jones, L.A. Lyon, Macromolecules 33 (2000) 8301. [8] G. Filipcsei, I. Csetneki, A. Szila´gyi, M. Zrı´nyi, Advances in Polymer Science 206 (2007) 137–189.
[9] R. De Palma, C.X. Liu, F. Barbagini, G. Reekmans, K. Bonroy, W. Laureyn, G. Borghs, G. Maes, Journal of Physical Chemistry C 111 (2007) 12227. [10] J. Popplewell, L. Sakhini, Journal of Magnetism and Magnetic Materials 149 (1995) 72. [11] D.K. Kim, Y. Zhang, W. Voit, K.V. Rao, M. Muhammed, Journal of Magnetism and Magnetic Materials 225 (2001) 30. [12] Z.L. Liu, Y.J. Liu, K.L. Yao, Z.H. Ding, J. Tao, X. Wang, Journal of Materials Synthesis and Processing 10 (2002) 83. [13] Y.L. Li, K.L. Yao, Z.L. Liu, Surface Science 601 (2007) 876. [14] W. Pei, H. Kumada, T. Natusme, H. Saito, S. Ishio, Journal of Magnetism and Magnetic Materials 310 (2007) 2375. [15] M.E. Khosroshahi, L. Ghazanfari, Physica E 42 (2010) 1824. [16] M.E. Khosroshahi, L. Ghazanfari, Surface Engineering 27 (2011) 573. [17] M.E. Khosroshahi, L. Ghazanfari, Materials Science and Engineering C 32 (2012) 1043. [18] B. Chertok, A.E. David, B.A. Moffat, V.C. Yang, Biomaterials 30 (2009) 6780. [19] K. Kamei, Y. Mukai, H. Kojima, T. Yoshikawa, M. Yoshikawa, Biomaterials 30 (2009) 1809. [20] F.G. Mondalek, Y.Y. Zhang, B. Kropp, R.D. Kopke, X. Ge, R.L. Jackson, K.J. Dormer, Journal of Nanobiotechnology 4 (2006) 1186/1477. [21] R. De Palma, C.X. Liu, F. Barbagini, G. Reekmans, K. Bonroy, W. Laureyn, G. Borghs, G. Maes, Journal of Physical Chemistry C 111 (2007) 12227. [22] N. Gribanov, E. Bibik, O. Buzunov, V. Naumov, Journal of Magnetism and Magnetic Materials 85 (1990) 7. [23] C. Murray, Sh. Sun, H. Doyle, T. Belley, MRS Bulletin (2001) 98. [24] D.K. Kim, Y. Zhang, W. Voit, K.V. Rao, M. Muhammed, Journal of Magnetism and Magnetic Materials 225 (2001) 30. [25] A. Gupta, S. Wells, IEEE Transactions on NanoBioscience 3 (2004) 66. [26] H. Yokoi, T. Kantoh, Bulletin of the Chemical Society of Japan 66 (1993) 1536. [27] M. Sairam, B.V.K. Naidu, S.K. Nataraj, B. Sreedhar, T.M. Aminabhavi, Journal of Membrane Science 283 (2006) 65. [28] B. Schopf, T. Neuberger, K. Schulze, A. Petri, M. Chastellain, M. Hofmann, H. Hofmann, B. von Rechenberg, Proceedings of the 5th International Conference on Scientific and Clinical Applications of Magnetic Carriers, Elsevier Science BV, Lyon, France, 2004, p. 411. [29] B. Xue, Y. Sun, Journal of Chromatography A 921 (2001) 109. [30] H. Pardoe, W. Chua-anusorn, T.G. St Pierre, J. Dobson, Proceedings of the 3rd International Conference on Scientific and Clinical Applications of Magnetic Carriers, Elsevier Science BV, Rostock, Germany, 2000, p. 41. [31] A. Chastellain, A. Petri, H. Hofmann, Journal of Colloid and Interface Science 278 (2004) 353. [32] R.A. Stratton, A.F. Butcher, Journal of Polymer Science Part B: Polymer Physics 11 (1973) 1747. [33] W. Gleissle, G. Astarita, G. Marrucci, L. Nicolais (Eds.), Rheology, in: Proceedings of the 8th International Congress on Rheology, vo1. 2, Plenum, New York, 1980. [34] D. Borin, A. Zubarev, D. Chirikov, R.M.€. uller, S. Odenbach, Journal of Magnetism and Magnetic Materials 323 (2011) 1273. [35] L. Pop, S. Odenbach, Journal of Physics Condensed Matter 18 (2006) S2785. [36] S. Odenbach, Colloidal Magnetic Fluids, Springer-Verlag, Berlin, Heidelberg, 2009. [37] D. Borin, S. Odenbach, Journal of Physics Condensed Matter 21 (2009) 246002. [38] S.D. Vlaev, I. Nikov, M. Martinov, Chemical Engineering Science 61 (2007) 5455. [39] A. Hassanpour, S.J. Antony, M. Ghadiri, Chemical Engineering Science 62 (2007) 935. [40] C.F. Welch, G.D. Rose, D. Malotky, S.T. Eckersley, Langmuir 22 (2006) 1544. [41] R.Y. Hong, Z.Q. Ren, Y.P. Han, H.Z. Li, Y. Zheng, J. Ding, Chemical Engineering Science 62 (2007) 5912. [42] L.M. Pop, J. Hilljegerdes, S. Odenbach, A. Wiedenmann, Applied Organometallic Chemistry 18 (2004) 523. [43] Z.P. Shulman, V.I. Kordonsky, E.A. Zaltsgendler, International Journal of Multiphase Flow 12 (1986) 935.
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Preparation and rheological studies of uncoated and PVA-coated magnetite nanofluid M.E. Khosroshahi a,b,n, L. Ghazanfari a a b
Amirkabir University of Technology, Faculty of Biomedical Engineering, Biomaterials Group, Laser and Nanobiophotonics Laboratory, Tehran, Iran Laser Optics and Photonics Research Center, Amirkabir University of Technology, Tehran, Iran
a r t i c l e i n f o
abstract
Article history: Received 27 December 2011 Received in revised form 1 July 2012 Available online 6 August 2012
Experimental studies of rheological behavior of uncoated magnetite nanoparticles (MNPs)U and polyvinyl alcohol (PVA) coated magnetite nanoparticles (MNPs)C were performed. A Co-precipitation technique under N2 gas was used to prevent undesirable critical oxidation of Fe2 þ . The results showed that smaller particles can be synthesized in both cases by decreasing the NaOH concentration which in our case this corresponded to 35 nm and 7 nm using 0.9 M NaOH at 750 rpm for (MNPs)U and (MNPs)C. The stable magnetic fluid contained well-dispersed Fe3O4/PVA nanocomposites which indicated fast magnetic response. The rheological measurement of magnetic fluid indicated an apparent viscosity range (0.1–1.2) pa s at constant shear rate of 20 s 1 with a minimum value in the case of (MNPs)U at 0 T and a maximum value for (MNPs)C at 0.5 T. Also, as the shear rate increased from 20 s 1 to 150 s 1 at constant magnetic field, the apparent viscosity also decreased correspondingly. The waterbased ferrofluid exhibited the non-Newtonian behavior of shear thinning under magnetic field. & 2012 Elsevier B.V. All rights reserved.
Keywords: Fe3O4 PVA Magnetic properties Rheology Non-Newtonian
1. Introduction Magnetic field-responsive materials are specific subsets of smart materials that can adaptively change their physical properties due to external magnetic field. Magnetic liquids or ferrofluids (FFs) are colloidal system of single domain magnetic nanoparticles (NPs) that are dispersed either in aqueous or organic liquids. Usually solid particles in these fluids are about 10 nm in size and a surfactant is used for stabilization. The most important advantage of these fluids over conventional mechanical interfaces is their ability to achieve a wide range of viscosity (several orders of magnitude) in a fraction of millisecond. This has been a source of various technical and clinical applications [1–8]. Based on the mesoscopic physical, tribological, thermal and mechanical properties of superparamagnetic iron oxide nanoparticles (SPION), they offer a variety of applications in different areas such as FFs, color imaging, and magnetic recording [9,10]. Various approaches have been explored for synthesis and characterization of high-quality magnetic iron oxide NPs [11–17]. There are, generally, two methods of producing NPs: i- physical methods such as gas phase deposition [18], electron beam lithography [19], microwave plasma [20] and laser pyrolysis [21,22] and ii- chemical method mainly using ferric and ferrous chloride as base source but
n Corresponding author at: Amirkabir University of Technology, Faculty of Biomedical Engineering, Biomaterials Group, Laser and Nanobiophotonics Laboratory, Tehran, Iran. Tel.: þ98 2164 542 398; fax: þ 98 216 646 8186. E-mail address: [email protected] (M.E. Khosroshahi).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.07.025
under different synthesizing conditions [23–25]. Applying an appropriate amount of polyvinyl alcohol (PVA) on SPIONs prevents their aggregation mostly via steric hindrance mechanism and gives rise to mono-dispersed NPs [26–31]. The measurement of the suspension’s viscosity can be used to characterize the microstructural state of a dispersion. The rheology of a magnetic particle dispersion is very complicated because such dispersions are multi-component systems consisting of magnetic particles and polymers. Moreover, the polymer is not only present in the solvent but is also adsorbed onto the particle surface [32–34]. Rheology is a major subject of investigating the flow and deformation of materials can be monitored by the application of a field, either magnetic or electric. Most studies have been focused on measurement of field-induced effects in FFs under shear flow [35–37], and shear stress versus shear rate [38–40]. Focusing on the change of fluid’s viscous behavior due to the action of an appropriate magnetic field seems to be the most prominent effect and accounts as a challenging topic in FF research. The aim of the research is to investigate the rheological properties of uncoated and PVA-coated Fe3O4 NPs synthesized in an oxidative environment and its possible impact in clinical applications.
2. Experimental 2.1. Materials All of analytic reagents were purchased from the indicated suppliers and used without further purification: Ferric chloride
4144
M.E. Khosroshahi, L. Ghazanfari / Journal of Magnetism and Magnetic Materials 324 (2012) 4143–4146
hexahydrate (FeCl3 6H2O) (99%), Ferrous chloride tetrahydrate (FeCl2 4H2O) (99%), sodium hydroxide (NaOH, 99%), hydrochloric acid (HCl, 37%), PVA of 30000–40000 g mol 1 nominal molecular weight and 86–89% degree of hydrolysis were purchased from Merck company. Milli-Q water (18.2 MO) was deoxygenated by bubbling N2 gas for 1 h prior to the use. 2.2. Synthesis Stock solutions of 1.28 M FeCl3 6H2O, 0.64 M FeCl2 4H2O and 0.4 M HCl were prepared as a source of iron by dissolving the respective chemicals in milli-Q water and deoxygenated by bubbling N2 gas to avoid possible oxygen contamination during the synthesis under vigorous stirring. In the same way, stock solutions of (0.9–1.5) M of NaOH were prepared as the alkali sources and the synthesized Fe3O4 samples were classified as S1–S4 using different NaOH concentration i.e. 0.9, 1.1, 1.3 and 1.5 M of NaOH, respectively. Aqueous dispersion of magnetite NPs (MNPs) was prepared by alkalinizing an aqueous mixture of ferric and ferrous salts with NaOH at room temperature. 25 ml of iron source was added drop-wise into 250 ml of alkali source under vigorous magnetic stirring (750 rpm) for 30 min at ambient temperature. A complete precipitation of Fe3O4 should be expected between 7.5 and 14 pH, while maintaining a molar ratio of Fe2 þ : Fe3 þ ¼1:2 under a non-oxidizing environment. The precipitated black powder was isolated by applying an external magnetic field, and the supernatant was removed from the precipitate by decantation. The powder was washed and the solution was decanted twice after centrifugation at 5000 rpm for 15 min. Then 0.01 M HCl was added to neutralize the anionic charge on the particle surface. The cationic colloidal particles were separated by centrifugation and peptized by watering. In order to prevent them from possible oxidation in air as well as from agglomeration, Fe3O4 NPs were coated with PVA shell. 4 g of PVA was dissolved in water and added to the magnetic solution, then it was heated at 90 1C for 30 min under magnetic stirring (750 rpm). The samples were classified as S5–S8 where each sample were synthesized using different NaOH concentration i.e. 0.9, 1.1, 1.3 and 1.5 M of NaOH corresponds to S5, S6, S7 and S8, respectively. The obtained magnetic product was collected by magnetic separation. After coating, the surfactant adsorbed physically on the particle surface was removed by washing, centrifugation and peptizing the solution for three times, then they freeze-dried at 60 1C.
Fig. 1. (a) TEM of Fe3O4 NPs (S1–S4) and (b) TEM of PVA-coated Fe3O4 NPs (S5).
2.3. Characterization Transmission electron microscopy (TEM) was performed using a Phillips CM-200-FEG microscope operating at 120 kV. The magnetorheological property of FFs was measured using a Physica MCR 300 rheometer (Anton paar company). The magnetic field intensity was 0.5 T, which was measured using a Hall-effect sensor.
3. Result and discussion 3.1. Structural properties TEM was used to observe the agglomeration state, particle size distribution, and morphology of uncoated and PVA-coated Fe3O4 NPs (Fig. 1). The mean particle size was examined by TEM imaging. As shown in Fig. 1a, the synthesized Fe3O4 powders consist of almost dispersed particles. However, because of the large specific surface area, high surface energy, and magnetization of Fe3O4 NPs, some of the primary NPs were aggregated into secondary particles during the process of drying. The variation of particle size with
Fig. 2. Particle size versus NaOH concentration for uncoated (S1–S4) and PVA coated (S5–S8) Fe3O4 NPs.
NaOH concentration is shown in Fig. 2, where the size of magnetite particles increases with increase in NaOH %. Thus, the lowest value of 7 nm was achieved for S5 at 0.9 M. As it is seen in this figure, growth of magnetite decreases at lower NaOH %. 3.2. Rheological property of magnetic fluid For the successful application of magnetic fluid, it is very important to obtain its rheological property. Assuming a suspension of MNPs under the influence of a shear flow, the particles will
M.E. Khosroshahi, L. Ghazanfari / Journal of Magnetism and Magnetic Materials 324 (2012) 4143–4146
rotate in the flow with their axis of rotation parallel to the vorticity of the flow. If the magnetic moment of the particles is fixed within the particle, then the Brownian relaxation time is shorter than the Neel time and thus the particles are magnetically hard. An anisotropic change of viscosity of the fluid depends on the strength and the direction of the field relative to the flow. The magnetic field can be applied in perpendicular or collinear direction. In the former case the magnetic field will tend to align the magnetic moment with the field direction. Thus, viscous torque exerted by the flow tries to rotate the particle, where as in the second case the magnetic moment will be aligned in the direction of the field. However, since this is identical with the axis of rotation of the particles no field influence will appear on the rotation of the particle. As a result no change of viscosity of the fluid will be observed. For perpendicular alignment of field and vorticity an absolute maximum of the relative change of viscosity is given by Rmax ¼
3 0 f 2
4145
Fig. 3. Variation of viscosity with shear rate at different applied magnetic field for uncoated (S1) and PVA-coated (S5) Fe3O4 NPs.
ð1Þ
where f0 denotes the volume fraction of the particles including the surfactant. Thus, in a suspension of diluted solution, magnetically hard particles with a volume fraction of magnetic material about 7 vol%, mean particle diameter of about 7 nm, the relative change of viscosity in a field cannot exceed about 40%. The viscosity of the fluid should be a function of the ratio (SR) of viscous to magnetic stress SR ¼
g: Z0 m0 M0 H
ð2Þ
Here g˙ denotes the shear rate, Z0 the viscosity of the solvent, m0 is the susceptibility of vacuum, M0 the spontaneous magnetization of the magnetic material, H the magnetic field strength. In diluted solutions, the ratio of the fluid’s viscosity under influence of H to that of the fluid for H¼0 is only a function of the stress parameter SR
Z ¼ Yðg: Z0 =m0 M 0 HÞ ZðH ¼ 0Þ
ð3Þ
The viscous behavior should be at a constant minimum level for large values of the stress parameter. In the intermediate range the viscosity is assumed to depend on shear and field. Albert Einstein proposed a simple equation to calculate the viscosity of particles suspension, Z ¼ (1þ2.5cs)ZL where, cs is the solid content of FFs and ZL is the viscosity of carrier liquid. The Bingham model takes account of the yield stress of fluids, ˙ : when the shear stress is less than the yield stress t0, t ¼ t0 þ Zg there is no fluid motion. But the Bingham model could not predict the shear-thinning/thickening behaviors of some fluids. The Carson model taking account of both the yield stress threshold and the shear-thinning behavior, adopts a relatively simple form, ˙ . The H–B model also takes account of both the Ot ¼Otc þOZc g yield stress threshold and the shear-thinning phenomenon, but takes a relatively complicated form: t ¼ t0 þk[g˙ n (t0/m0)n], and Z ¼kg˙ n 1. For most of the fluids, the H–B equation can be simplified as: t ¼ t0 þkg˙ n. The consistency index (k) and the shear-thinning exponent (n) are influenced by the intensity of applied magnetic field, particle content in FFs, magnetic properties of MNPs and the dosages of surfactants [41]. There are two kinds of viscosity variations for FFs when the shear rate increases: Newtonian and shear-thinning behaviors. Fig. 3 shows that the viscosity of both uncoated and PVA coated Fe3O4 solution increases with decreasing the shear rate for a given magnetic field. However, at constant shear rate the viscosity increases with increase in the magnetic field (Fig. 4) until it reaches a saturation point which is in agreement with the Pop et al. findings [42]. For FF, the viscosity is determined by the viscosity of carrier liquid (water) and the interaction of MNPs. The viscosity of water
Fig. 4. Viscosity versus applied magnetic field at different shear rate for uncoated (S1) and PVA-coated (S5) Fe3O4 NPs.
is not affected by applied magnetic field, while the MNPs were polarized by magnetic field and arranged their orientation along the direction of magnetic field. The viscosity of magnetic fluid shows the tendency to increase because of the magneto-viscous effect of magnetic solution. As it can be seen in Fig. 4, increasing the magnetic field causes an increase in apparent viscosity which is more significant at lower shear rate. It is also known that under an applied magnetic field orderly microstructures are formed. As the shear rate increases, these microstructures are disoriented under the shear stress, thus, the viscosity of high-concentration FFs diminishes rapidly. Shulman et al. [43] suggested that magnetorheological suspensions under applied magnetic field could be described using the Bingham model. Our results showed that the characteristics of FFs gradually deviated from the Bingham model when the intensity of applied magnetic field increased. Figs. 3 and 4 demonstrate the shear-thinning behavior of FFs under different magnetic fields and without magnetic field. It is seen that the viscosity and shear stress of FFs increases gradually as the intensity of applied magnetic field increases. This is due to the fact that the MNPs are arranged to form a chaining structure along the applied magnetic field and hence the attraction among these micro-chains increases with the intensity of applied magnetic field, and the viscosity. This results in an increase of yield stress of FFs compared with the case without magnetic field. The shear-thinning exponent (n) and viscous coefficient (or called consistency index, k) were correlated from the experimental data, while the yield stress was derived theoretically. It was reported by Hong et al. [41] that chaining microstructures are formed in FFs when the intensity of applied magnetic field is strong enough. The H–B model takes account of both the yield stress and the shear-thinning behavior of FFs. Therefore, this model is currently used for describing the rheological properties FFs with or without applied magnetic field. The results corresponding to the relationship between shear stress and shear rate in different magnetic fields are shown in Fig. 5a and b for without and with PVA, respectively when
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M.E. Khosroshahi, L. Ghazanfari / Journal of Magnetism and Magnetic Materials 324 (2012) 4143–4146
Fig. 5. Shear stress versus shear rate at different applied magnetic field for (a) uncoated (S1) and (b) PVA coated (S5) Fe3O4 NPs.
the ratio of solid concentration of magnetite/PVA is 7%. Similar rheological properties were obtained in the case of PEG-coated Fe3O4 NPs as reported by Hong et al. [41]. The PVA not only acted as an outer surfactant shell but also enhanced the viscosity of the carrier fluid which led to a remarkable reduction of sedimentation velocity. The sedimentation velocity of the particles/aggregates in FF (S5) was about 6.0 mm/month obtained from Stokes law v¼
ðr1 r2 Þgd 18Z
2
ð4Þ
where r1 is the density of solid particles (5.18 g/cm3), r2 is density of carrier fluid (0.998 g/ml), g is the gravitational acceleration (9.81 m/s2), d is the diameter of aggregates (31.8 nm for S5), and Z is the viscosity of the carrier fluid (1.00 mPa s). The density of aggregates is much less than that of the magnetite particles. That is to say, the above Stokes equation overpredicted sedimentation rate of aggregates.
4. Conclusions In this research, Fe3O4/PVA nanocomposites with least diameter value of 7 nm was synthesized. A systematic study of the rheological properties of Fe3O4/PVA core/shell was performed. The obtained magnetic fluid was non-Newtonian (shear-thinning) at volume concentration of 7%. The viscosity of magnetic fluid increased with increasing the applied magnetic field, but decreased with increasing of shear rate. A remarkable magnetoviscous effect was seen in FF. The unexpected increase of magnetoviscous effect at low shear rates was related to the casual agglomeration in the absence of shear rate and more interactions aroused from rearrangement of particles after shear rate increase. It is believed that these binary NPs with their magnetic potential can be considered for relevant biomedical and industrial applications. References [1] T. Ohmori, H. Takahashi, H. Mametsuka, E. Suzuki, Physical Chemistry 2 (2000) 3519. [2] L.H. Huo, W. Li, L.H. Lu, H.N. Cui, S.Q. Xi, J. Wang, B. Zhao, Y.C. Shen, Z.H. Lu, Chemistry of Materials 12 (2000) 790. [3] P. Tartaj, M. del Puetro Morales, S. Veintemillas-Verdaguer, T. Gonza´lez˜ o, C.J. Serna, Journal of Physics 36 (2003) 182. Carren [4] X.Q. Xu, C.H. Deng, M.X. Gao, W.J. Yu, P.Y. Yang, X.M. Zhang, Advanced Materials 18 (2006) 3289. [5] A. Tanimoto, S. Eur.Kuribayashi, Journal de Radiologie 58 (2006) 200. [6] H.Y. Park, M.J. Schadt, L.Y. Wang, I.I.S. Lim, P.N. Njoki, S.H. Kim, M.Y. Jang, J. Luo, C.J. Zhong, Langmuir 23 (2007) 9050. [7] C.D. Jones, L.A. Lyon, Macromolecules 33 (2000) 8301. [8] G. Filipcsei, I. Csetneki, A. Szila´gyi, M. Zrı´nyi, Advances in Polymer Science 206 (2007) 137–189.
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