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Dielectric, magnetic, and rheologi- cal characteristics of these fluids are considered. The characteristic features of the formed structures depending on the.
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Synergistic effect in magnetoelectrorheological fluids with a complex dispersed phase

Journal of Intelligent Material Systems and Structures 23(9) 963–967 Ó The Author(s) 2011 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/1045389X11429174 jim.sagepub.com

E V Korobko1, Z A Novikova1, M A Zhurauski1, D Borin2 and S Odenbach2

Abstract In this article, the results of investigation of structural interactions, generated by simultaneous impact of electric and magnetic fields on fluids containing a two-component dispersed phase, are presented. Dielectric, magnetic, and rheological characteristics of these fluids are considered. The characteristic features of the formed structures depending on the filler type, deformation conditions, and intensity of application of external fields are discussed. Keywords magnetorheological, electrorheological, control, synergistic effect

Introduction In structure-reversed fluids based on dispersed phase (magnetoelectrorheological fluids (MERF)) that are sensitive to electric and magnetic fields, one can perform separate or simultaneous electrical or magnetic structurization of particles by applying independent fields. In prospect, this will allow changing the character of particle interaction, controlling the structure more flexibly, and broadening the range of regulation of the mechanical properties of a material. It means that the induced increase in the shear stress at two fields is higher than the sum of increments of the intensities, induced separately by the electric and magnetic fields (Gorodkin et al., 1991; Kordonsky et al., 1994; Koyama, 1996; Minagawa et al., 1994). As the index characterizing the value of the synergistic effect, we will use j = DtEH =(DtE + DtH ), which is the ratio of the shear stress increment on simultaneous application of electric and magnetic fields DtEH to the sum of increments on separate application of the electric Dt E and on separate application the magnetic Dt H fields. In our recent article (Korobko et al., 2009), the rheological behavior of MERF with a complex dispersed phase based on two components was investigated. It was found that on simultaneous application of two fields, the synergistic effect also appeared, and that its characteristics surpassed the results obtained earlier by other authors for fluids based on a single component dispersed phase. The aim of the present study is to perform a more detailed study of the characteristic peculiarities of a

two-field impact on MERF with a complex (two-component) dispersed phase. This investigation related not only to the study of rheological response, but also to the dielectric and magnetic properties of fluids for determination of the influence of these parameters on the appearance of synergistic effect.

Materials and Methods We have investigated MERF with compositions consisting of such fillers as carbonyl iron (CI or flakeshaped CI (F-CI)), which is sensitive only to a magnetic field; AerosilTM activated by some additives (F-Si), which is sensitive only to an electric field; and oxides aFe2O3 (F-a) and g-Fe2O3 (F-g), which are sensitive to both fields in different degrees. Mineral oil was used as a dispersion medium. Fluids with a concentration C of a dispersed phase of 5 vol.% and 10 vol.% with only one filler and MERF with a complex filler, consisting of 5 vol.% CI and 5 vol.% SiO2 (F-CI/Si) or 5 vol.% a-Fe2O3 (F-CI/a) or 5 vol.% g-Fe2O3 (F-CI/g) have been investigated. It was earlier stated (Kordonski 1

A. V. Luikov Heat and Mass Transfer Institute, National Academy of Sciences of Belarus, Minsk, Belarus 2 Technische Universita¨t Dresden, Dresden, Germany Corresponding author: E. V. Korobko, A. V. Luikov Heat and Mass Transfer Institute, Laboratory of Rheophysics and Macrokinetics, National Academy of Sciences of Belarus, 15 P. Brovka Street, Minsk 220072, Belarus. Email: [email protected]

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maximal lies in the range 102–105 Hz, then such fluids display a high electrorheological activity. From the results shown in Figure 1, it is seen that fluids consisting of one filler F-Si and F-g have the strongest interphase polarization. With increase in the filler concentration of one type, the values of e#, e$ also increase without a qualitative change in the frequency dependencies (Figure 1, curve 6). The fluid F-a has a small value of De#, and the value of fr does not fall within the area of interphase polarization, thus indicating the absence of relaxational interphase polarization, and one cannot expect a high electrorheological response for such a fluid. If we compare the electrorheological effect for these three fluids (Figure 2(b)–(d)), then it is seen that the MERF F-a is least sensitive to an electric field. The function t (E) changes a little bit more strongly in fluid with F-g. F-Si has the strongest electrorheological response. It is noted that the presence of CI in MERF in addition to all investigated oxides qualitatively does not change the character of the dielectric spectra and electrorheological response t (E), e# (f), e$ (f).

Figure 1. Dielectric spectra of fluids: (a) 1: F-a; 2: F-Si; 3: F-CI/ Si; 4 and 6: F-g; 5: F-CI/g. (b) 1, 2, and 4: C = 5 vol.%, 6: 10 vol.%.

Magnetic Properties and Magnetorheological Effect et al., 1998) that the biggest increment of rheological characteristics in the magnetic field is given exactly by CI, which was chosen by us as a constant component of the dispersed phase of all complex compounds. The rheological properties were carried out using rotational viscometer modified for measurements under two fields. The dielectric properties of MERF were determined in the range of frequencies from 102 to 105 Hz at a temperature of 20° C. The magnetic properties of MERF were determined by means of the standard technique for measuring magnetization with two Hall sensors.

Main Results Dielectric Properties and Electrorheological Effect To evaluate the capability of dispersed phase particles to be polarized in an external electric field and to form rigid structural bridges in a dielectric dispersion medium, we have measured the components of the complex dielectric permittivity (e#,e$) for fluids with single and complex fillers. For the chosen fillers SiO2, g-Fe2O3, a-Fe2O3, like for other oxides, the interphase polarization mechanism is most characteristic. It is known (Hao et al., 1998; Ikazaki et al., 1998) that for fluids with such fillers, if the difference between e# at frequencies 102 and 105 Hz (De#) is large and the relaxation frequency fr at which the value of e$ is

In Figure 3, the magnetization curves of the fluids under investigation are presented. For compositions with single dispersed phase, the highest magnetization is displayed by the F-CI fluid (curves 5 and 9). The use of only a-Fe2O3 as a dispersed phase provides the saturation magnetization Js not higher than 4 kA/m (at 10 vol.%), for the fluid with 10 vol.%g-Fe2O3 Js reaches 25 kA/m, for the fluid with 10 vol.% CI Js reaches 80 kA/m. The F-a and F-g fluids reach magnetic saturation at Hs ’ 50–60 kA/m in contrast to the F-CI fluid for Hs . 350 kA/m. When using a complex dispersed phase in F-CI/a (Figure 3, curve 7), saturation magnetization Js is increased substantially (above 70 kA/m), and the threshold value of the magnetic field at which the saturation occurs (Hs = 380 kA/m) also increases. It should be noted that all compositions of MERF containing 5 vol.% of CI have similar magnetization curves that differ little when using different oxide fillers. Their saturation magnetization varies in the range of Js = 60–70 kA/m in a magnetic field with H . 350 kA/m. Comparing the magnetic characteristics of MERF with single dispersed phase with the magnitude of the magnetorheological effect has shown that the largest magnetorheological effect is in the fluid with CI (Figure 2(a)). By increasing the intensity up to H = 60 kA/m, that is, up to reaching the saturation magnetization (Figure 3), one can also note the growth in the rheological characteristics of the F-a and F-g fluids.

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Figure 2. Flow curves for fluids with different fillers in electric and magnetic fields: (a) F-CI (1: H = 0; 2: H = 80 kA/m; 3: H = 100 kA/ m); (b) F-Si (1: E = 0; 2: E = 1.8 kV/m); (c) F- a(1: E = 0, H = 0; 2: E = 1.8 kV/m, H = 0; 3: E = 0, H = 100 kA/m; 4: E = 1.8 kV/mm, H = 100 kA/m); (d) F- g(1: E = 0, H = 0; 2: E = 1.8 kV/mm, H = 0; 3: E = 0, H = 100 kA/m; 4: E = 1.8 kV/mm, H = 100 kA/m). C = 5 vol.%.

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Figure 3. Magnetization curves for fluids: 1 and 2:F-a; 3 and 4:F-g; 5 and 9: F-CI; 6: F-CI/g; 7: F-CI/a; 8: F-CI/Si. 1, 3, and 5: C = 5 vol.%; 2, 4, and 9: 10 vol.%.

Comparing the values of magnetization and magnetorheological response MERF with complex dispersed phase with analogous characteristics for a fluid F-CI (Figure 3, curve 5) allows us to assume that the growth of t is due to the high sensitivity of CI to the magnetic field. In using a complex dispersed phase, a maximum magnetorheological effect was revealed in F-CI/g. The values of increments in the shear stress in a magnetic field are smaller and differ slightly for fluids F-CI/a or F-CI/Si. Thus, a magnetic field forms weaker structures

in F-CI/a in contrast to its influence on F-CI/g, since the intensity of magnetization of the a-Fe2O3 particles is much lesser. Due to the high magnetorheological activity of CI, the fluid F-CI/Si displays an increase in t (H) in a magnetic field (Figure 4(c)). However, its magnitude is not so much larger than the increment in t (E) in contrast to other fluids in which this difference is more appreciable.

Rheological Characteristics in Two Fields The rheological response to the electric and magnetic fields of fluid F-a is very weak, and maximal values of t reach only 20 Pa. For fluids with F-g, they are several times higher; moreover, the shear stress grows substantially. In the case of the influence of two fields, the rheological response in the range of shear rates g . 10 s21 for fluid F-g appears to be lower than that under the impact of only a magnetic field (Figure 2(d), curve 4). Addition of 5 vol.% CI to the composition leads to a change in the rheological characteristics of MERF in a magnetic field and two fields (Figure 4(a)–(c)). The presence of one component insensitive (SiO2) or slightly sensitive (a-Fe2O3) to magnetic effect enhances the rheological response MERF in the two fields.

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Figure 4. Flow curves: (a) F-CI/a, b) F-CI/g, c) F-CI/Si (1: H = 0, E = 0; 2: H = 0, E = 1.36 kV/mm; 3: H = 100 kA/m, E = 0; 4: H = 100 kA/m, E = 1.36 kV/mm).

Possibly, this occurs because of the action of electric field and creation of the most beneficial polarization conditions for maximum structuring of each of the dispersed phase components separately. The magnetosensitive g-Fe2O3 and a-Fe2O3 particles depending on the relationship between the intensities of the two fields form agglomerates with CI particles, which are stronger than those formed only by the magnetic field, since having united structures, they are capable of being attached to the electrodes. These peculiarities of structuring for MERF with different fillers create a peculiar nature for every composition that displays the synergistic effect as shown in Figure 5. For fluid F-CI/a, synergistic effect is increased continuously. For F-CI/g only when H . 60 kA/m, at H \ 20 kA/m, the synergistic effect is most attributable to the electric field. F-CI/Si displays (Figure 5(c)) the highest synergistic effect under all intensities of electric and magnetic fields. It was found that for MERFs with a-Fe2O3 or g-Fe2O3, the decrease in Dt EH in comparison with D tH occurs at higher shear rates. On increasing the shear rate, the transformations in the electrorheological structures and magnetorheological structures are intensified differently depending on the relationship between the electric and magnetic forces, as well as on the number of particles involved in the aggregates. It appears possible to not only increase but also lower the induced stress

by varying the level of intensities and hydrodynamic regimes. Probably this occurs because part of ferromagnetic particles from these structures are entrained into the common, with-CI magnetic structures increasing their strength not only due to their own magnetic properties, but also because of their attachment to the electrodes, just as in the case of the electrorheological effect.

Conclusion Investigations have shown that an MERF containing a complex dispersed phase in which one of the components is highly sensitive only to the impact of an electric field (SiO2) and the other one, that is, CI, is highly sensitive only to the impact of a magnetic field, shows the strong synergistic effect in the entire investigated range of intensities of electric and magnetic fields. For such types of fluids, the formation of separate strong electric and magnetic structures is typical. An MERF containing ferromagnetic particles sensitive to the impact of an electric field displays an increasing synergistic effect while reaching saturation magnetization. The second region of the synergistic effect in such fluids is created by fields due to the appearance of strong electrostructures from g-Fe2O3 particles in weak magnetic fields. It disappears on increase in the magnetic field strength. The MERFs containing a complex dispersed phase in which one

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Figure 5. Shear stress of MERF versus the magnetic field strength: (a) F-CI/g; (b and d) F-CI/a; (c) F-CI/Si; (a–c) g = 2.2 s21; (d) g = 36 s21; 1: E = 0, 2: E = 1.8 kV/mm.

component is weakly sensitive to the actions of both electric and magnetic fields (a-Fe2O3) and the other component is CI have only one zone of synergistic effect. For such types of fluids, the formation of structures that are identical for the particles of the two fillers is also typical. Acknowledgment The authors acknowledge Belarusian Fund for Fundamental Research for financial support of investigations (Project No. T09MS-018).

References Gorodkin S, Kordonsky W and Medvedeva E (1991) Rheological measurements of structure-reversible medium sensitive to the electrical and magnetic fields. In: Problems of Heat and Hass Transfer-91, Minsk, HMTI NAS Belarus, 48–51. (in Russian). Hao T, Kawai A and Ikazaki F (1998) Mechanism of the electrorheological effect: Evidence from the conductive, dielectric, and surface characteristics of water-free electrorheological fluids. Langmuir 14(5): 1256–1262.

Ikazaki F, Kawai A, Uchida K, Kawakami T, Edamura K, Sakurai K, et al. (1998) Mechanisms of electrorheology: The effect of the dielectric property. Journal of Physics D: Applied Physics 31: 336–347. Kordonsky W, Gorodkin S and Medvedeva E (1994) First experiments on magnetoelectro-rheological fluid (MERF). Proceedings of the 4th International Conference on ER Fluids, Feldkirch, Austria, July 20–23, 1993, 23–36. Kordonski WI, Gorodkin SR and Novikova ZA (1998) The influence of ferroparticle concentration and size on MR fluid properties. Proceedings of the 6th International Conference on Electro-Rheological Fluids, Magneto-Rheological Suspensions and Their Applications, Yonezawa, Japan, July 22–25, 1997, 535–542. Korobko EV, Zhurauski MA, Novikova ZA and Kuzmin VA (2009) Rheological properties of magnetoelectrorheological fluids with complex disperse phase. Journal of Physics: Conference Series 149: 012065. Koyama K (1996) Rheological synergistic effects of electric and magnetic fields in iron particle suspension. International Journal of Modern Physics B 10(23–24): 3067–3072. Minagawa K, Watanabe T, Koyama K and Sasaki M (1994) Significant synergistic effect of superimposed electric and magnetic fields on the rheology of iron suspension. Langmuir 10: 3926–3928.