Langmuir 1994,10, 3926-3928
3926
Significant Synergistic Effect of Superimposed Electric and Magnetic Fields on the Rheology of Iron Suspension Keiji Minagawa,? Tom Watanabe,? Kiyohito Koyama,*>?and Makoto SasakiS Department of Materials Science and Engineering, Yamagata University, Yonezawa 992, Japan, and Nippon Oil Co., Ltd., Naka-ku, Yokohama 231, Japan Received May 27, 1994. In Final Form: September 12, 1994@ The effects of electric and magnetic fields on the rheological properties of a suspension of iron particles was studied with a parallel plate rheometer. The suspension exhibited a large increase in its yield stress upon application of the electric and/or magnetic fields. In particular, the combination of the electric and magnetic fields induced a drastic increase of the yield stress owing to the aggregation of the dispersed particles. With this eledromametorheological (EMR) fluid, the electric and magnetic fields exhibited a significant synergism to increase the yield stress.
I. Introduction Polarizable particles dispersed in a nonpolar solvent aggregate to form clusters when subjected to a strong electric field. The a g e g a t i o n of the particles causes a reversible increase of appment viscosity of the suspension. This phenomenon is known as an electrorheological (ER) and has be6n attracting much attention because of the possible application as a new device for rapid, direct transformation of electric signals into mechanical force. A similar effect induced by a magnetic field is referred to as magnetorheological (MR) effect,4ss though the studies of MR effect have been not so many. In order to utilize these rheological effects, the control of yield stress by the external field is important. Since the yield stress is governed by the resistance of the particle aggregation against shear deformation, it is desired to enhance the interacting force among the particles. The structure of particle cluster has been extensively discussed elsewhere. The induced particle chains can be microscopically observed3s6for some suspensions and are simulated using appropriate The shear flow also affects the aggregation of the particles and makes the interpretation more c~mplicated.~ Some reports describe the growth of particle chains into thick column s t r u c t ~ r e s , ~ which J~J~ cause the enhancement of the yield stress. The control of aggregated structure is thus essential for larger stress increase. Recently we have suggested an attempt to facilitate the particle interaction by simultaneous application of electric and magnetic fields to a single fluid and termed the fluid responsive to the both fields as a n electromagnetorheo-
* To whom correspondence should be addressed. + Yamagata
University.
* Nippon Oil Co., Ltd. @
Abstract published inAdvance ACSAbstracts, October 15,1994.
(1)Block, H.; Kelly, J. P. J . Phys. D 1988,21,1661. (2)Jordan, T. C.; Shaw, M. T. IEEE Trans. Ekctr. Insul. 1989,24,
849. (3)Halsey, T.C. Science 1992,258,761. (4)Schulman, Z. P.;Kordonsky, V. I.; Zaltsgendler, E. A.; Prokhorow, I. V.; Khusid, B. M.; Demchuk, S. A. Int. J . Multiphase Flow 1986,12, 1935. ( 5 ) Lemaire, E.; Bossis, G. J . Phys. D 1991,24,1473. ( 6 ) Klingenberg, D. J.; Zukoski, C. F. Langmuir 1090,6, 15. (7) See, H.; Doi, M. J . Rheol. 1992,36, 1033. (8) Takimoto, J. In Proceedings of the 3rd International Conference on ER Fluids; Tao, R., Ed.; World Scientific: Singapore, 1992;p 53. (9)Lemaire, E.; Bossis, G . ; Grasselli, Y. Langmuir 1992,8, 2957. (10)Sprecher, A.F.; Chen, Y.; Choi, Y.; Conrad, H. In Proceedrngs of the 3rd International Conference on ER Fluids; Tao, R., Ed.; World Scientific: Singapore, 1992;p 142. (11)Tanaka, K.; Yoshida, T.; Koyama, K. In Proceedings of the 3rd International Conference on ER Flurds; Tao, R., Ed.; World Scientific: Singapore, 1992;p 289.
logical (EMR)fluid.12 We reported the principal strategy for utilization ofEMFt fluids together with the development of a n apparatus capable of quantitative measurement of the EMR effect. In the previous work, however, the EMR effect observed was still smaller than the ER effect of other ER fluids, owing to the nature of the suspension used. It is necessary to find a proper suspension in which the particles efficiently interact with each other responding to both the electric and magnetic fields. Here we present a novel fluid which has a remarkable response to the both fields. The behavior of the EMR fluid depending on the external field conditions is discussed in terms of the change of particle aggregation induced by electric andlor magnetic fields.
11. Materials and Method The preparation of the particle was carried out at Nippon Oil Co.,Ltd. Iron particles with the diameter of 1- lOpm were used. The surface of these particles was covered with titania as the nonconducting skin layer. The coercivity of the particle was 100 Oe. The particles were suspended in a 400 cSt silicone oil at concentrations of 2, 6, and 11 vol %. The rheological measurements were carried out by use of a parallel-plate rheometer which we developed in our previous work.12 The electric field was applied to the fluid by the electrodes attached at the surfaces of the two parallel plates. The gap between the two plates was 0.5 mm. The bottom plate was slid to one direction and the displacement of the upper plate was detected by a U-gage. Detected signals were digitized with an analogto digital converter,and the data were stored in a personal computer. With this detection system, any force influencing the displacement of the upper plate, e.g., a force perpendicular to the plate, can be detected as the change of the stress value. Two electromagneticcoils sandwiched the parallel plates from upper and lower sides. The distance between the two coils was 6 mm. The electric field strength was varied from 0 to 2 kV/mm and a magnetic field ranging from 0 to 600 Oe was applied by the coils. The sample was sheared at a constant rate of 2.8 s-1. 111. Results and Discussion Parts a and b of Figure 1show the response of the shear stress to the 2 kV/mm electric field and to the 600 Oe magnetic field, respectively. The shear stress increased when the fluid was subjected to the electric field or the magnetic field, which revealed that the iron suspension has the characteristics of both the ER and MR fluids. Under the present condition, the stress value did not reach a constant value after the yield; the stress increased under the electric field and decreased under the magnetic field as the fluid was sheared. The increase and decrease of ~
~~
(12)Minagawa, K.;Watanabe, T.; Munakata, M.; Koyama, K. J.NonNewtonian Fluid Mech. 1994,52,59.
0743-7463/94/2410-3926$04.50/00 1994 American Chemical Society
Letters
Langmuir, Vol. 10,No. 11, 1994 3927 2000
-5
2 1000 v
z e
Eon
800-
(d
Eoff
w
@
ER effect (ZkV/mm)
1000
v!
Figure 1. Stress response of the EMR fluid to (a) 2kV/mm dc electric field and (b) 600 Oe magnetic field, under a constant shear flow. Shear rate p = 2.8 s-l. H, E, and p represent the magnetic field, electric field, and shear, and subscript on and off indicate the switching-on and switching-off,respectively.
H on
; ; i 2000
,
0
Time (sec)
5
0
MR effect
/
I/
1
200
1
,
400
)
600
H @e) Figure 3. Comparison between the stress increase due to the EMR effect (OEEMX,circles) and the MR effect (UMR, triangles) plotted against the magnetic field strength. The electric field was 2 kV/mm for the EMR measurements. The broken line indicates the UEMR values estimated assuming that the UEMR is represented by the simple addition of the ER and MR effects. The deviation of UEMR line from the broken line indicates the extra stress due to the synergistic effect of the superimposed fields.
E;
15"[ I000
e,
500
1
V
Shear
w Time (sec)
Figure 2. Response of the EMR fluid to a superimposed magnetic and electric fields with different application procedures: (a) electric field was applied before magnetic field and (b)magnetic field was applied before electric field. p = 2.8 s-l, E = 2 kV/mm, H = 600 Oe.
the stress are explained as the growth of the particle chains into thick columns and as the particle aggregation a t one of the plates, respectively. Similar phenomena have been discussed in our recent study13of the different effects of the ac and dc fields on a n ER suspension with needle-like iron particles. Although the differencein the stress change is interesting for the understanding of the ER and MR effects, here we deal with only the yield stress to focus on the general feature of the ER, MR, and EMR effects. Figure 2 shows the stress responses to the superimposed application of electric and magnetic fields under shear deformation with different application order. In Figure 2a, the shear stress was increased upon application of the electric field and was increased further when the magnetic field was applied in the presence of the electric field. Similar two-step stress increase was observed in the case that the fields were applied with opposite order (Figure 2b). In this case, however, the stress decreased after reaching maximum under the superimposed fields. Therefore, it would be better to apply electric field before magnetic field in order to obtain large steady stress value. It is found from the comparison of the Figures l a , lb, and 2a that the total increase of the stress value, which is regarded as the EMR effect,is much larger than the values due to the ER or MR effect. It should be noted that the stress values must be corrected for the effects of the fields on extending or narrowing the gap between parallel plates, which are (13)Kumakura,Y.;Watanabe, T.; Minagawa, K.; Koyama, K. Polym. Prepr. (Am. Chem. SOC.,Diu. Polym. Chem.) 1994,35 (2),362.
Figure 4. Plausible behaviors ofthe particles, which generate the synergisticstress increase under the superimposedelectric and magnetic fields.
detected as the changes of apparent shear stress. In fact, the apparent stress value increased when the magnetic field was applied to the fluid even without shear. This effect is due to the characteristic of the detection system in the apparatus, which was discussed in our previous paper.12 The stress values describing the MR and EMR effects were obtained by subtracting the magnetically increased stress under no shear from the total stress increase. We discuss the EMR effect by comparing the increase of yield stress on the application of the magnetic field in the absence and presence of the electric field. In Figure 3, the values of stress increase from the base line (in the steady shear) to the maximum stress under the magnetic field (UR) and the stress increase under 2 kV/mm electric field ( u E ~are ) plotted against the strength of magnetic field. Here the data for EMR effect are those obtained with the procedure in which electric field was applied prior to magnetic field. Assuming that the EMR effect is the simple addition of the ER and MR effects, the 2 kV/mm field should give a shift of the plot parallel to the UMR line, as indicated by the broken line. The actual UEMR values, however, deviate from the prediction. The superimposed fields gave drastic increase of the stress value, indicating the existence of a significant synergism in the EMR effect. The deviation of the two U E lines ~ shown in Figures 3 represents the degree of synergism in the EMR effect. The synergistic effect is more enhanced with the increase of magnetic field strength. Figure 4 schematically illustrates a possible explanation of the synergistic EMR effect. The particles form chains upon applying the electric field and the subsequent application of the magnetic field facilitates the more aggregation of the particle chains. The resulting clusters resist against the shear deformation and cause the large stress increase. Recent studies on ER fluids have elu-
3928 Langmuir, Vol. 10, No. 11, 1994 cidated the formation of thick columns by particle chains during the application of electric field.3J0J1 The yield stress of ER fluids is increased when such thick clusters are induced. Our interpretation on the EMR effect is based on the speculation that the magnetic field facilitates the particle interaction, and the resulting structure to be stronger. In the case of opposite application procedure of the fields as exemplified in Figure 2b, the effect of electric field may be too weak to make such strong cluster structure as that illustrated in Figure 4. It is needed to clarify the aggregated structure of particles formed under each field condition in order to understand the mechanism. It is also of interest to consider the role of titania surface layer of the particle, which is possibly important in the significant EMR effect. Further studies on the mechanism of the synergistic EMR effect are in progress to elucidate
Letters the aggregated structure of particles. Although detailed mechanism of the phenomenon is still unclear yet, the extension of the stress region available with a single fluid as well as the large absolute stress value is expected to be useful for various applications. In conclusion, the suspension of titania-coated iron particle exhibits a significant EMR effect, which is much larger than the addition of the ER and MR effects. The synergistic rheological effect on the stress increase by the superimposed electric and magnetic fields is interpreted as the facilitation of the particle aggregation.
Acknowledgment. We wish to thank Mr. Yoshiki Kumakura for valuable help. This work was partly supported by the Grant-in-Aid from the Ministry of Education, Science and Culture, Japan.
