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A novel apparatus for rheological measurements of electro-magneto-rheological fluids
59
Journal of Non-Newtonian Fluid Mechanics, 52 (1994) 59-67 Elsevier Science B.V.
A novel apparatus for rheological measurements electro-magneto-rheological fluids K. Minagawa, T. Watanabe,
M. Munakata
and K. Koyama
of
*
Faculty of Engineering, Yamagata University, Yonezawa 992 (Japan) (Received September 26, 1993; in revised form December 6, 1993)
Abstract A parallel-plate rheometer equipped with electrodes and magnetic coils was developed for rheological measurements of electro-magneto-rheological (EMR) fluids which respond to both electric and magnetic fields. The effects of electric and magnetic fields perpendicular to the direction of shear flow were examined. It has been found that the shear stress of the fluid depends not only on the strength of the fields but also on the order of application. The apparent viscosity of the EMR fluid under shear flow increased when the electric and magnetic fields were applied simultaneously. The increase in viscosity was lower when only an electric or magnetic field was applied. The dependence of EMR fluid behavior on the external field conditions is discussed in terms of the difference between cluster structures induced by electric and/or magnetic fields. Keywords: electro-magneto-rheological measurement
(EMR) fluids; parallel-plate
rheometer;
rheological
1. Introduction The electrorheological (ER) effect is known as a reversible change of the apparent viscosity of fluid under an external electric field [l-4]. This phenomenon is expected to give us various possibilities for utilization because the viscosity can be reversibly controlled by changing external field conditions. Despite many attempts at utilization, however, there has been no report on the practical application of any ER fluid as yet. Typical ER fluids reported are suspensions consisting of polarizable particles and insulating solvent. The particles aggregate and form fibrous * Corresponding
author.
0377-0257/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDI 0377-0257(93)01220-X
60
K. Minagawa et al. /J. Non-Newtonian Fluid Mech. 52 (1994) 59-67
cluster structures under an external electric field [ 1,2,5-81. The viscosity change, of several orders of magnitude, is interpreted as the resistance of the fibrous structure against shear deformation. The fibers are broken when the shear stress exceeds a yield stress. It is thus important to make a very aggregated structure for any practical applications of ER fluids. To create an aggregated structure in ER fluids we suggest the use of a magnetic field together with an electric field. It is known that magnetic fields also affect the rheological properties of fluids [9]. For such magnetic fluids, however, the viscosity change is not so drastic as that for ER fluids. Consequently, we suggest affecting the rheological properties of such fluids, which respond to electric and magnetic fields, by simultaneous application of both fields. We term these fluids electro-magneto-rheological (EMR) fluids. The application of a magnetic field together with an electric field is an attempt to obtain a more aggregated and organized structure; for example a network structure of the fluid, resulting in a more drastic change of the fluid viscosity. Fujita et al. reported an attempt at applying both electric and magnetic fields to control the rheological properties of a fluid [lo]. In their system, however, the magnetic field is not fully utilized for a fine control of the rheological properties because the field direction is inhomogeneous within the fluid. The magnetic force in this case mainly helps to decrease the sedimentation of dispersed particles. The effect of a magnetic field is expected to be significant if the field direction is maintained homogeneous, because the particle orientation and resulting cluster structure can be controlled by changing the external field conditions. It may be possible to build up a structure, e.g. a network structure, which would enhance the viscosity change more substantially. Based on this idea, we have designed a novel parallel-plate rheometer in which the electric and magnetic fields are applied simultaneously in the fluid. In this apparatus, the electrodes are placed on the surface of two parallel plates, and the magnetic coils sandwich the plates. The direction of the electric field is perpendicular to the shearing flow, while the magnetic field is applied along the direction of (parallel-field type, Fig. l(a)), or perpendicular (crossed-field type, Fig. l(b)) to, the direction of the electric field by changing the position of the coils. Fields with different directions and/or duration times are expected to be suitable for the control of cluster structure formation and have the rheological properties of fluids. In the present work, we introduce the parallel-plate rheometer in which the directions of both electric and magnetic fields are parallel and perpendicular to the shearing flow (Fig. l(a)). The measurement of rheological properties of an EMR fluid is also demonstrated.
K. Minagawa et al. 1 J. Non-Newtonian Fluid Mech. 52 (1994) 59-67
61
0a
Fig. 1. Schematic illustration of parallel-plate viscometer with electrodes and electromagnetic coils: (a) parallel-field type; (b) crossed-field type.
2. Experimental 2.1 Preparation of EA4R @id Needle-like iron particles with a long-axis length of 0.2-0.3 ,um were used. The ratio of long and short axes of the particle was about 10. The coercive force and the specific gravity of the particles were 1625 Oe and 5.2 g cmm3, respectively. The surface of the particle was covered with a thin layer of iron oxide. The particles were dispersed in a 30 cSt silicone oil at a concentration of 4 vol.%. 2.2 Apparatus A rheometer suitable for the rheological measurement of EMR fluids was developed by modifying our home-made parallel-plate rheometer [5] used for ER fluids. The fluid sample between the plates was sheared at a constant shear rate. The gap between the two plates was 0.5 mm. The bottom plate was slid in one direction and the displacement of the upper plate was detected by a U-gage. Detected signals were digitized with an A/D converter, and the data were stored in a personal computer. With this detection system, any force influencing the displacement of the upper plate can be detected [5]. For example, a force perpendicular to the plate is detected as a change in the stress. Thus it is possible to evaluate the effects by extending or narrowing the gap between the two plates. Two electromagnetic coils were placed at positions sandwiching the two plates from both upper and lower sides. The electromagnetic coils were made of urethane coated copper wire of diameter 2 mm. The number of
62
1% ~~na~awa et al. / J. eon-~ewton~n
Fluid Meek. 52 (1994) 59-67
turns was 200. The coils can be cooled by water, if necessary, to avoid any over-heating arising from a large current. The distance between the two coils was 6 mm. Maximum field strength obtained was 2 kOe. A blank test, i.e. measurement without sample fluid, was carried out prior to the experiments with the EMR fluid under electric and magnetic fields. It was found that the response to a 2-kOe ma~etic field without fluid is negligibly small, indicating no apparent magnetic influence on the detection system. It has also previously been confirmed that the reliability of measurements under an electric field is sufficient [5]. The viscosity of the EMR fluid was measured under a 3 kV mm d.c. electric field and 2-kOe magnetic field. The shear rate was kept constant at either 2.8 or 4.0 s-l. 3. Results and discussion Figure 2 shows the response of the EMR fluid to shear deformation. Although some high frequency noise is detected in the stress values, this is a problem of the data processing and the noise can be reduced by using appropriate filtering techniques. The stress exhibited a small yield value at the beginning of the shear defo~ation, and after that, the stress was almost constant, reaching several tens of Pascal under a steady shear rate of 4.0 s-‘. The viscosity estimated from the constant region of the shear stress value in *Fig. 2 was about 10 Pa.s. We checked the characteristics of the detection system of the apparatus under a magnetic field, before carrying out the measurements with the
TIMEl (set)
TIMEI (set)
Fig. 2. Response of EMR fluid to shear flow; shear rate 9 = 4.0 s-l. it,,, and foff indicate the switching-on and switching-off of the shear strain, respectively. Fig. 3. Response of EMR fluid to magnetic field (first cycle); A&= 2 kOe. A& and Noff indicate the switching-on and switching-off of the magnetic field, respectively.
K. Minagawa et al. 1 J. Non-Newtonian Fluid Mech. 52 (1994) 59-67
63
600
-200
0
3
6
TIME (set)
9
TIME (set)
Fig. 4. Response of EMR fluid to magnetic field (second cycle); A4 = 2 kOe. Fig. 5. Response of EMR fluid to shear flow after removing magnetic field; 1; = 4.0 SC’.
various combinations of shear flow and electric and magnetic fields. The stress values were measured under magnetic field without shear (Figs 3-5). The stress values changed during the application of the magnetic field. The stress change is interpreted as the change in the vertical position of the parallel plates due to the normal stress induced by the magnetic field. Although the stress value is detected in the horizontal direction of the parallel plates, a small change of the vertical position of the two plates is detected also as the stress values change because of the high sensitivity of the detection system. In fact, we have confirmed that similar, but opposite, stress is observed by pressing the upper plate down to approach the lower one. The characteristic features of the response to a magnetic field are as follows. As shown in Fig. 3, the initial application of a magnetic field to the fluid results in an abrupt increase of stress up to 200 Pa, followed by a decrease to a constant value of about 20 Pa. When the field is removed, the stress value decreases further, giving a negative stress value of about - 100 Pa. This result indicates that the dispersed particles aggregate to form a cluster structure under a magnetic field and that the organized structure still remains even though the magnetic field is removed. There should be some difference between the cluster structures formed in the presence and absence of a magnetic field. The different cluster structures would create normal forces with opposite directions, i.e. the structure under a magnetic field expands and the structure after removing the field contracts the plates. The stress values observed without shear are interpreted as being due to these normal forces. The above interpretation is supported by the results of repeated measurements under the same magnetic field. Figure 4 shows the result of the
K. Minagawa et al. 1 J. Non-Newtonian Fluid Mech. 52 (1994) 59-67
64
600
-200
3
6
TIME (set)
9
TIME (set)
Fig. 6. Response of EMR fluid to shear flow under magnetic field; M = 2 kOe, J = 4.0 s-l. Fig. 7. Response of EMR fluid to shear flow under electric field. The strength of the applied electric field E = 3 kV mm-‘, and v = 2.8 s-‘. Eon and Eoff indicate the switching-on and switching-off, respectively, of the electric field.
second measurement. It is found from comparison of Figs 3 and 4 that the stress values during and after the application of the magnetic field were quite similar. The repeated measurements gave similar results to those shown in Fig. 4. These results indicate that the fluid has two stable states ,depending on the magnetic field. The stress value returned to zero when a large shear deformation was applied to the sample, as shown in Fig. 5, indicating that the cluster structure was destroyed by shear flow. As a next step, we examined the stress response of the EMR fluid under shear flow, in the presence of a magnetic and/or electric field. Figure 6 shows the response to shearing flow under a 2-kOe magnetic field. The shear was applied after the stress had decreased to a constant value, (i.e. at 3 s in Fig. 6). The stress immediately increased up to about 200 Pa, responding to shear. During the shearing flow, the stress value was almost constant (200 Pa). The higher stress in comparison with the value when no magnetic field was applied is due to the magnetically induced tight structure of the particles. Figure 7 shows the response to shear flow under a 3 kV 111111-lelectric field. The stress value slightly increases when the electric field is switched on. When the shear is applied (ion) in the presence of the electric field, the stress increases abruptly up to 200 Pa, followed by a gradual decrease down to about zero. The decrease of stress from yen to E,,, is speculated to be due to some structural change of the particle cluster. Figures 8 and 9 show the stress response under combined application of electric and magnetic fields without shear flow. In Fig. 8, the stress was measured under magnetic and electric fields applied in this order. Figure 9
K. ~~naga~a et al. / J. Nan-Ngwt~n~n Fluid Mech. 52 (1994) 59-67
TIME (set)
65
TIME (see)
Fig. 8. Response of EMR fluid to magnetic field followed by electric field; B# = 2 kOe, E=3kVmn--‘. Fig. 9. Response of EMR fluid to electric field followed by magnetic field; M = 2 kOe, E=3kVmm-‘.
shows the result obtained with the two fields applied in the opposite order to that of Fig. 8. As shown in Fig. 8, the stress change was rather small when the magnetic field was applied first. On the other hand, when the magnetic field was applied after the electric field, a marked increase of the stress was observed (Fig. 9). As we have mentioned above, the stress value obtained without shear flow is due to the force normal to the parallel plates of the rheometer. In both the above cases the detected stress values reflect the normal stress induced by the formation of cluster structures. The different responses would be due to the difference in mobility of the particles within the ordered cluster structures induced by electric and by magnetic fields. It is speculated that the particles under a magnetic field can hardly move to form any organized structure. On the other hand, the electrically induced structure is rater loose and can easily move in response to the magnetic field, resulting in the large “normal stress” detected. The two different structures induced by electric and magnetic fields show different responses to the shear flow. Figures 10 and 11 show the response measured in two different ways. In Fig. 10, the electric field, magnetic field, and shear flow of shear rate 2.8 s-’ were applied in that order. In Fig. 11, the magnetic field was applied first, followed by shear flow and electric field. The combination of the electric and magnetic fields gave much higher stress values, up to about 400 Pa, than those obtained under either electric or magnetic field above. For both cases, the particles aggregate and form a highly ordered rigid structure. The stress decrease is due to some structural change of the cluster structure under the combined fieIds, This behavior is interpreted in terms of the cluster structures as follows.
66
K. Minagawa
et al. /J. Non-Newtonian
1
-200 ;
I
I
I 9
600
I
-200 I,
Fluid Mech. 52 (1994) 59-67
-
I 3 TIME
6 (W
Fig. 10. Response of EMR fluid to shear flow under electric and magnetic fields; M = 2 kOe, E = 3 kV mm-‘, j = 2.8 s-‘. Fig. 11. Response of EMR fluid to electric field under magnetic M = 2 kOe, E = 3 kV mm-‘, d = 2.8 SK’.
field and shear flow;
In the usual ER fluids, the dispersed particles orient and aggregate to clusters after application of an external electric field [ 1,2,5-81. The drastic increase in the apparent viscosity of ER fluids is due to the bridging of the electrodes by the clusters. The similar cluster formation induced by a magnetic field is also known for magnetic fluids [9]. The structure of the cluster depends on the material properties and/or field conditions. Since the induced magnetic and electric dipoles have different directions in the needle-like particles in the case of our EMR fluid, the response of orientation under electric and magnetic fields should differ from each other. In addition, the orientational response to an electric field is faster than the response to a magnetic field under the present conditions. The rapid response of the electric polarization may cause an induced dipole whose direction is independent of the particle orientation. On the other hand, the magnetic field induces dipoles along the long axis of the particle, which makes the particles reorient along the field direction. The orientation of the long axis parallel to the field direction has the effect of increasing the measured stress, including the normal stress to the plates. When the magnetic field is removed, the highly oriented clusters are broken up, but the particle aggregation still remains, due to the remaining magnetization. The aggregated structure remaining after removing the magnetic field exhibits a more compact and contracted form, which results in a decrease in the stress. As shown, the EMR fluid has an enhanced response to simultaneous application of electric and magnetic fields. Such a feature provides the possibility for fine control of the rheological properties of fluids.
K. Minagawa et al. /J. Non-Newtonian Fluid Mech. 52 (1994) 59-67
67
4. Conclusions A parallel-plate rheometer suitable for measurements of rheological properties of fluids under electric and magnetic fields has been developed. The shear stress of the EMR fluid depended not only on the strength of the fields but also on the application order. The various behaviors of our EMR fluid depend on the external field conditions, indicating the difference between cluster structures induced by electric and magnetic fields. The novel rheometer is useful for the study of various EMR fluids which are expected to be high-performance materials because of the abundance of controllable parameters. Acknowledgments The authors would like to thank Professor T. Masuko and Mr. M. Kudo for providing the sample fluid, and Mr. Y. Seino for valuable help in setting up the experimental apparatus. The authors are also grateful to Dr. P. Riha for his helpful suggestions on the manuscript. References 1 2 3 4 5 6 7 8 9 10
W.M. Winslow, J. Appl. Phys., 20 (1949) 1137-1140. T.C. Jordan and M.T. Shaw, IEEE Trans. Electr. Insul., 24 (1989) 849-878. K. Tanaka, A. Fujii and K. Koyama, Polym. J., 24 (1992) 995-998. K. Koyama, K. Minagawa, T. Yoshida, N. Kuramoto and K. Tanaka, Proc. 4th Int. Conf. ER Fluids, in press. K. Tanaka, T. Yoshida and K. Koyama, in R. Tao (Ed.), Proc. 3rd Int. Conf. ER Fluids, World Scientific, 1992, pp. 289-299. A.F. Sprecher, J. D. Carlson and H. Conrad, Mater. Sci. Eng., 95 (1987) 187-197. D.J. Klingenberg and CF. Zukoski, Langmuir 6, (1990) 15-24. H. See and M. Dol, J. Rheol., 36 (1992) 1033-1055. S. Kamiyama and A. Satoh, J. Colloid Interface Sci., 127 (1989) 173-188. T. Fujita, J. Mochizuki and I.J. Lin, J. Magn. Magn. Mater., 122 (1993) 29-33.
Journal of Non-Newtonian Fluid Mechanics, 52 (1994) 59-67 Elsevier Science B.V.
A novel apparatus for rheological measurements electro-magneto-rheological fluids K. Minagawa, T. Watanabe,
M. Munakata
and K. Koyama
of
*
Faculty of Engineering, Yamagata University, Yonezawa 992 (Japan) (Received September 26, 1993; in revised form December 6, 1993)
Abstract A parallel-plate rheometer equipped with electrodes and magnetic coils was developed for rheological measurements of electro-magneto-rheological (EMR) fluids which respond to both electric and magnetic fields. The effects of electric and magnetic fields perpendicular to the direction of shear flow were examined. It has been found that the shear stress of the fluid depends not only on the strength of the fields but also on the order of application. The apparent viscosity of the EMR fluid under shear flow increased when the electric and magnetic fields were applied simultaneously. The increase in viscosity was lower when only an electric or magnetic field was applied. The dependence of EMR fluid behavior on the external field conditions is discussed in terms of the difference between cluster structures induced by electric and/or magnetic fields. Keywords: electro-magneto-rheological measurement
(EMR) fluids; parallel-plate
rheometer;
rheological
1. Introduction The electrorheological (ER) effect is known as a reversible change of the apparent viscosity of fluid under an external electric field [l-4]. This phenomenon is expected to give us various possibilities for utilization because the viscosity can be reversibly controlled by changing external field conditions. Despite many attempts at utilization, however, there has been no report on the practical application of any ER fluid as yet. Typical ER fluids reported are suspensions consisting of polarizable particles and insulating solvent. The particles aggregate and form fibrous * Corresponding
author.
0377-0257/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDI 0377-0257(93)01220-X
60
K. Minagawa et al. /J. Non-Newtonian Fluid Mech. 52 (1994) 59-67
cluster structures under an external electric field [ 1,2,5-81. The viscosity change, of several orders of magnitude, is interpreted as the resistance of the fibrous structure against shear deformation. The fibers are broken when the shear stress exceeds a yield stress. It is thus important to make a very aggregated structure for any practical applications of ER fluids. To create an aggregated structure in ER fluids we suggest the use of a magnetic field together with an electric field. It is known that magnetic fields also affect the rheological properties of fluids [9]. For such magnetic fluids, however, the viscosity change is not so drastic as that for ER fluids. Consequently, we suggest affecting the rheological properties of such fluids, which respond to electric and magnetic fields, by simultaneous application of both fields. We term these fluids electro-magneto-rheological (EMR) fluids. The application of a magnetic field together with an electric field is an attempt to obtain a more aggregated and organized structure; for example a network structure of the fluid, resulting in a more drastic change of the fluid viscosity. Fujita et al. reported an attempt at applying both electric and magnetic fields to control the rheological properties of a fluid [lo]. In their system, however, the magnetic field is not fully utilized for a fine control of the rheological properties because the field direction is inhomogeneous within the fluid. The magnetic force in this case mainly helps to decrease the sedimentation of dispersed particles. The effect of a magnetic field is expected to be significant if the field direction is maintained homogeneous, because the particle orientation and resulting cluster structure can be controlled by changing the external field conditions. It may be possible to build up a structure, e.g. a network structure, which would enhance the viscosity change more substantially. Based on this idea, we have designed a novel parallel-plate rheometer in which the electric and magnetic fields are applied simultaneously in the fluid. In this apparatus, the electrodes are placed on the surface of two parallel plates, and the magnetic coils sandwich the plates. The direction of the electric field is perpendicular to the shearing flow, while the magnetic field is applied along the direction of (parallel-field type, Fig. l(a)), or perpendicular (crossed-field type, Fig. l(b)) to, the direction of the electric field by changing the position of the coils. Fields with different directions and/or duration times are expected to be suitable for the control of cluster structure formation and have the rheological properties of fluids. In the present work, we introduce the parallel-plate rheometer in which the directions of both electric and magnetic fields are parallel and perpendicular to the shearing flow (Fig. l(a)). The measurement of rheological properties of an EMR fluid is also demonstrated.
K. Minagawa et al. 1 J. Non-Newtonian Fluid Mech. 52 (1994) 59-67
61
0a
Fig. 1. Schematic illustration of parallel-plate viscometer with electrodes and electromagnetic coils: (a) parallel-field type; (b) crossed-field type.
2. Experimental 2.1 Preparation of EA4R @id Needle-like iron particles with a long-axis length of 0.2-0.3 ,um were used. The ratio of long and short axes of the particle was about 10. The coercive force and the specific gravity of the particles were 1625 Oe and 5.2 g cmm3, respectively. The surface of the particle was covered with a thin layer of iron oxide. The particles were dispersed in a 30 cSt silicone oil at a concentration of 4 vol.%. 2.2 Apparatus A rheometer suitable for the rheological measurement of EMR fluids was developed by modifying our home-made parallel-plate rheometer [5] used for ER fluids. The fluid sample between the plates was sheared at a constant shear rate. The gap between the two plates was 0.5 mm. The bottom plate was slid in one direction and the displacement of the upper plate was detected by a U-gage. Detected signals were digitized with an A/D converter, and the data were stored in a personal computer. With this detection system, any force influencing the displacement of the upper plate can be detected [5]. For example, a force perpendicular to the plate is detected as a change in the stress. Thus it is possible to evaluate the effects by extending or narrowing the gap between the two plates. Two electromagnetic coils were placed at positions sandwiching the two plates from both upper and lower sides. The electromagnetic coils were made of urethane coated copper wire of diameter 2 mm. The number of
62
1% ~~na~awa et al. / J. eon-~ewton~n
Fluid Meek. 52 (1994) 59-67
turns was 200. The coils can be cooled by water, if necessary, to avoid any over-heating arising from a large current. The distance between the two coils was 6 mm. Maximum field strength obtained was 2 kOe. A blank test, i.e. measurement without sample fluid, was carried out prior to the experiments with the EMR fluid under electric and magnetic fields. It was found that the response to a 2-kOe ma~etic field without fluid is negligibly small, indicating no apparent magnetic influence on the detection system. It has also previously been confirmed that the reliability of measurements under an electric field is sufficient [5]. The viscosity of the EMR fluid was measured under a 3 kV mm d.c. electric field and 2-kOe magnetic field. The shear rate was kept constant at either 2.8 or 4.0 s-l. 3. Results and discussion Figure 2 shows the response of the EMR fluid to shear deformation. Although some high frequency noise is detected in the stress values, this is a problem of the data processing and the noise can be reduced by using appropriate filtering techniques. The stress exhibited a small yield value at the beginning of the shear defo~ation, and after that, the stress was almost constant, reaching several tens of Pascal under a steady shear rate of 4.0 s-‘. The viscosity estimated from the constant region of the shear stress value in *Fig. 2 was about 10 Pa.s. We checked the characteristics of the detection system of the apparatus under a magnetic field, before carrying out the measurements with the
TIMEl (set)
TIMEI (set)
Fig. 2. Response of EMR fluid to shear flow; shear rate 9 = 4.0 s-l. it,,, and foff indicate the switching-on and switching-off of the shear strain, respectively. Fig. 3. Response of EMR fluid to magnetic field (first cycle); A&= 2 kOe. A& and Noff indicate the switching-on and switching-off of the magnetic field, respectively.
K. Minagawa et al. 1 J. Non-Newtonian Fluid Mech. 52 (1994) 59-67
63
600
-200
0
3
6
TIME (set)
9
TIME (set)
Fig. 4. Response of EMR fluid to magnetic field (second cycle); A4 = 2 kOe. Fig. 5. Response of EMR fluid to shear flow after removing magnetic field; 1; = 4.0 SC’.
various combinations of shear flow and electric and magnetic fields. The stress values were measured under magnetic field without shear (Figs 3-5). The stress values changed during the application of the magnetic field. The stress change is interpreted as the change in the vertical position of the parallel plates due to the normal stress induced by the magnetic field. Although the stress value is detected in the horizontal direction of the parallel plates, a small change of the vertical position of the two plates is detected also as the stress values change because of the high sensitivity of the detection system. In fact, we have confirmed that similar, but opposite, stress is observed by pressing the upper plate down to approach the lower one. The characteristic features of the response to a magnetic field are as follows. As shown in Fig. 3, the initial application of a magnetic field to the fluid results in an abrupt increase of stress up to 200 Pa, followed by a decrease to a constant value of about 20 Pa. When the field is removed, the stress value decreases further, giving a negative stress value of about - 100 Pa. This result indicates that the dispersed particles aggregate to form a cluster structure under a magnetic field and that the organized structure still remains even though the magnetic field is removed. There should be some difference between the cluster structures formed in the presence and absence of a magnetic field. The different cluster structures would create normal forces with opposite directions, i.e. the structure under a magnetic field expands and the structure after removing the field contracts the plates. The stress values observed without shear are interpreted as being due to these normal forces. The above interpretation is supported by the results of repeated measurements under the same magnetic field. Figure 4 shows the result of the
K. Minagawa et al. 1 J. Non-Newtonian Fluid Mech. 52 (1994) 59-67
64
600
-200
3
6
TIME (set)
9
TIME (set)
Fig. 6. Response of EMR fluid to shear flow under magnetic field; M = 2 kOe, J = 4.0 s-l. Fig. 7. Response of EMR fluid to shear flow under electric field. The strength of the applied electric field E = 3 kV mm-‘, and v = 2.8 s-‘. Eon and Eoff indicate the switching-on and switching-off, respectively, of the electric field.
second measurement. It is found from comparison of Figs 3 and 4 that the stress values during and after the application of the magnetic field were quite similar. The repeated measurements gave similar results to those shown in Fig. 4. These results indicate that the fluid has two stable states ,depending on the magnetic field. The stress value returned to zero when a large shear deformation was applied to the sample, as shown in Fig. 5, indicating that the cluster structure was destroyed by shear flow. As a next step, we examined the stress response of the EMR fluid under shear flow, in the presence of a magnetic and/or electric field. Figure 6 shows the response to shearing flow under a 2-kOe magnetic field. The shear was applied after the stress had decreased to a constant value, (i.e. at 3 s in Fig. 6). The stress immediately increased up to about 200 Pa, responding to shear. During the shearing flow, the stress value was almost constant (200 Pa). The higher stress in comparison with the value when no magnetic field was applied is due to the magnetically induced tight structure of the particles. Figure 7 shows the response to shear flow under a 3 kV 111111-lelectric field. The stress value slightly increases when the electric field is switched on. When the shear is applied (ion) in the presence of the electric field, the stress increases abruptly up to 200 Pa, followed by a gradual decrease down to about zero. The decrease of stress from yen to E,,, is speculated to be due to some structural change of the particle cluster. Figures 8 and 9 show the stress response under combined application of electric and magnetic fields without shear flow. In Fig. 8, the stress was measured under magnetic and electric fields applied in this order. Figure 9
K. ~~naga~a et al. / J. Nan-Ngwt~n~n Fluid Mech. 52 (1994) 59-67
TIME (set)
65
TIME (see)
Fig. 8. Response of EMR fluid to magnetic field followed by electric field; B# = 2 kOe, E=3kVmn--‘. Fig. 9. Response of EMR fluid to electric field followed by magnetic field; M = 2 kOe, E=3kVmm-‘.
shows the result obtained with the two fields applied in the opposite order to that of Fig. 8. As shown in Fig. 8, the stress change was rather small when the magnetic field was applied first. On the other hand, when the magnetic field was applied after the electric field, a marked increase of the stress was observed (Fig. 9). As we have mentioned above, the stress value obtained without shear flow is due to the force normal to the parallel plates of the rheometer. In both the above cases the detected stress values reflect the normal stress induced by the formation of cluster structures. The different responses would be due to the difference in mobility of the particles within the ordered cluster structures induced by electric and by magnetic fields. It is speculated that the particles under a magnetic field can hardly move to form any organized structure. On the other hand, the electrically induced structure is rater loose and can easily move in response to the magnetic field, resulting in the large “normal stress” detected. The two different structures induced by electric and magnetic fields show different responses to the shear flow. Figures 10 and 11 show the response measured in two different ways. In Fig. 10, the electric field, magnetic field, and shear flow of shear rate 2.8 s-’ were applied in that order. In Fig. 11, the magnetic field was applied first, followed by shear flow and electric field. The combination of the electric and magnetic fields gave much higher stress values, up to about 400 Pa, than those obtained under either electric or magnetic field above. For both cases, the particles aggregate and form a highly ordered rigid structure. The stress decrease is due to some structural change of the cluster structure under the combined fieIds, This behavior is interpreted in terms of the cluster structures as follows.
66
K. Minagawa
et al. /J. Non-Newtonian
1
-200 ;
I
I
I 9
600
I
-200 I,
Fluid Mech. 52 (1994) 59-67
-
I 3 TIME
6 (W
Fig. 10. Response of EMR fluid to shear flow under electric and magnetic fields; M = 2 kOe, E = 3 kV mm-‘, j = 2.8 s-‘. Fig. 11. Response of EMR fluid to electric field under magnetic M = 2 kOe, E = 3 kV mm-‘, d = 2.8 SK’.
field and shear flow;
In the usual ER fluids, the dispersed particles orient and aggregate to clusters after application of an external electric field [ 1,2,5-81. The drastic increase in the apparent viscosity of ER fluids is due to the bridging of the electrodes by the clusters. The similar cluster formation induced by a magnetic field is also known for magnetic fluids [9]. The structure of the cluster depends on the material properties and/or field conditions. Since the induced magnetic and electric dipoles have different directions in the needle-like particles in the case of our EMR fluid, the response of orientation under electric and magnetic fields should differ from each other. In addition, the orientational response to an electric field is faster than the response to a magnetic field under the present conditions. The rapid response of the electric polarization may cause an induced dipole whose direction is independent of the particle orientation. On the other hand, the magnetic field induces dipoles along the long axis of the particle, which makes the particles reorient along the field direction. The orientation of the long axis parallel to the field direction has the effect of increasing the measured stress, including the normal stress to the plates. When the magnetic field is removed, the highly oriented clusters are broken up, but the particle aggregation still remains, due to the remaining magnetization. The aggregated structure remaining after removing the magnetic field exhibits a more compact and contracted form, which results in a decrease in the stress. As shown, the EMR fluid has an enhanced response to simultaneous application of electric and magnetic fields. Such a feature provides the possibility for fine control of the rheological properties of fluids.
K. Minagawa et al. /J. Non-Newtonian Fluid Mech. 52 (1994) 59-67
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4. Conclusions A parallel-plate rheometer suitable for measurements of rheological properties of fluids under electric and magnetic fields has been developed. The shear stress of the EMR fluid depended not only on the strength of the fields but also on the application order. The various behaviors of our EMR fluid depend on the external field conditions, indicating the difference between cluster structures induced by electric and magnetic fields. The novel rheometer is useful for the study of various EMR fluids which are expected to be high-performance materials because of the abundance of controllable parameters. Acknowledgments The authors would like to thank Professor T. Masuko and Mr. M. Kudo for providing the sample fluid, and Mr. Y. Seino for valuable help in setting up the experimental apparatus. The authors are also grateful to Dr. P. Riha for his helpful suggestions on the manuscript. References 1 2 3 4 5 6 7 8 9 10
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