Encyclopedia of Smart Materials (Vols 1 and 2) - M. Schwartz (2002) Episode 6 pot

P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-E-drv January 12, 2002 1:3 376 ELECTRORHEOLOGICAL MATERIALS 7. H. Block, J.P. Kelly, A. Qin, and T. Watson, Langmuir 6:6 (1990). 8. R.A. Anderson, Langmuir 10: 2917 (1994). 9. T. Garino, D. Adolf, and B. Hance, in Proc. Int. Conf. ER Fluids, R. Tao, ed. World Scientific, Singapore, 1992, p. 167. 10. L.C. Davis, J. Appl. Phys. 72(4): 1334 (1992). 11. L.C. Davis, J. Appl. Phys. 73(2): 680 (1993). 12. L.C. Davis, Appl. Phys. Lett. 60(3): 319 (1992). 13. Y. Otsubo and K. Watanabe, J. Soc. Rheol. Jpn. 18: 111 (1990). 14. C.F. Zukoski, Annu. Rev. Mater. Sci. 23, 45 (1993). 15. P. Attten, J N. Foulc, and N. Felici, Int. J. Mod. Phys. B 8: 2731 (1994). 16. J N. Foulc, P. Attten, and N. Felici, J. Electrostatics 33: 103 (1994). 17. X. Tang, C.Wu, and H. Conrad, J. Rheol. 39(5): 1059 (1995). 18. X. Tang and H. Conrad J. Appl. Phys. 80(9): 5240 (1996). 19. C. Wu and H. Conrad, J. Phys. D: Appl. Phys. 29: 3147 (1996). 20. C. Wu and H. Conrad, J. Appl. Phys. 81(12): 8057 (1997). 21. C. Wu and H. Conrad, J. Appl. Phys. 81(1): 383 (1997). 22. A. Inoue, in Proc. Int. Conf. ER Fluids, J.D. Carlson, A.F. Sprecher, and H. Conrad, eds., Technomic, Lancaster-Basel, 1990, p. 176. 23. B. Khusid, and A. Acrivos, Phys. Rev. E 52: 1669 (1995). 24. J. Trlica, O. Quadrat, P. Bradna, V. Pavlinek, and P. Saha, J. Rheol. 40(5): 943 (1996). 25. H. See and T. Saito, Rheol. Acta 35: 233 (1996). 26. H. Ma, W.Wen, W.Y. Tam, and P.Sheng, Phys. Rev. Lett. 77(12): 2499 (1996). 27. W. Wen, H. Ma, W.Y. Tam, and P. Sheng, Phys. Rev.E 55(2): R1294 (1997). 28. U. Treasurer, L.H. Radzilowski, and F.E. Filisko, J. Rheol. 35(6): 1051 (1991). 29. F.E. Filisko, in Progress in Electrorheology, K.O. Havelka and F.E. Filisko, eds., Plenum Press, NY, 1994, p. 3. 30. A.W. Schubring and F.E. Filisko, in Progress: in Electrorheo- logy, K.O Havelka and F.E. Filisko, eds., Plenum Press, NY, 1994, p. 215. 31. A. Kawai, K. Uchida, K. Kamiya, A. Gotoh, S. Yoda, K. Urabe, and F. Ikazaki, Int. J. Modern Phys. B 10: 2849 (1996). 32. F. Ikazaki, A. Kawai, T. Kawakami, K. Edamura, K. Sakurai, H. Anzai, and Y. Asako, J. Appl. Phys. D 31: 336 (1998). 33. F. Ikazaki, A. Kawai, T. Kawakami, M. Konishi, and Y. Asako, Proc. Int. Conf. Electrorheological Magnetorheological Fluids, K. Koyama and M. Nakano, eds., World Scientific, Singapore, 1998, p. 205. 34. Z.Y Qiu, H. Zhang, Y. Tang, L.W. Zhou, C. Wei, S.H. Zhang, and E.V. Korobko, Proc. Int. Conf. Electrorheological Magne- torheological Fluids, K. Koyama and M. Nakano, eds., World Scientific: Singapore, 1998, p. 197. 35. S.O. Morgan, Trans. Am. Electrochem. Soc. 65: 109 (1934). 36. R.W. Sillars, J.I.E.E. 80: 378 (1937). 37. T. Hanai. N. Koizumi, and R. Gotoh, Proc. Symp. Rheol. Emul- sion., P. Sherman ed., Pergamon, Oxford, 1963, p. 91. 38. D.A.G. Bruggeman, Ann. Phys. 24: 636 (1935). 39. T. Hao and Y. Xu, J. Colloid Interfacial Sci. 181: 581 (1996). 40. F.E. Filisko, Proc. Int. Conf. ER Fluids, R. Tao, ed., World Scientific, Singapore, 1992, p. 116. 41. H. Frohlich, Theory of Dielectrics. Clarendon Press, Oxford, 1958, p. 80. 42. K.D. Weiss, D.A. Nixon, J.D. Carlson, and A.J. Margida, in Progress in Electrorheology, K.O. Havelka and F.E. Filisko, eds., Plenum Press, NY, 1994, p. 207. 43. H. Conrad and Y. Chemn, in Progress in Electrorheology, K.O. Havelka and F.E. Filisko, eds., Plenum Press, NY, 1994, p. 55. 44. M. Whittle, W.A. Bullough, D.J. Peel, and R. Firoozian, Phys. Rev. E 49(6): 5249 (1994). 45. K.D. Weiss and J.D. Carlson, Proc. 3rd Int. Conf. Electrorheo- logical Fluids, R. Tao, ed., World Scientific, Singapore, 1992, p. 264. 46. C.P. Smyth, Dielectric Behavior and Structure. McGraw-Hill, NY, Toronto, London, 1955, p. 201. 47. G.I. Skanavi, Dielectric Physics, translated by Y. Chen. High Educational Press, Beijing, 1958, Chaps II and IV. 48. B. Gross, J. Chem. Phys. 17: 866 (1949). 49. T. Hao, A. Kawai, and F. Ikazaki, Langmuir 14: 1256 (1998). 50. W. Wen and K. Lu, Appl. Phys. Lett. 68(8): 1046 (1996). 51. This is a common way to express the yield stress based on the interparticle force. For example, see J. Rheol. 41(3): 769 (1997). 52. T. Hao, Z. Xu, and Y. Xu, J. Colloid Interfacial Sci. 190: 334 (1997). 53. H. Conrad, Y. Li, and Y. Chen, J. Rheol. 39(5): 1041 (1995). 54. T. Hao, H. Yu, and Y. Xu, J. Colloid Interfacial Sci. 184: 542 (1996). 55. T. Hao, A. Kawai, and F. Ikazaki, Langmuir 16: 3058 (2000). 56. T. Hao, J. Colloid Interfacial Sci. 206: 240 (1998). ELECTRORHEOLOGICAL MATERIALS FRANK FILISKO University of Michigan Ann Arbor, MI INTRODUCTION Electrorheological (ER) materials are materials whose rheological properties, flow and deformation behavior in response to a stress, are strong functions of the electric field strength imposed upon them. The materials are typ- ically fluids in the absence of an electric field (although they may be pastes, gels, or elastomers) but under con- stant shear stress at high enough fields, the materials can solidify into viscoelastic solids. In their solid state, various properties of the solid such as shear and tensile strengths and damping capacity, internal friction, and the ability to adsorb energy under impact are also strong functions of the electric field. Further, all physical and mechani- cal changes induced by the applied field are virtually in- stantaneously reversible upon removal of the field; such a material can almost instantaneously be solidified and liquefied by applying and removing the electric field. In the liquid state during flow, these materials exhibit re- sistivity to flow (or apparent viscosity) which can be in- creased by hundreds or thousands of times by applying an electric field. The materials can be compounded so that viscosities are near that of water under zero fields but approach infinity at very low shear rates under the P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-E-drv January 12, 2002 1:3 ELECTRORHEOLOGICAL MATERIALS 377 influence of fields of the order of a few thousand volts/mm. In the solid state, these materials can have shear strengths of the order of 5,000–10,000 Nt/m 2 (1-2 lb/in 2 )infields around 5000 volts/mm. In brief, these are materials whose mechanical properties and physical state are determined at any instant by the electric field to which they are exposed. ER materials are typically dispersions of fine hygro- scopic particles in a hydrophobic nonelectrically conduct- ing dispersion medium (1). Particle sizes in the range of 0.1–10 µm are common, although particles much larger have demonstrated ER effectiveness, and certain macro- molecules in solution exhibit the effect. For example, materials that work well as the dispersed phase include such diverse materials as corn starch, various clays, sil- ica gel, talcum powder, and various polymers. The fluid phase also may consist of a very wide range of liquids or greases which have the common properties of high electrical resistivity (so that high fields may be applied over the fluids without significant currents) and hydropho- bicity. Liquids such as kerosene, mineral oil, toluene and silicone oil work well as do many other fluids. With few very significant exceptions, the vast majority of systems also require that significant amounts of water (10–30%) or other activators be adsorbed onto the particulate phase. This requirement severely limited the potential use of these materials. Dry particulate systems will be discussed later. Although it is not necessary for an appropriate disper- sion to demonstrate ER activity, various other types of additives, called activators, have been reported and are commonly incorporated into the mixtures, including var- ious surfactants, to enhance the effect and to increase the stability of the dispersions. How they work is unclear, but as will be discussed later, they most certainly affect the particulate surface, the dispersing liquid, and the water on the particles. BACKGROUND The phenomenon which ultimately became known as elec- trorheology was first observed in the late 1800s by Duff (2) and others, but it was not until the work of Winslow (3,4) in the 1940s, 1950s, and 1960s that the engineering potential and application of these materials began being fully recog- nized. The most immediate and obvious applications in- cluded torque transmission and damping or vibration con- trol. Upon attempting to use these materials, however, it was soon realized that a seemingly insurmountable obsta- cle prevented their widespread use; the dispersed phase required significant amounts of water to be adsorbed onto the particles (1,4). Work proceeded to resolve this problem by replacing the water with other substances such as gly- cerol (5) and silanol (6), but the ER effect was substantially reduced. In effect, it began to be accepted that adsorbed water was necessary. The problem was resolved in the late 1980s (1,7) resulting in a tremendous increase in activity within this field over the next decade and a considerable increase in understanding the physics and chemistry of ER suspensions. 70 60 50 40 30 20 10 0 024 Voltage (kV) Dried TiO 2 Wet TiO 2 Torque(Nt.m × 0.0001) 68 Figure 1. Rheological data for titanium dioxide/paraffin oil (5 g/20 cm 3 ) based ER fluids. The wet TiO 2 is as-received and the dried powder was maintained at 160 ◦ C for 5 hours under a liquid N 2 trapped vacuum. One of the first models was proposed by Winslow (4) and follows from simple observations that particles in an ER fluid align between the electrodes under an electric field in static conditions. He hypothesized that under shear, these chains would become distorted and break but would reform again very rapidly. This could account for the increased stresses but does not address fundamental questions con- cerning the mechanism of interactions between particles, although a polarization mechanism is mentioned. Klass and Martinek (8) question this model because ER mate- rials show activity in high-frequency ac fields and these chains could not re-form at such speeds. Brooks et al. (9) reported a timescale for fibrillation of around 20 s which is much greater than the submillisecond responses reported (8,10). Other notable discrepancies arise from the fact that chaining is a trivial consequence of polarization of the par- ticles, and therefore it would seem straightforward to in- crease yield strengths by using particles of higher polariz- abilities. However, as illustrated in Fig. 1, for an ERM that contains TiO 2 particulates and has a permittivity of 200, when dried, chaining still occurs, but the fluid loses its ER activity. Another discrepancy arises from the fact that the strength of particle interactions due to polarization ef- fects in chaining is related to the permittivity difference between the particles and liquid phase (11). This would suggest that metal particles (if appropriately insulated) would result in the strongest interactions between par- ticles. Although this probably occurs, it is not reflected in stronger ER activity. Although there are some reports that metal particle systems are ER active, the strengths are quite low. This and other information suggest that mechanisms other than particle bulk polarizabilities must be involved in a major way in ER activity. An extension of this idea proposes that the particles that interact coulombically flow as clusters, but in static situations, will bridge the electrodes (12,13). Neither of these ad- dresses the basis for particulate interactions on a molecu- lar level. P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-E-drv January 12, 2002 1:3 378 ELECTRORHEOLOGICAL MATERIALS The particles by themselves and/or in conjunction with the dispersing media must interact with the electric field for the particles to align, provide yield stress, and hold the clusters together. The particles and liquids can inter- act independently with the field by virtue of their inherent electrical and dielectric properties, and/or the components can act cooperatively by virtue of the electrical double layer (14,15) which develops around colloidal particles in a dis- persing liquid, and/or by virtue of interfacial polarization which develops due to mobile charges at the interface of the two materials (16,17). The latter situations are most commonly considered related to ER activity, but it is not clear to what extent these two are interrelated or in fact may be part of the same mechanism. Part of the confusion comes from the fact that although the basis for interfacial polarization is fairly well understood, theories related to electrical double layers [which are well developed for sus- pensions in electrolytic fluids (18)], are poorly understood (19) for a nonconducting dispersing medium of which ER fluids are an example. Klass and Martinek (8,10) were the first to involve elec- trical double layers in their explanation of ER activity. They proposed that the diffuse portion of the double layers would become polarized under the influence of the elec- tric field and that the electrostatic interactions of these distorted double layers require additional energy during flow, especially in concentrated suspensions where the lay- ers overlap. This energy is required due to repulsion of the double layers, so that the particles cannot simply move in a streamline but must have a transverse component that gives rise to the additional dissipation of energy. They do not explicitly discuss the function of the adsorbed water even though without it, there would be no ER effect in these systems; yet, double layers would still exist (19). An interesting observation, based upon the relative permit- tivities of the systems of particles they used and the rela- tive ER effectiveness, is that the bulk dielectric properties of the dispersed particles does not play an important role. Interfacial and surface properties of the particles are much more important in ER activity. This finding is also sup- ported by others (8,10,20). Schul’man and Deinega (21) focus more on orientation of the particles and structures that may form in the electric field. They invoke electrical double layers and associate them with a surface-conducting layer on the particles (i.e., water) in a nonconducting fluid where ion exchange with the fluid is presumably negligible. In this case, the mo- bile charges responsible for the Maxwell–Wagner–Sillars interfacial polarization also involve this water layer. The charge carriers can move along this conductive film un- der the influence of the electric field and give rise to an MWS polarization. The moisture here serves an essential function. Ion extension into the surrounding medium, the dispersed double layer, may extend to various degrees, de- pending on among other factors, the degree of conducti- vity of this medium. In reality, we may speculate that both mechanisms are probably involved in the ER phe- nomenon. What is certain, however, is that if either of them are correct, then the surface charge conductivity introduced via the water certainly has a dramatic effect on the character of the double layer. This must actually be the case because the bulk conductivities of the sys- tems increase many orders of magnitude for wet versus dry particles (7), thereby suggesting significant ion trans- fer to the medium when water is present versus without it. Uejima (20) presented dielectric measurements that provided the most direct support for these mechanisms. In these studies, he followed loss factor and dielectric con- stant versus frequency for ER materials composed of cel- lulose particles and various amounts of adsorbed water. Specifically, he was observing the MWS interfacial disper- sion that shifts to higher frequencies as the amount of wa- ter on the particles is increased. This is reasonable for this type of polarization (16,22) but has a number of other impli- cations. The first is that the charge carriers involved in this dispersion are characterized by a relaxational spectrum whose characteristic times, temperatures, and presumably distribution depend strongly on the amount and type of wa- ter present (23). Whether the MWS dispersion disappears as all the water is removed is an interesting mechanistic question because in these inherently heterogeneous sys- tems, a MWS dispersion should still exist (16,21,23), but charge carriers may be of a different type. Deneiga and Vinogradov (18) who also made dielectric and rheological measurements, characterized this water layer further by suggesting that upon increasing tempe- rature and field, there is a corresponding rise in ER ac- tivity and in the permittivity of the dispersions. However, these quantities peak at some point, and beyond this peak, the bulk electrical conductivity of the system begins to increase dramatically. They suggest that a breakdown of the hydrate layer occurs from both temperature and field and results in lowering of the activation barrier for flow of carriers between particles. A very important point implied here is that the bulk conductivity may not be related to ER activity and the preferred situation is to contain charges on the particles by an infinite activation barrier between particles, if this is possible. This speculation is further supported by the work of Deinega and Vinogradov (18) on, the relationships among ER activity, adsorbed water, and bulk conductivity. Using various modifications and extensions, virtually all investigations continued to refine the basic concepts of the electrical double layer extending into the liquid phase, a conductive surface layer of water (or other surfactant) on the particles that gives rise to lateral mobility of ions, which are responsible for the classical Maxwell–Wagner– Sillars interfacial polarization. All of them imply the pres- ence of a conductive layer on the particles, most commonly ions in the water, but none explained why the ER effect disappears when the water is removed, even though the double layer and the MWS interfacial polarization still pre- sumably remain. A major advancement occurred based on reports of par- ticulate systems that produce ER active materials without the need for adsorbed water or any water (7,24). This is crit- ical in resolving the model because either the same mecha- nism is operating both with and without water or, less likely, that different mechanisms are operating. The impli- cation here is very important because it suggests that the mechanism responsible for ER activity can be an intrinsic P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-E-drv January 12, 2002 1:3 ELECTRORHEOLOGICAL MATERIALS 379 0.04 0.03 0.02 0.01 −0.01 −0.02 −0.03 −0.04 −0.05 −0.06 −0.07 −0.08 −0.09 0 02468 Number Electrophoretic mobility 10 12 14 Figure 2. Electrophoretic mobility for an ER material illustra- ting one that has a net negative EPM. The best ER fluids have curves whose maximum is around zero. characteristic of the chemistry and physics of materials, not due solely to extrinsic factors such as water. The models proposed for this activity are similar to those previously discussed but are modified inthattheelec- trical double layer is probably less dominant and the mo- bile charge carriers are not a consequence of an adsorbed electrolyte. In an article by Block and Kelly (25), more em- phasis is put on the particle polarization which is really identical to MWS interfacial polarization. Partially, sup- port of this deemphasis away from the double layer is a consequence of electrophoretic mobility (EPM) measure- ments on the actual fluids, which indicate that the mate- rials can be very active electrorheologically yet show no significant EPM. Further, it appears that systems that show significant EPM are less active electrorheologically, (Fig. 2). A sug- gestion here is that those mechanisms responsible for EPM, fixed surface charges and a diffuse ion layer, are different from those responsible for ER activity and in fact oppose each other to some extent. Permanent fixed surface charges cause attraction or drift of the particles toward one electrode and create an oil or particle-free layer adjacent to the other electrode, thereby giving an apparent viscosity decrease. This also opposes formation of particle-mediated shear transfer between the electrodes which is necessary for ER activity. The mechanism res- ponsible for ER activity, consistent with most others, is the presence of mobile charges (ions or electrons) asso- ciated with the particles that can move somewhat freely within the particles but cannot move off of them, that is, a low activation barrier for migration within a par- ticle but an infinite barrier for motion away from the particle. The explanation for the activity of these dry systems is based essentially on the presence of mobile charge carriers intrinsic to the molecular character or chemistry of the particles. This local mobility of the carriers on the parti- cles is high, but mobility between particles should be very low. In the anhydrous materials of Block and Kelly (1,25), the carriers are presumably electrons because the particles are semiconductors; in aluminosilicate systems, the charge carriers are ions that are intrinsic to the chemistry of the particles and are located on the surfaces (26). Surface here includes the walls of the extensive interparticulate net- work of channels and cavities inherent in the morphology of the particles, which can constitute more than 97% of the total surface area (27). An important distinction between the two systems is that the bulk currents in the semicon- ductor systems are very high (1,25), presumably because electrons can more easily jump or tunnel between the par- ticles and all are available to the outside surface of the particles by standard conduction mechanisms. However, in the zeolite systems, ions on the outer surfaces have a much greater activation barrier to overcome to jump parti- cles, but more important, most are contained within the in- ternal structure of the particles and cannot migrate to the outside surface; yet, they are mobile within the internal labyrinth of channels and cavities afforded by the tremen- dous porosity inherent in the morphology of the materials. Apparent Maxwell–Wagner–Sillars interfacial dispersions are observed in all dried zeolite systems, as illustrated in Fig. 3. They all occur at very low temperatures. Anhydrous polyelectrolyte systems are presumed to be ER active by virtue of locally mobile ions that can move within the environment of the chain coils or along a chain (28) but are not free to move easily between chains. It has been shown that ER activity is primarily a function of the pK of various polyelectrolytes (29). MATERIALS Although the number of materials that can produce ER ac- tive suspensions is almost infinite, it is well understood that for most, the phenomenon is a consequence of an extrinsic component, most commonly adsorbed water (or some other electrolyte) that contains various surfactants added, and has nothing or little to do with the chemistry of the particles. There are, of, course properties of the ma- terials that are beneficial in producing better ER materi- als such as particle porosity, high surface areas, and high affinity for water, but the ER mechanisms are not related to the particle chemistry. Such “wet” or extrinsic systems, most of which were summarized by Block and Kelly (1), were known for many years, and it was as well realized that the water severely limited the potential application of ER technology. Some of the reasons are listed here. 1. Thermal runaway currents, although small, but at the high voltages required cause i 2 R heating which drives off some water, which, in turn, increases the current which increases i 2 R heating which drives off more water which increases the current, etc., until virtually all the water is off, and the fluid no longer works. 2. Relatively high currents and therefore high powers needed. 3. Limited operating temperature range due to freezing and boiling of the water. 4. Electrolysis P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-E-drv January 12, 2002 1:3 380 ELECTRORHEOLOGICAL MATERIALS 0.3 0.2 0.1 0.0 .01 .1 1 10 Freq (kHz) Loss factor 100 1000 0 C 25 C 50 C 75 C 100 C 125 C 150 C Crystalline Alumino-Silicate Figure 3. Dielectric dispersion due presumably to MWS interfacial polarization for 4A zeolite. 5. Corrosion of devices containing fluids 6. Instability of fluids with time and operation 7. Irreproducibility of different batches 8. Solid mat formation upon settling due to interparti- culate hydrate bond formation. Other important considerations certainly exist, which will emerge as applications are developed, such as cost, environmental acceptability, raw material availability, settling in low shear applications, plating of particles on one or both electrodes, sealing, effect on pumps, breakdown of particles upon shear of polymeric particles, adsorption of water, and contamination. The items listed before, how- ever, are those associated directly with the presence of ad- sorbed water. Because of the discovery of water free or “intrinsic” sys- tems, a number of immediate improvements were realized; many had to do specifically with the water. Some of these are listed here: 1. Thermal runaway eliminated, because no water or other adsorbent is required. 2. Low currents. Currents 10 3 to 10 6 lower as a result of dryness; currents are in the range of microamps/cm 2 , or nanoamps/cm 2 instead of milliamps/cm 2 for alumi- nosilicate particulate systems. Drastic reductions in current do not occur in semiconductor systems. 3. Expanded temperature range. Zeolite-based fluids operate from −60 to 350 ◦ C when dispersed in sili- cone oil. 4. Electrolysis eliminated. Electrolysis does not occur because water is not present and is not needed. 5. Corrosion eliminated. Corrosion does not occur be- cause water is not present. 6. Instability improved. A major cause of instability is loss of water due to operation or heating. 7. Irreproducibility substantially improved. Variability of water is a major source of difficulty in formulating water-based fluids. 8. Solid mat formation and settling substantially re- duced. An important additional consequence of these discover- ies implies that mechanisms responsible for ER activity can be associated with the basic chemistry and physics of the particles. Thus, once these mechanisms are under- stood, materials can be synthesized specifically to optimize these mechanisms and improve ER properties in an intel- ligent manner. It is realized that many continue to work on “wet” sys- tems,mostlybecauseitisrelativelyeasytoimproveproper- ties to a limited extent. However, we consider that this is a very limited and interim approach because prior attempts to use this method to improve properties substantially in the 1950s and 1960s resulted in virtually complete failure. However it was also realized then that the water made the materials impractical for the most part. Extrinsic ER (Wet) Systems Extrinsic ER fluids are suspensions that require adding some substance other than the particles and matrix liquid to make the ER fluid function. This specifically refers to the particulate phase because most solids in suspension do not themselves result in ER active suspensions. Substances that are added to make a suspension ER active are some- times called activators and may include many additives such as surfactants which are added to stabilize a suspen- sion against settling. The most well known and effective is water, although many others have been reported (1). P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-E-drv January 12, 2002 1:3 ELECTRORHEOLOGICAL MATERIALS 381 Although virtually any particle can be made ER active by adsorbing sufficient water onto it, the actual function of the water is still not known. Theories proposed include that water bridges form that tie the particles together. An- other proposes that the high dielectric constant (30) creates a stronger dipolar interaction between particles. Another suggests that water modifies the electrical double layer, and another that it increases the current. Whatever the reason for the effect of adsorbed water, it undoubtedly is the most effective activator. Yet, as explained previously, adsorbed water severely limits the commercial value of the phenomenon. Intrinsic ER(Dry)Systems Electrorheological fluids that operate without the need for adsorbed water on the particulate phase can be classified as follows: 1. ionic conductors 2. semiconductors 3. polyelectrolytes 4. solutions Ionic Conductors. The main particulate systems in this category are alumino silicates, or zeolites. The particles are highly porous and contain numerous cavities and in- terconnecting channels such that around 97% of the total surface area of the particles is contained within the par- ticles, that is, the walls of the cavities and channels (26). The dimensions of the cavities and channels can be varied by synthetic methods and by varying the aluminum/silicon ratio. Cationic charge carriers arise from the requirement for stoichiometry when some tetravalent Si atoms are re- placed by trivalent Al without disrupting the crystal struc- ture. Thus, an Al at the center of a tetrahedron that has oxygens at the vertices can bind to only three of these oxygens, leaving one unbonded and a net negative charge in the structure. This negative charge is balanced by in- troducing of cations into the system. These cations, how- ever, cannot fit into the closely packed crystal structure and therefore must reside on the surfaces of the cavities and channels. Thus, the cations are present as a conse- quence of the chemistry of zeolites (not the presence of an electrolyte such as water), and they are mobile be- cause they are on surfaces that are primarily internal and not confined within the crystal structure. Common uses of zeolites are as molecular sieves because they can syn- thetically control the channel dimensions, and as ion ex- change materials due to the presence of unbonded cations that readily exchange with other cations in an aqueous suspension. The intrinsic ER activity of these materials is associ- ated with the presence of these cations which presumably can move locally under the influence of an electric field. Such materials are susceptible to modification partly by varying the Si/Al ratio, by incorporating atoms other than aluminum, and by varying the types of cations. These materials have been available commercially for years as molecular sieves. Semiconductors. Most of the work in this area has been performed by Block and associates (24,25) using various polyacene quinone radicals (PAQR) and recently polyani- line. PAQRs are not available commercially but can be pre- pared by the method described by Pohl (12). The mech- anism of activity for these materials is associated with the electronic charge carriers that can move locally un- der the influence of an electric field. A characteristic of these materials, associated with electron-mediated ER ac- tivity, is relatively high bulk current due to the relative ease with which electrons may jump or tunnel between particles. Although there are many types of semiconductors that can be used to make ER materials, many are ineffective or only a few are effective when dried. The reasons for this may be related to the size of the energy band gaps, the charge mobility, and/or charge concentration, although I know of no studies reported in this regard. Notable materials that fall into this category are the commercially available “carbonaceous” ER fluids of Bridgestone. The particulates in these fluids are presum- ably sythesized by the controlled pyrolysis of polymer(s). The most commonly used, although it is unknown what is used in the Bridgestone fluid, is polyacrylonitrile. Photoconductors represent an interesting group of semi- conductors that can be used to make ER materials. In this instance, many fluids that are inactive or weakly ac- tive can show much enhanced ER activity when exposed to the correct frequency of light (30,31). Phenothiazine demonstrates this rather dramatically, even when dried. Photoelectrorheological materials (PHERM) represent the clearest proof of the relationship between charge mobility and ER activity because exposing such materials to light produces a tremendous increase in the number of free elec- tronic charge carriers. Polyelectrolytes. Although many types of polyelec- trolytes have been used in ER materials (poly lithium methacrylate is the most well documented), most require adsorbed water to function, presumably to dissociate the cations from the macroions. Treasurer (29) evaluated var- ious polyelectrolytes that are commercially available as ion exchange resins and reported that many function with greatly reduced amounts of water. These were all dried at 120 ◦ C under vacuum for 4 days but retain between 0.1 and 2% water. No correlation was found between the ER acti- vity and residual water; however, a strong correlation ex- ists between the dissociation constant pK and ER activity. Another interesting observation was that some systems were ER active at 23 ◦ C and 100 ◦ C, some were inactive at both temperatures, and some were inactive or weak at 23 ◦ C but showed much enhanced activity at 100 ◦ C. Ma- terials that have high and low pKs demonstrated activity at both temperatures. Materials that have intermediate pKs showed partial or no activity, and materials that were acidified, that is, contained no cations, showed activity at 100 ◦ C but not at 23 ◦ C. A common feature of these latter materials was that they all contained quatenary ammo- niums, but no reason is given for the relationship to ER activity. P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-E-drv January 12, 2002 1:3 382 ELECTRORHEOLOGICAL MATERIALS The mechanism for ER activity in these dried materials is presumably due to cations which, even in the absence of an electrolyte, can move locally within the confines of a chain coil in the presence of an electric field but cannot move outside the coil into the surrounding liquid (28). Di- rect evidence for this has not been obtained, but dielectric dispersions associated with interfacial polarization have been detected and are presumably due to these locally mo- bile ions. Systems that contain polymers as the dispersed phase are very popular because the particles are soft and there- fore reduce abrasion, because of the relatively low density which will aid the settling problem, and because of the vast body of knowledge on latex suspensions. Additionally, poly- mers represent an almost infinite range of systems that can be chemically customized for ER materials, once the basic chemical mechanisms for ER activity in “intrinsic ”systems are better understood. Two commercially available ER fluids fall into this cat- egory. One is from Nippon Shokubai Co., Ltd., which pro- duces an ER fluid based on sulfonated polystyrene. The particles are prepared in a unique way, so that the sul- fonation appears at the surface of the particles. Another commercially available material is made by Bayer-Silicone. This is not strictly a polyelectrolyte but is more correctly referred to as a polymeric electrolyte. The particles are block copolymers of a polyurethane and polyethylene ox- ide. Solid polyethylene oxide has an interesting capability of dissolving or ionizing small amounts of salts. Presum- ably, incorporating it into the polyurethane gives this ma- terial the capacity of producing ions which, as charge car- riers, are considered important in ER activity. Because the material is a good ER fluid, it supports the charge mobility hypothesis. Solutions. Two of the most common are solutions of poly-γ -benzyl- L-glutamate (PBLG) in various solvents (31) and poly(hexyl isocyanate) (PHIC) in various solvents (32). Difficulties encountered with the PBLG systems include achieving high concentrations before gelling occurs and the better solvents are polar thus resulting in high currents. Nonetheless, these solutions showed very significant in- creases in viscosity upon applying a field. Further, the ef- fectiveness increased significantly with temperature, the limit is the boiling point of the solvent used. PHIC systems, on the other hand, are soluble at much greater concentrations and in nonpolar solvents. The ER activities are also significantly higher. The discovery of ER active solutions represents an- other very significant advance in the field of ER. One rea- son is that it would resolve the problem of settling which has remained a major concern in some device designs. These solutions, however, will have unique disadvantages such as the greater toxicity and aggressiveness of the sol- vents, limited upper operating temperatures due to sol- vents and thermal degradation of the polymers, and gen- erally higher costs. What is most important, however, is that this discovery emphasizes the enormous versatility in the compositions of ER active materials, as well as the complexity in attempting to ascribe the behavior to a single mechanism. One ER fluid that may be put into this category is not a suspension of solid particles but a mixture of a high and low viscosity liquid. Both phases are siloxane backbone poly- mers, but the high viscosity material contains liquid crys- tallizable side chains that are responsible for its ER acti- vity. Similar to the PHICs in solution, the LC side chains are induced to form nematic structures by the electric field that is reportedly responsible for its ER activity (33–35). MECHANICAL (RHEOLOGICAL) PROPERTIES OF ER MATERIALS The rheological behavior of ER materials under the in- fluence of an electric field is commonly characterized by observing their properties during steady-state flow (8,10,21,36). Under these conditions, the flow properties of ERM can be adequately described as Bingham bodies. ER materials are usually fluids when subjected to uni- directional shearing and under zero field conditions. How- ever, when shearing conditions are maintained constant, the shear stress increases with increasing applied elec- tric field strength. It is commonly reported that the shear stress dependence is proportional to the field squared (4,8,10,21), but many other types of behavior are ob- served (25,37). According to idealized Bingham behav- ior (Fig. 4), ER materials are fluids under zero field but are solids under a nonzero field up to a certain criti- cal shear stress (S c ) and liquids at shear stresses above S c . Although adequate in steady-flow situations where transient or “start up” effects are neglected or unimpor- tant, this model is not applicable when the transient behavior is important or under dynamic loading (i.e., rapid or impact stresses or in damping applications). In these situations, the Bingham model completely over- looks the properties of the materials at stresses less than S c (38). A more complete description of the beha- vior of ER materials is illustrated by a plot of stress versus strain, as in Fig. 5. Under these deformation conditions, ER materials can be described as viscoelastic solids below a certain criti- cal yield stress or yield strain and as viscous liquids at stresses at or above S c and strains greater than the yield strain. If characterized in this manner, ER materials can be described in terms of their overall rheology as viscoelas- tic perfectly plastic materials in which S c and the yield 5 4 3 2 1 10 0 4000 volts 3000 volts 2000 volts 1000 volts 0 volts Shear rate or RPM Shear stress or torque Figure 4. Bingham body illustration of rheological behavior of an ER material. (Oversimplified because nothing is in preyield). P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-E-drv January 12, 2002 1:3 ELECTRORHEOLOGICAL MATERIALS 383 Yield stress Yield strain Increasing electric field Flow or deformation Pre- yield Post- yield Stress 0 field-liquid state Figure 5. Illustration of more correct rheological behavior of an ER fluid. strain are strong functions of the electric field strength. The yield stress S c is highly variable and depends strongly on numerous factors, including the ER material composi- tion. Furthermore, the energy dissipation mechanisms in the preyield region are different from those mechanisms present in steady flow. In the simplest case, the rheological behavior of ER materials in the preyield region can be characterized by a modulus and a yield stress (S c ), in contrast to the postyield region (i.e., liquid state) in which the material is characterized by an apparent viscosity (η a ). Further- more, the behavior of the ER material is linear viscoelastic when deformation is restricted to the preyield state and the ER material is characterized by time constants and damping factors that are complex functions of the field strength. ERM in Steady-State Flow (Postyield Behavior) In steady-state flow at a shear rate of “˙γ ,” ER materials are characterized by an apparent viscosity “η a ” which is defined as η a = [S c (E) + S o ( ˙γ )]/ ˙γ a (1) where S c is a function of the electric field strength (E) and S o is a function only of the shear rate and temperature and is material specific. Two common criteria for evaluating ER materials in flow include (1) the magnitude to which the viscosity can be increased and (2) by what factor it can be increased. The second criterion is more important because it indicates how effective an electric field strength is on the rheology of the material. Regarding the latter point, we can define a fluid effectiveness factor K as K = η a (E)/η o = [(S o + S c )/ ˙γ ]/(S o / ˙γ ) = 1 + S c /S o (2) where S c is essentially a constant at a certain field strength but S o increases continuously with increasing shear rate. Therefore, this suggests that K decreasestoward one as the shear rate increases. This is an important first-order rela- tionship for understanding the characteristics of ER ma- terials under flow because it implies that to make a more effective fluid requires making S c as large as possible while keeping S o as small as possible. The second parameter, S o , is a function of the ER material composition and flow con- ditions. Thus, ER materials of low or high viscosity can be made by varying the solid concentration or the viscosity of the dispersing liquid. However, though the maximum shear stresses can be increased by making the zero-field materials thicker, the K factor can become small, so that the field-induced change in S c becomes insignificant. Fur- thermore, it has been reported that S o is a much stronger function of concentration than S c (39), and it appears that S c is a linear function of concentration (18). ERM in Oscillatory Shearing (Three Rheological Regions) The response of ER materials to dynamic loadings can be discussed in terms of three distinct rheological regions: preyield, yield, and postyield regions. In the previous sec- tion, discussion was limited to conditions of steady-state flow in which the transient effects of the preyield region were not considered. The preyield region can be effectively studied when oscillatory stresses are applied to the ER ma- terial such as may occur in vibration damping, as first re- ported in detail by Gamota and Filisko (40,41). Under these straining conditions, the amplitude of the shear stress response is a strong function of the applied field strength. However, a limiting shear stress value exists beyond which the shear stress response no longer follows the shape of the shear strain function, but becomes “cutoff” or truncated (40,42). The value of the shear stress at the onset of trun- cation is a function of the field strength and is also related to S c . Rheologically, the appearance of the cutoff is an in- dication that the material is beginning to flow. During an oscillatory shear strain, the ER material may deform as a linear viscoelastic solid over part of the deformation cycle and as a liquid over the other part. A representative series of stress responses for the ER material when subjected to a sinusoidal shear strain is presented in Fig. 6. Curve a is the applied sinusoidal shear strain of frequency 15 Hz and amplitude 0.25. Curve b is the shear stress response when the material is subjected to a zero-strength electric field. The shear stress response appears sinusoidal, and the phase an- gle between the applied shear strain and shear stress (a) (b) (c) (d) Strain 0 kV 1 kV 2 kV data at 1/4 scale Figure 6. Shear stress response to a constant strain amplitude at various electric fields. Curve a is the strain, b the shear stress where E = 0, c the shear stress where E = 1 kV/mm, d the shear stress where E = 2.5 kV/mm. (D is one-quarter scale.) P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-E-drv January 12, 2002 1:3 384 ELECTRORHEOLOGICAL MATERIALS response is 90 ◦ , suggesting that the material is deform- ing as a viscous body. Subjecting the ER material to an electric field whose strength is 1.0 kV/mm yields Curve c; the shear stress response amplitude increases, and the phase angle decreases. Thus, when the ER material is subjected to a nonzero electric field, the material be- haves as a viscoelastic material. If the strength of the electric field is increased to 2.5 kV/mm, the shear stress response deviates from sinusoidal behavior (Curve d). A nonsinusoidal response suggests that the material is be- having as a nonlinear viscoelastic material. In addition, as the material is subjected to a 2.5-kV/mm field, the funda- mental harmonic of the shear stress response increases, and the phase angle between the fundamental harmonic of the shear stress response and the applied shear strain decreases. Thus, as the strength of the applied electric field is increased from 0.0 to 2.5 kV/mm, the ER material trans- forms from a viscous to a linear viscoelastic to a nonlinear viscoelastic body. Moreover, the energy storing and energy dissipating properties of the ER material are strong func- tions of the applied electric field strength. The linear viscoelastic parameters, shear storage modu- lus (G’) and shear loss modulus (G”), are strong functions of the applied electric field, strain amplitude, strain fre- quency, and material composition (9,18,41). In addition, it was shown (18,41) that the shear storage modulus is a stronger increasing function with increasing electric field strength compared to the shear loss modulus. Further- more, it is of particular interest to note that ER materials become greater energy storing bodies as they simultane- ously become greater energy dissipating bodies. A second technique for observing the effect of the electric field under cyclic loadings is to observe shear stress—shear strain loops (hysteresis loops) for these materials at a constant strain frequency and amplitude, while varying the strength of the electric field. A sequence of hysteresis loops generated when the ER material is de- forming as a linear viscoelastic body is shown in Fig. 7. As the strength of the electric field increases, both the area within the hysteresis loops and the angle that the major axis of the hysteresis loop makes with the abscissa increase. The hysteresis loops are elliptical which is indica- tive of a linear viscoelastic response. The viscous compo- nent (energy dissipated) is determined by the area within the loop, and the elastic component (stored energy) is de- termined by the major axis inclination. The existence of a deformation transition limits the applicability of linear viscoelastic mathematics for quan- tifying the energy storing and energy dissipating prop- erties of an ER material. The amount of energy dissi- pated by the ER material during one deformation cycle, irrespective of a linear or nonlinear viscoelastic response, can be obtained by generating a hysteresis loop. The en- ergy dissipated by an ER material is found by calculat- ing the area within the loop. The recorded hysteresis loop for an ER material subjected to a strain of moderate fre- quency, moderate amplitude, and zero-strength electric field is elliptical, and the major axis of the hysteresis loop is parallel to the abscissa; this response is indicative of a viscous material (Fig. 8a). When the ER material is subjected to a field strength of 1.0 kV/mm, the area within (a) (b) (c) Figure 7. Hysteresis loops for an ER material when subjected to a strain of amplitude of 0.001 radian at 300 Hz. Loop ‘a’ is under a zero strength field, loop b: E = 1.0 kV/mm, and loop c: E = 2.0 kV/mm. ‘a’ is a hysteresis loop for viscous behavior; b and c represent hysteresis loops indicating viscoelastic behavior where the viscous component of b < c and the elastic component of b < c. the hysteresis loop increases, and the angle between the major axis of the hysteresis loop and the abscissa in- creases (Fig. 8b). The increased area within the hystere- sis loop suggests that the ER material dissipates more energy and the increased angle is related to the energy storing properties of the ER material. Continuing to in- crease the strength of the electric field yields hysteresis loops that encompass greater areas, suggesting that the material dissipates more energy (Figs. 8c and 8d). How- ever, the loops are no longer elliptical, and thus the ER (a) (b) (c) (d) Figure 8. Actual hysteresis loops recorded for an ER mate- rial under various electric fields. a: E = 0, b: E = 1 kV/mm, c: E = 2 kV/mm, d: E = 3 kV/mm. 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