Ind. Eng. Chem.Res. 1995,34, 3303-3306
3303
Protein-Enhanced Electrorheological Fluids? Daniel J. Klingenberg,* Peyman Pakdel, Young Dae Kim, Brett M. Belongia, and Sangtae Kim Department of Chemical Engineering and Rheology Research Center, University of Wisconsin, Madison, Wisconsin 53706
Electrorheological (ER) fluids are typically suspensions of solid particles in electrically nonconducting liquids that undergo dramatic changes in their rheological properties upon application of large electric fields; apparent viscosities can increase several orders of magnitude for electric field strengths on the order of 1 kV/mm. This technology has many possible applications in new types of stress transfer and damping devices. We present experimental results showing that the addition of small amounts of proteins to these materials can dramatically enhance their ER response. These enhanced ER fluids may represent a practical means for controlling material properties and optimizing formulations for applications.
1. Introduction First studied extensively by Winslow (1949),electrorheology (ER) refers to the rapid, reversible rheological changes of suspensions of solid particles in electrically nonconducting liquids due to the application of external electric fields (Deinega and Vinogradov, 1984; Block and Kelly, 1988;Gast and Zukoski, 1989; Halsey, 1992;Weiss et al., 1993). Under typical electric field strengths of 0.5-4.0 kV/mm, ER suspensions demonstrate a field-dependent yield stress and orders of magnitude increases in their small shear rate viscosities. This rheological effect is accompanied by a dramatic change in the suspension microstructure, with particle strands forming in the direction of the electric field. The origin of the ER response is still debated, although it is generally accepted that it begins with particle polarization and hence is intimately related to the suspension dielectric properties; there is less agreement as to what subsequently occurs to produce the rheological behavior (Block and Kelly, 1988;Weiss et al., 1993). The steady-shear response of ER fluids is commonly modeled approximately as that of a Bingham fluid (Bird et al., 1960)with the yield stress and small shear-rate apparent viscosity strong increasing functions of the electric field strength, increasing with the square of the electric field in most systems (Deinega and Vinogradov, 1984;Block and Kelly, 1988;Gast and Zukoski, 1989; Halsey, 1992; Weiss et al., 1993). This ability to electronicallycontrol stress transfer has many applications in stress transfer devices. Suggested applications include clutches and brakes, damping devices such as engine mounts and shock absorbers, and electronically controlled valves (Deinega and Vinogradov, 1984). Development of ER fluids and devices has been driven largely by the impressive potential market, estimated to be around $20 billion per year (US. DOE Report, 1993). Although many ER devices have been brought successfully to the prototype stage, and despite much industrial activity in the United States and abroad, there are essentially no commercially available devices. The main limitation of ER technology development is a
* To whom correspondence should be addressed.
*
Submitted for publication in a special issue in recognition of the 35th anniversary of the publication of Transport Phenomena by Bird, Stewart, and Lightfoot.
lack of “good”fluids (Hartsock et al., 1991;Weiss et al., 1993; U S . DOE Report, 1993). Most applications require fluids that possess a large field-induced yield stress, are stable to settling and irreversible aggregation, are environmentally benign, and draw limited current. Our inability to design such acceptable fluids stems largely from a lack of fundamental understanding of the mechanisms that control ER behavior. This situation is further complicated by the many variables that are known to influence ER activity, such as component types and concentrations, electric field strength and frequency, the dielectric properties of the phases, and temperature, and by a lack of fundamental studies on well-characterized systems. Effects of various components and process parameters on macroscopic ER properties (yield stress, current density, stability) must be understood to develop materials suitable for practical devices (Weiss et al.,1993). Various chemicals, termed “activators”, have been employed t o enhance the ER response, in particular, to increase the yield stress at a given electric field strength. It is well-known that polar molecules such as water can impart ER activity to suspensions that display no ER response in their anhydrous state (Deinega and Vinogradov, 1984;Block and Kelly, 1988;Gast and Zukoski, 1989;Weiss et al., 1993). Surfactants have also been shown to significantly enhance the ER response (Kim and Klingenberg, 1994). In this paper, we report experimental results demonstrating the enhancement of the ER response by the addition of proteins. We find that some suspensions can realize increases in their yield stress of several thousand percent by the addition of small amounts (51 wt %) of certain proteins. The mechanisms for this enhancement are unknown at this time but are likely related to the increased polarizability of particles containing adsorbed proteins. Many different types of proteins are available in nature. In addition, progress in the biological sciences has provided us with tools for synthesizing proteins with specific properties. Hence, we believe that protein-enhanced ER fluids represent a very attractive avenue for tailoring formulations for various ER applications.
2. Experimental Section Suspensions consisted of various particles [neutral alumina (Aldrich),hollow silica spheres (PQIndustries), and zeolite A (obtained from T. W. Root, University of
0888-5885/95/2634-3303$09.00/00 1995 American Chemical Society
3304 Ind. Eng. Chem. Res., Vol. 34,No. 10,1995
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no protein
a
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Figure 1. Shear stress versus shear rate for 20 wt % alumina suspensions for an electric field strength of 1 kV/mm and a field frequency of 500 Hz.
Wisconsin)] dispersed in silicone oil (General Electric, SF-96). The particle diameters were in the range 5360 pm. Proteins investigated included ,9-lactoglobulin (b-LG, Sigma), a-lactalbumin (a-LA, Sigma), bovine alumin (99% pure, BAlb; fraction V, BAlb-W, globulinfree, BAlb-Gf; Sigma), a-casein (a-C, Sigma), K-casein (k-C, Sigma), chicken albumin (Ch-Alb,Sigma), and soy protein mixtures (obtained from M. R. Etzel, University of Wisconsin). Proteins were used as received and kept in desiccators a t 4 "C prior t o sample preparation. The samples were prepared by first dispersing a known amount of protein in the silicone oil and then adding the solid particles (20 wt %). The proteins were dispersed in the oil by vigorous stirring a t room temperature for several hours until visually uniform (cloudy) dispersions were produced. The particles were dried under vacuum a t 60 "C for 2 h to remove loosely bound water and then allowed to cool in a desiccator prior to adding them to the oillprotein solution. Rheological experiments were performed on a Bohlin VOR rheometer with a parallel-plate geometry, modified to allow the application of large electric fields as described by Parthasarathy (1994). Potential differences were supplied by a function generator (Stanford Research Systems, DS345) and amplified with a n ac amplifier (Trek, 10110). The ER response was characterized by measuring the dynamic yield stress, TO, as a function of suspension composition and electric field strength. The suspensions were sheared a t constant shear rate under the applied electric field and the steady-state shear stress transmitted by the suspension was recorded. Experiments were performed a t decreasing shear rates, and the dynamic yield stress was determined by extrapolating the shear stress-shear rate data t o zero shear rate (see Figure 1).The temperature was held constant a t 25 "C for all experiments. The applied field frequency was 500 Hz (with the exception of the data presented in Figure 31, and the electric field strength was varied in the range 0.0-1.5 kV/mm.
3. Results Typical results of shear stress versus shear rate are shown in Figure 1. This figure illustrates how the dynamic yield stress is obtained from an experiment, as well as the significant enhancement that can be obtained by the addition of a small amount of protein activator to the suspension. Results for the enhancement of the ER response of alumina suspensions with various protein activators are
.
Protein Activator
Figure 2. Dynamic yield stress for various protein-activated alumina suspensions. Alumina and protein concentrations are 20 and 0.1 wt %, respectively, the electric field strength is 1 kV/mm, and the field frequency is 500 Hn. Abbreviations for proteins are defined as follows: b-LG, 0-lactoglobulin; a-LA, a-lactalbumin; BAlb, bovine albumin, 99%pure; BAlb-W, bovine albumin. fraction V; BAlb-Gf, bovine albumin. globulin-free; a-C, a-casein; k-C, x-casein; Ch-Alb, chicken albumin; and soy, soy protein mixtures.
Table 1. The Ratio of Yield Stresses with and without Protein for 20 wt % Suspensions with 0.1 wt % &Lactoglobulin at 1 kV/mm and 500 Hz TO (with proteinllro
disperse phase
(without protein)
alumina zeolite fly ash
20 1.1 2.6
summarized in Figure 2, where the dynamic yield stress a t 1 kV/mm is plotted as a function of protein type. For each data point, the alumina and protein concentrations are 20 and 0.1 wt %, respectively. The yield stress increases by as much as 20-30 times upon the addition of protein, with the smallest enhancement being several hundred percent. We note that the suspensions containing the caseins possessed a yield stress in the absence of an electric field, suggesting that these suspensions were flocculated. Also, as expected for hydrophilic systems (Block and Kelly, 1988), we found that small amounts of water enhanced the response even further but also dramatically increased the suspension conductivity. Table 1compares the enhancement for several types of particulate materials using p-lactoglobulin as the activator. The largest enhancement was observed for the porous alumina system where the average pore size is 58 A. In contrast zeolites which typically have much smaller pores ( ~ 5show a weaker enhancement. A series of adsorption experiments verified that the proteins adsorb strongly to the particle surfaces. These observations suggest that the incorporation of a significant amount of the protein through the volume of the particles may be important for realizing an enhancement. Figure 3 presents the dependence of the dynamic yield stress on the electric field frequency, for alumina suspensions with and without a protein activator (plactoglobulin). The yield stress of both suspensions decreases with increasing frequency over the range 101000 Hz; the yield stress of the protein-activated suspension is larger a t small frequencies and decreases more rapidly with increasing frequency. This variation indicates that the frequency is a useful parameter for controlling the magnitude of the ER response and may
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Ind. Eng. Chem. Res., Vol. 34, No. 10, 1995 3305 10.0
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also provide insight into the underlying activation mechanisms.
4. Possible Mechanisms
It is generally accepted that the ER phenomenon is initiated by the electric polarization of the disperse phase particles. Thus the ER response depends critically upon the suspension dielectric properties. A simple model for electrorheology that treats the disperse and continuous phases as “simple” dielectric materials (Klingenberg et al., 1989, 1991a,b) predicts that the dynamic yield stress is an increasing function of the difference in dielectric constants of the two phases; that is, the yield stress increases with the ability of the particles to polarize relative to the continuous phase. Particles can polarize in an electric field by several mechanisms: electronic, atomic, dipolar, nomadic, and migration (or interfacial) polarization can each alter a material’s charge distribution (von Hippel, 1954; Weiss et al., 1993). No consensus of the polarization mechanism(s) dominant in ER fluids has been reached (Block and Kelly, 1988; Weiss et al., 1993);however, considerable evidence has been mounting in favor of migration polarization, where polarization is dominated by the large scale migration of charged species. Indeed, most, if not all, ER formulations appear to contain mobile charge carriers (ions, electrons, or holes), including anhydrous systems developed by Block and Kelly (1988) and Filisko and Radzilowski (1990). Thus we suspect that the role of proteins in enhancing the ER response is to increase the polarizability of the disperse phase particles, perhaps by increasing the number or mobility of mobile charge carriers. Further evidence for this hypothesis is discussed below. In the absence of proteins, the yield stress decreased with frequency over the range 10-104 Hz (see Figure 3); such a low-frequency dispersion is indicative of a migration polarization mechanism (Block and Kelly, 1988; Weiss et al., 1993). With added protein, the decrease was more pronounced and occurred over the same frequency range, consistent with an increase in the number of mobile charges. The influence of protein on the yield stress was also sensitive to the water content. In nonaqueous dispersions, water is believed to enhance the particle polarizability by creating mobile charges on the particle surface or within pores (Dukhin, 1970). The precise mechanism(s) of increased mobile charge carrier number or mobility is unclear at this time. The proteins may act as surfactants, solvating tightly bound ions and allowing them to participate in the polarization process. Alternatively, the proteins may simply donate
their bound ions or bound water to the particles. Determining the role of proteins on the polarization processes a t the microscopic level will provide useful information for understanding ER in general as well as help us optimize formulations for practical applications.
5. Conclusion Our results show that the ER response can be enhanced significantly by the addition of small amounts of proteins to the suspensions. These results suggest a promising future; proteins are designed by nature to be polarizable, many are plentiful and inexpensive, and many more can be synthesized by a variety of modern techniques t o possess desirable attributes. A more thorough study is clearly warranted in order to understand the underlying mechanisms and to ultimately exploit this phenomenon. Although the preliminary results are encouraging, it is important to be aware of possible problems and to estimate our ability to deal with them in order to eventually commercialize this technology. One possible concern is protein denaturation. If denaturation alters the ER response significantly, application temperature ranges will be limited. At this stage, we do not know how denaturing affects ER enhancement. However, the enhancement was found to decrease with time for some systems suggesting that denaturation may be detrimental to the response. Ifthis hypothesis is true, techniques could be employed to keep the proteins from denaturing such as trapping the proteins in the particle pores or embedding the proteins within the solid matrix. Alternatively, proteins extracted from hyperthermophiles may offer a simple solution t o this potential problem. Another possible problem arises in that the migration polarization mechanism inherently produces suspension conductivity, which, if excessive, would be detrimental to applications. However, it is possible to affect (or control) the suspension conductance with the field frequency and the extent of ion binding. The large body of previous research in protein properties and design may be exploited to optimize formulations and obtain effective ER suspensions that simultaneously satisfy such criteria as large field-induced yield stresses and limited conduction. Developing these capabilities and determining their limitations require a more thorough understanding of the underlying mechanisms.
Acknowledgment This work was supported by the University of Wisconsin Graduate School and College of Engineering,
Literature Cited Bird, R. B.; Stewart, W. E.; Lightfoot, E. N. Transport Phenomena; Wiley: New York, 1960. Block, H.; Kelly, J. P. Electro-rheology J.Phys.D:Appl Phys. 1988, 21,1661-1677. Deinega, Y. F.; Vinogradov, G. V. Electric Fields in the Rheology of Disperse Systems Rheol. Acta 1984,23,636-651. Dukhin, S. S. Dielectric Properties of Disperse Systems J. Sug. Colloid Sci. 1970,3,83-165. Filisko, F. E.; Radzilowski, L. H. An Intrinsic Mechanism for the Activity of Alumino-silicate Based Electrorheological Materials. J. Rheol. 1990,34,539-552. Gast, A. P.; Zukoski, C. F. Electrorheological Fluids as Colloidal Suspensions Adv. Colloid Interface Sci. 1989,30,153-202. Halsey, T.C. Electrorheological Fluids Science 1992,258,761766. Hartsock, D. L.;Novak, R. F.; Chaundry, G. J. ER Fluid Requirements for Automotive Devices J. Rheol. 1991,35,1305-1326.
3306 Ind. Eng. Chem. Res., Vol. 34,No. 10,1995 Kim, Y. D.; Klingenberg, D. J. Surfactant-activated Electrorheological Suspensions Polym. Prepr. (Am. Chem. Soc.,Diu. Polym. Chem.) 1994,35,389-390. Klingenberg, D. J.;van Swol, F.; Zukoski,C. F. Dynamic Simulation of Electrorheological Suspensions J. Chem.Phys. 1989,91, 7888-7895. Klingenberg, D.J.;van Swol, F.; Zukoski, C. F. The Small Shear Rate Response of Electrorheological Suspensions. I. Simulation in the Point-dipole Limit J . Chem. Phys. 1991a,94,6160-6169. Klingenberg, D. J.;van Swol, F.; Zukoski, C. F. The Small Shear Rate Response of Electrorheological Suspensions. 11. Extension Beyond the Point-dipole Limit J. Chem. Phys. 1991b,94,61706178. Parthasarathy, M.; Ahn, K. H.; Belongia, B. M.; Klingenberg, D. J. The Role of Suspension Structure in the Dynamic Response of Electrorheological Suspensions. Znt. J.Mod. Phys. B 1994, in press.
US.Department of Energy. Electrorheological (ER) Fluids. A Research Needs Assessment Final Report. Report No. DE-ACOP91ER30172,May, 1993. Weiss, K. D.; Carlson, J. D.; Coulter, J. P. Material Aspects of Electrorheolodcal Systems. J. Znt. Mater. SYS.Struct. 1993. 4,13-34. Winslow, W. H. Induced Fibration of Suspensions. J.Appl. Phys. 1949,20,1137-1140. von Hippel, A. R. Dielectrics and Waves; Wiley: New York, 1954. I
Received for review December 28, 1994 Accepted May 2 , 1995@
IE9407691 Abstract published in Advance ACS Abstracts, August 15, 1995. @
