Relaxation Processes in the Electrorheological Response
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RELAXATION PROCESSES IN THE ELECTRORHEOLOGICAL RESPONSE P. Katsikopoulos and C. Zukoski, Department of Chemical Engineering, University of Illinois, Urbana, Illinois 61801
I.. INTRODUCTION The electrorheological (ER) response is characterized by a reversible increase in suspension viscosity on application of large electric fields. Suspensions displaying this behavior are typically composed of a polar solid phase suspended in a low conductivity oil (1-3). In the past five years considerable work investigating the mechanisms controlling the ER response has established that increases in viscosity are associated with the formation of electrode spanning particulate structures which are degraded by shear (4-8). The structures are the result of polarization interactions produced by the dielectric mismatch between the solid and continuous phases.' Extensive modelling studies have shown that much of the rheological behavior of ER suspensions can be understood in terms of a balance of viscous and electrical polarization forces as written in terms of the Mason number, Mn = TiY i/(2eo•E,(jE) 2) (9). Here TI, is the continuous phase viscosity, Y is the shear rate, and E is the applied field strength. The relative polarizability of the particulate phase is given in terms of P3= (1E-)/(eP+2e5 ) where EPis the particle dielectric constant and E, is the continuous phase dielectric constant. The relative viscosity of many suspensions can be reduced to a single universal function of Mason number and often display a yield stress which scales with E2 . Many experimental studies are carried out in the limit of large particles and electric fields such that thermal motion is unimportant (i.e., a3 Eoe, (13E) 2/kaT >>I, a is the particle radius) and under D.C. field conditions. Models to understand the ER behavior observed in this limit were developed assuming linear isotropic dielectric particles suspended in a nonconducting continuous phase and, as a result, predict that the ER response grows with P3 such that the largest response will be observed with conducting particles. While many of the predictions of the polarization model have been supported experimentally, there is considerable confusion over the importance and mechanisms of particle polarizability. For example, Block et al. (10) argue that the ER response passes through a maximum as the particle conductivity increases while other evidence suggests that the ER response increases monotonically with particle dielectric constant and conductivity (11). Difficulties in probing the ER response of suspensions containing conducting particles arise from the ability of structures produced by the electric field to carry substantial currents thus overloading power supplies or causing thermal instabilities in the suspension. One solution to this problem is to block Faradaic or resistive currents by coating the electrodes with an insulating layer of paint. In principle, if the paint has a low dielectric constant and the suspension has a conductivity of zero, the loss of electric field in the i
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