Relaxation Processes in the Electrorheological Response

Abstract

AbstractThe 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 = ηc ṙ/(2ε0εc(βE)2) (9). Here ηc, is the continuous phase viscosity, ṙ is the shear rate, and E is the applied field strength. The relative polarizability of the particulate phase is given in terms of β = (εp−εc)/(εp+2εc) where εp is the particle dielectric constant and γc 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.