Dielectric response of electrorheological fluids: Application to the electrical characterization of particles suspended in liquid medium

Electrorheological (ER) fluids are suspensions of fine particles dispersed in a dielectric liquid. Upon application of an electric field, the solid phase (particles) fibrillates along the field lines between the electrodes. Thus, the rheological properties of the suspension are drastically modified (strongly dependence of the viscosity of the suspension on the applied field strength). To understand and model the ER phenomenon, it is necessary to take into account the dielectric properties (conductivity and permittivity) of the solid and liquid phases. For the experimental investigation, the main difficulty is to measure the conductivity and the permittivity of the micro (or nano)-sized particles. In this paper, we propose results based on two alternative methods allowing obtaining the electrical parameters of the solid particles. The first one is made up of the arrangement of the solid phase (particles) in fibres on high voltage condition. The measurement of the resistive and capacitive parts of the ac time-varying current allows us to obtain the dielectric constants of the solid phase and then to deduce the conductivity and permittivity of the particles. The second method consists in measuring the complex permittivity epsiv\´ and epsiv" of the heterogeneous suspensions (random solid-phase dispersed system) and then, using the dielectric constants of the liquid medium, to infer the particle conductivity and permittivity. Measurements are done with a low voltage dielectric spectrometer. A comparison of results obtained with the two methods is proposed and discussed.