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The electrorheological effect is characterized by an increase in viscosity of two to three orders of magnitude when an electric field is applied. This feature opens a variety of potential applications for electromechanical devices. However, for practical applications, it is important to know the required time to reach the effective or working viscosity of the system. In fact, in a good electrorheological fluid (ERF), the response time is expected to be as rapid as a few milliseconds. Neverheless, no agreement has been reached in the literature on either what response time in ERFs really means or how to measure this quantity experimentally. Accordingly, a mechanical measurement of the characteristic times (or time constants) of different ERFs, both commercial and laboratory made, is presented, aiming to determine under which conditions a steady viscosity state is reached in every case. Suspensions of silica particles (63 µm in diameter) in various dielectric fluids with different permitivities were prepared, to study the effect of these variables on the mechanical response of the system. The experiments were performed by using a modified torsion cell of a universal mechanical testing instrument.

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