Abstract
While the magnitude of the field-induced shear stress is a critical parameter in designing semiactive electrorheological (ER) devices, the stress response time is also relevant. Furthermore, sharp transients in the stress response may lead to perceptions of harshness that is undesirable, for example, in automotive applications. In light of the importance of these transient effects, a variety of time-dependent rheological phenomena were explored experimentally in several model ER fluid systems.1,2 The character of the transient stress response was found to depend strongly on the composition of the ER system under test. The detailed time dependence of the stress and its variation with material properties can be understood semiquantitatively by using a simple model that accounts for the polarization dynamics of the suspended particles. The character of the transient response depends largely on whether particle polarization involves primarily a particle-fluid conductivity mismatch or a dielectric constant mismatch. The transient stress response is therefore a powerful tool to uncover the mechanisms by which ER activity arises. The shear-rate dependence of the stress response was also investigated; perhaps counterintuitively, the characteristic times for stress growth and decay were largest at low shear rates and varied inversely with shear rate at high shear rates. This relationship is qualitatively consistent with the contributions of particle polarization and the growth and strain of the field-induced structure to the observed transient stresses.2
Published Version
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