Rheological properties of a model colloidal suspension under large electric fields of different waveforms

An investigation of the suitable characteristics of the electric field that maximize the ER effect has been carried out in this work. Rheological parameters (shear stress and viscosity) were determined in dependence of electric fields of different waveforms (square, sinusoidal and triangular), frequency, ν, and strength, E, for hematite/silicone oil suspensions under steady shear conditions. It was seen that the ER performance was highly dependent on the waveform of the external field, with a square signal producing the optimal response. The yield stress, τy, was almost independent of the field at high frequencies but it sharply increased below 50 Hz. Additionally, it correlated with a power-law dependence, τy = aEb, where b strongly depends on ν: it is b ≈ 2, but it gradually reduces when the frequency increases. The experimental data could be qualitatively explained according to the interfacial polarization mechanism. However, significant deviations from its predictions appeared, which could be only justified when other considerations, in addition to the polarization mechanism are taken into account, specifically, the period of time for which samples are subjected to the maximum voltage, the effect of the particle rotation under shear flow and the frequency-dependent behaviour of the electrical properties of the suspension.