Electrorheology of suspensions of elongated goethite particles

We describe the rheology of moderately concentrated suspensions of elongated goethite (β-FeOOH) particles with axial ratio around 8, both in the absence and presence of high-strength DC electric fields (up to 4.3 kV/mm). The selected liquid medium was a silicone oil with 1 Pa s nominal viscosity. The aim of this work is the evaluation of the electrorheological (ER) effect of suspensions containing highly anisotropic particles and the comparison with that exhibited by samples made of less anisotropic particles of similar chemical composition (hematite, α-Fe2O3). Under the application of large electric fields, goethite suspensions changed their rheological behavior, as expected, from Newtonian – at zero field – to shear thinning, thus displaying electrorheological response. A well defined yield stress (σy) was observed in the electrified suspensions, that increases with both the field strength and particle concentration ϕ, although following different trends to those predicted by the classical chain models. σy was found to depend on ϕ in a parabolic fashion, as a consequence of the fact that field-induced structures in the suspensions do not consist of individual chains as the classical models consider, but of much more complex particle aggregates. The yield stress was found to be almost linearly dependent on the field strength, contrary to the predictions of the polarization model (σy ∝ E2). The field-induced enhancement of the conductivity of the host oil, leading to saturation of the electrical forces among polarized particles, is required to explain this deviation. The goethite suspensions were also analyzed under oscillating shear stresses for investigation of their viscoelastic properties as well. The results indicate that the ER effect was only noticeable for sufficiently high field strength and particle concentrations, typically >1 kV/mm and >4% in volume fraction, respectively. In such conditions the elastic modulus G′ was independent of the shear frequency as corresponds to an elastic solid-like structure. Suspensions of goethite particles display an ER effect with the same characteristics as hematite dispersions (same tendencies of σy with both E and ϕ2), indicating that the physical mechanism responsible of such effects is the same in both cases. However, suspensions containing elongated particles produce a more efficient response to the electric field than those made of irregularly shaped solids, since the former give rise to higher yield stress for the same field strength, and exhibit a lower viscosity in absence of external excitation.