Electrorheology and shear dependent reversible gelation of elongated α-FeOOH suspensions

We study the effect of applying an external electric field (E) on the rheology and microstructure of α-FeOOH, length (L) = 488 ± 20 nm and an aspect ratio of (5 ± 0.8) – in – silicone oil suspensions (α-FeOOH/silicone oil). The α-FeOOH/silicone oil suspensions show Newtonian response up to α-FeOOH particle volume fraction of 0.039 when no E is applied. The suspensions exhibit τ<inf>y</inf> ~ E² and E^1.5 scaling in the low and high E regimes respectively and the normalized τ<inf>y</inf> versus E data of the suspensions collapses onto single master curve, irrespective of the particle loading and applied E. The solid-like behavior of the suspensions is also evident from the frequency-independent storage modulus, and the plateau in relaxation modulus with applied E. The optical microscopy reveals the E driven irreversible network formation of α-FeOOH particles, which does not relax to a uniform dispersion of α-FeOOH particles when the E is switched off. The suspensions demonstrate a reversible ER response once they are sheared at constant shear rate of 10 s⁻¹.