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 alpha-FeOOH, length (L) = 488 +/- 20 nm and an aspect ratio of (5 +/- 0.8) - in - silicone oil suspensions (alpha-FeOOH/silicone oil). The alpha-FeOOH/silicone oil suspensions show Newtonian response up to alpha-FeOOH particle volume fraction of 0.039 when no E is applied. The suspensions exhibit tau(y) - E-2 and E-1.5 scaling in the low and high E regimes respectively and the normalized tau(y) 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 alpha-FeOOH particles, which does not relax to a uniform dispersion of alpha-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).