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

Abstract

We study the effect of applying an external electric field (E) on the rheology and microstructure of �-FeOOH, length (L) = 488 (plus or minus) 20 nm and an aspect ratio of (5(plus or minus) 0.8) -in -silicone oil suspensions (�-FeOOH/silicone oil). The �-FeOOH/silicone oil suspensions show Newtonian response up to �-FeOOH particle volume fraction of 0.039 when no E is applied. The suspensions exhibit �y E2 and E1.5 scaling in the low and high E regimes respectively and the normalized �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 �-FeOOH particles, which does not relax to a uniform dispersion of �-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.