Parallel superposition rheology on the yielding behaviors of ferrofluids

Abstract

The microscopic mechanism behind the yielding behaviors of ferrofluids is still blurred, lacking a research method to monitor the structural evolution under a varying steady shear stress. In the present work, parallel superposition rheometry was first used to evaluate the viscoelastic behaviors of ferrofluids during the yielding process. The existence of a linear viscoelastic region in storage moduli curve under very low steady shear stresses demonstrates that gap-spanning structures dominate the rheological behaviors of ferrofluids under weak hydrodynamic interaction. The disappearance of the peak in loss moduli of ferrofluids with relative low particle volume concentration was spotted, indicating that the decomposition of gap-spanning structures can be completed through different paths in ferrofluid system. A sudden drop in the storage moduli was observed under the sweep of steady stress, magnetic field strength or temperature respectively, which is regarded as the sign of transition from thick columnar structures to single chains. By examining the combined effect of magnetic field strength and temperature, a fixed yield stress of ferrofluids was thought to reflect a critical contrast between the thermal and magnetic interaction level.