We use particle level dynamic simulations, ultrasonic characterization, and rheomicroscopy to investigate the yielding behavior of magnetorheological (MR) fluids under oscillatory shear in both dilute and concentrated regimes. Dilute suspensions exhibit a single peak in the elastic stress that is associated to the breaking of the field-induced structures at the flow point (G′ = G″). On the other hand, more concentrated suspensions demonstrate a two-step yielding that is associated to the existence of short-ranged attractions between the particles, possibly coming from remnant magnetization or van der Waals forces. This two-step yielding is demonstrated by introducing additives in the formulation of the MR fluidsand performing particle level simulations that include R-shifted Lennard-Jones potentials of interaction.
ACKNOWLEDGMENTS
This work was supported by MINECO MAT 2010-15101, DPI2010-17716, PIB2010BZ-00570, MAT2013-44429-R projects (Spain), and by Junta de Andalucía (P10-FQM-5977, P10-RNM-6630, and P11-FQM-7074 projects). J.R.L. is grateful to a CSIC JAE fellowship.