J. C. Fernández-Toledano

Two-step yielding in magnetorheology

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 fluids and performing particle level simulations that include R-shifted Lennard-Jones potentials of interaction.

Model magnetorheology: A direct comparative study between theories, particle-level simulations and experiments, in steady and dynamic oscillatory shear

We investigate model magnetorheological (MR) fluids (inverse ferrofluids) under both steady and dynamic oscillatory shear. Analytical theories, particle-level simulations, and magnetorheometry are used in an attempt to obtain universal master curves. Steady shear flow data can be collapsed when plotted as a function of a dimensionless Mason number. The critical Mason number associated with the transition from magnetostatic to hydrodynamic control of the suspension structure is demonstrated to linearly increase with particle concentration which is in good agreement with theories and our simulations. Experimental linear viscoelastic moduli are in good agreement with micromechanical and macroscopic models in the dilute regime. However, upon increasing particle concentration, theoretical predictions underestimate experimental data while particle-level simulations are in good agreement. The accordance with particle-level simulations suggests that the mean (average) magnetization approximation gives a good pred...

Simulations of polydisperse magnetorheological fluids: A structural and kinetic investigation

A simulation method is proposed to explore the effect of particle size polydispersity in magnetorheology including Brownian motion. The method aims to extend the classical particle-level simulation methodology developed by Klingenberg et al. [J. Chem. Phys. 91, 7888–7895 (1989)] for the case of polydisperse magnetorheological (MR) fluids. The simulation study concerns the aggregation kinetics at rest as well as the rheological behavior under start-up of steady shear and dynamic oscillatory shear tests at increasing strain amplitudes. Results demonstrate that the effect of polydispersity is only relevant at the transition regime between magnetostatic to hydrodynamic control of the suspension structure. The yielding behavior is correlated to the structural characteristics (radial distribution functions, pair correlation functions, and angular connectivities) of the MR fluids before the onset of flow. A more abrupt transition is observed for polydisperse MR fluids because interparticle links are weaker in th...