J. P. Segovia-Gutiérrez

Dynamic rheology of sphere- and rod-based magnetorheological fluids

The effect of particle shape in the small amplitude oscillatory shear behavior of magnetorheological (MR) fluids is investigated from zero magnetic field strengths up to 800 kA/m. Two types of MR fluids are studied: the first system is prepared with spherical particles and a second system is prepared with rodlike particles. Both types of particles are fabricated following practically the same precipitation technique and have the same intrinsic magnetic and crystallographic properties. Furthermore, the distribution of sphere diameters is very similar to that of rod thicknesses. Rod-based MR fluids show an enhanced MR performance under oscillatory shear in the viscoelastic linear regime. A lower magnetic field strength is needed for the structuration of the colloid and, once saturation is fully achieved, a larger storage modulus is observed. Existing sphere- and rod-based models usually underestimate experimental results regarding the magnetic field strength and particle volume fraction dependences of both storage modulus and yield stress. A simple model is proposed here to explain the behavior of microrod-based MR fluids at low, medium and saturating magnetic fields in the viscoelastic linear regime in terms of magnetic interaction forces between particles. These results are further completed with rheomicroscopic and dynamic yield stress observations.

Effect of particle shape in magnetorheology

Magnetorheological (MR) properties were investigated for sphere, plate, and rod-like iron particles in suspension under the presence of magnetic fields to ascertain the effect of particle shape in MR performance. A novel two-step synthesis route for micrometer sized iron particles with different morphologies is described in detail. Small-amplitude dynamic oscillatory and steady shear flow measurements were carried out in the presence of external magnetic fields. Finite element method calculations were performed to explain the effect of particle shape in the magnetic field-induced yield stress. Compared to their sphere and plate counterparts, rod-like particle based MR fluids present a larger storage modulus and yield stress. The effect of particle shape is found to be negligible at large particle content and/or magnetic field strengths.

Nonlinear viscoelasticity and two-step yielding in magnetorheology: A colloidal gel approach to understand the effect of particle concentration

The yielding behavior of conventional magnetorheological (MR) fluids is revisited for a wide range of magnetic fields and particle concentrations under a colloidal gel perspective. A two-step yielding behavior is found at intermediate magnetic fields (∼10 kA/m) that can be explained as a transition from a strong-link to a weak-link (or transition) regime upon increasing the particle concentration in the MR fluid. This two-step yielding behavior is reminiscent of the classical concepts of static (frictional) and dynamic (Bingham) yield stress. By relating macroscopic elastic properties to a scaling fractal model, we could identify the prevalent gelation regime in MR fluids.

Squeeze flow magnetorheology

This paper is concerned with an investigation of the rheological performance of magnetorheological fluids under squeeze flow. Preliminary results on Newtonian fluids are first compared to Stefan’s equation. Then, unidirectional monotonic compression tests are carried out in the presence of uniaxial external magnetic fields at slow compression rates under constant volume operation. Results are compared to Bingham plastic, biviscous, and single chain micromechanical squeeze flow models. Measurements using combined deformation modes (compression+small-strain oscillatory shear) suggest a compression-induced shear strengthen effect up to strains of ∼0.5. Particle-level dynamic simulations are in qualitatively good agreement with experimental observations.

Brownian dynamics simulations in magnetorheology and comparison with experiments

The rheological behaviour of unsheared magnetorheological fluids is studied using Brownian dynamics simulations and experiments. In the simulations, we use monodisperse and polydisperse systems, and study the structure formation and the stress autocorrelation function that provide the shear moduli and shear viscosity of the system. Whereas the monodisperse system crystallizes, as identified by the pair distribution function, polydispersity hinders crystallization and allows a comparison with the experiments. These are performed with carbonyl iron particles in different Newtonian solvents (silicone oils and glucose syrup). Special attention is paid to the equilibration of the samples. A rescaling of the viscosity is introduced that collapses data from different systems and shear rates, leaving solely the dependence on the external magnetic field. The simulation data can be collapsed onto the same curve if the magnetic field is also rescaled, due to the approximations involved. The master curve shows the expected quadratic dependence on the external field. The shear moduli from simulations and experiments agree qualitatively; both moduli develop a shoulder at low frequencies, indicating a slow mechanism of stress relaxation connected to structural relaxation.

Average particle magnetization as an experimental scaling parameter for the yield stress of dilute magnetorheological fluids

We propose an experimental parameter for the scaling of the yield stress (τy) of magnetorheological (MR) fluids: the average particle magnetization Mp as estimated from magnetization curves of the MR suspensions. When τy was expressed as a function of this scaling parameter, the curves for MR suspensions prepared with particles of different saturation magnetization and even different morphology collapsed together. In addition, the collapse worked reasonably well for a wide range of magnetic fields: from weak fields below which the sensitivity of our magnetorheometer could not detect the τy, to fields close to particle saturation. The collapse failed for particles of highly anisotropic morphology, which must be indicative of non-magnetostatic contributions to the yield stress.

On the effect of particle porosity and roughness in magnetorheology

We report a study on the mechanical properties of magnetorheological (MR) fluids prepared with porous iron particles with rough surfaces. These particles were obtained by reducing a magnetite precursor in a H2 atmosphere at 400 °C. Small-amplitude dynamic oscillatory and steady shear flow measurements were carried out in the presence of external magnetic fields. Results were compared with those obtained for MR fluids prepared with conventional solid carbonyl iron particles of comparable size. We found significant differences between the rheology of both types of suspensions, and, more importantly, we found that simple available models can predict quantitatively those differences as long as the average density of the particles is known and is used to calculate their effective volume magnetization and the real volume fraction of the MR fluids prepared with them. By doing so, we obtained for both the porous iron suspensions and the solid iron suspensions a single master curve of the dimensionless storage mod...

Extensional rheometry of magnetic dispersions

This work presents a technique and develops an apparatus that allows the application of homogeneous external magnetic fields (parallel or perpendicular to the deformation axis) to a fluid sample undergoing extensional flow kinematics while measuring the filament thinning using the commercial version of the capillary breakup extensional rheometer (Haake™ CaBER™ 1, Thermo Scientific). We also present innovative rheological measurements of several commercial ferrofluids (FFs) and one magnetorheological fluid (MRF) under uniaxial extensional flow. The experimental results demonstrate that FFs exhibit a Newtonian-like behavior in the absence of magnetic fields. When a magnetic field is applied perpendicular to the extensional flow, no significant effects are observed similar to shear experiments. However, when the external magnetic field is aligned with the extensional flow, the filament takes longer to break up but otherwise behaves as a Newtonian fluid. In the case of the MRF, due to the higher concentration of particles and larger particle size, the differences in the extensional behaviors are much more dramatic regardless of the orientation of the magnetic field compared to the case when no magnetic field is applied.

Faceted particles: An approach for the enhancement of the elasticity and the yield-stress of magnetorheological fluids

Faceted particles have been used to prepare dilute magnetorheological (MR) fluids with enhanced aggregate strength. The measured storage modulus of these suspensions is significantly larger than that of the MR fluids prepared with spherical particles, and comparable to that of the rod-based fluids, whereas no sign of formation of a percolated system was observed at the largest concentration we studied (5 vol. %). Finite element method calculations confirm that the more intimate surface contacts between faceted particles lead to larger magnetic interparticle forces than the point contacts associated with the spherical particles. The contribution of friction is expected to be significant but remains unknown.

Brownian dynamic simulations and experiments of MR fluids

The use of computational techniques in magnetorheology is not new. I general, these approaches assume dipolar magnetic interactions, hard sphere repulsions, and no-slip conditions. In this contribution we focus on the dynamics of the equilibrium state in the presence of uniaxial DC fields. To achieve this goal we make use of Brownian Dynamic Simulations. We highlight the importance of the Brownian forces versus magnetic dipolar interaction in the range of low magnetic field strengths. We monitor the formation of columnar structures and their dynamics, in competition with the Brownian motion, until a hexatic crystal phase appears at high field strengths for monodisperse systems. The shear viscosity is computed from the Einstein relation and eventually compared with experimental data at very low-shear rates. A reasonably good agreement between both data sets is observed.