Fernando Vereda

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.

Physical Properties of Elongated Magnetic Particles: Magnetization and Friction Coefficient Anisotropies

Anisotropy counts: A brief review of the main physical properties of elongated magnetic particles (EMPs) is presented. The most important characteristic of an EMP is the additional contribution of shape anisotropy to the total anisotropy energy of the particle, when compared to spherical magnetic particles. The electron micrograph shows Ni-ferrite microrods fabricated by the authors.We present an overview of the main physical properties of elongated magnetic particles (EMPs), including some of their more relevant properties in suspension. When compared to a spherical magnetic particle, the most important characteristic of an EMP is an additional contribution of shape anisotropy to the total anisotropy energy of the particle. Increasing aspect ratios also lead to an increase in both the critical single-domain size of a magnetic particle and its resistance to thermally activated spontaneous reversal of the magnetization. For single-domain EMPs, magnetization reversal occurs primarily by one of two modes, coherent rotation or curling, the latter being facilitated by larger aspect ratios. When EMPs are used to prepare colloidal suspensions, other physical properties come into play, such as their anisotropic friction coefficient and the consequent enhanced torque they experience in a shear flow, their tendency to align in the direction of an external field, to form less dense sediments and to entangle into more intricate aggregates. From a more practical point of view, EMPs are discussed in connection with two interesting types of magnetic colloids: magnetorheological fluids and suspensions for magnetic hyperthermia. Advances reported in the literature regarding the use of EMPs in these two systems are included. In the final section, we present a summary of the most relevant methods documented in the literature for the fabrication of EMPs, together with a list of the most common ferromagnetic materials that have been synthesized in the form of EMPs.

Oxidation of ferrous hydroxides with nitrate: A versatile method for the preparation of magnetic colloidal particles

Slightly over 30 years ago, Sugimoto and Matijević published an article in this journal on the synthesis of uniform magnetite particles by the partial oxidation of ferrous hydroxide gels. The article has become a widely-used reference for the preparation of magnetite particles in aqueous media. A reason for this was the thoroughness of their study and the versatility of the process: the authors described conditions under which cubic nanometric (30-100 nm) crystals, or larger (0.4-1.1 μm) spherical particles, the latter having either a smooth or a rough surface, could be obtained. Further work by Matijević and other authors has shown that small modifications of the process, such as the addition of divalent cations other than Fe(2+) to the system or the superposition of a magnetic field, can be used for the preparation of ferrite particles or rod-like particles, respectively. In this article we present a short description of the synthesis process and a brief overview of subsequent work carried out by other researchers that illustrates the versatility and the potential of this method.

Synthesis of Ni ferrite and Co ferrite rodlike particles by superposition of a constant magnetic field

We report the fabrication of micron-sized rodlike particles of nonstoichiometric Co and Ni ferrites by aging coprecipitated Fe(OH) 2 and M(OH) 2 —where M is either Ni or Co—at 90 °C in the presence of an external magnetic field (B ≈ 405 mT). Potassium nitrate was used as a mild oxidant. Resultant particles were analyzed by means of electron microscopy, x-ray powder diffraction (XRD), magnetometry, energy dispersive x-ray (EDX) spectrometry, and atomic absorption spectroscopy. Rodlike particles of both types of ferrite exhibited a relatively uniform thickness, an average aspect ratio close to 10, and have a spinel crystalline structure. EDX spectrometry and atomic absorption spectroscopy confirmed the incorporation of Ni 2+ and Co 2+ in the respective ferrite particles. The incorporation of Co 2+ led to non-negligible remanence and coercivity. The incorporation of Ni 2+ led to a lower saturation magnetization, whereas the remanence and coercivity of the Ni ferrite were very low, still typical of a soft ferrimagnetic material. The mechanism of formation of the rodlike particles was investigated by the time-dependent observation of growing Ni ferrite rods.

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...

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.

Control of surface morphology and internal structure in magnetite microparticles: from smooth single crystals to rough polycrystals

Magnetite particles in the micrometer range have been obtained by the oxidative aging of ferrous hydroxide with KNO3. The surface morphology and the number of crystallites that constitute each particle can be controlled by adjusting the Fe2+ excess in the reaction media. Thus, for a relatively low [Fe2+]Exc we obtained smooth polyhedral single-crystal particles, whereas for larger [Fe2+]Exc the particles had rough surfaces and a raspberry-like appearance due to their polycrystalline nature. The differences in the surface morphology of the particles are intimately related to the differences in the internal structure, which are the outcome of particular growth mechanisms. These mechanisms of particle formation can therefore also be controlled and can be qualitatively explained in terms of the interparticle electrostatic interactions after the initial nucleation. Magnetic properties were also connected to the internal structure of the particles. Because of the relatively large size of the crystalline domains, magnetization reversal took place by magnetic domain wall motion and all the particles we obtained were magnetically soft at room temperature. At 5 K the more complex structure of the rough particles resulted in a larger coercivity.

Effect of surface roughness on the magnetic interaction between micron-sized ferromagnetic particles: Finite element method calculations

We report a finite element method study on the effect of surface roughness on the field-induced magnetization of micrometric iron particles and on the interparticle magnetostatic forces between them. Calculations were carried out for two-dimensional geometries in which particles were modelled as discs. Roughness was introduced as semicircular protrusions or as triangular- or square-wave profiles. Interestingly, we found that increasing amplitudes of the triangular- or square-wave profiles facilitated the magnetization of the particles, resulting in larger interparticle forces at fields below saturation. The effect of the semicircular protrusions and of the spatial frequency of the wave profiles was comparatively small, suggesting that in real systems the effect of particle roughness on magnetic properties may depend on the specific surface morphology. The permeability of the particles also influenced the extent to which roughness facilitated the magnetization process: a larger permeability resulted in larger differences between the magnetization curves of the smooth and the rough particles. Results are relevant to magnetorheological fluids, since we show that surface roughness can affect the magnetic interactions between particles.