Modesto T. López-López

Rheological study of the stabilization of magnetizable colloidal suspensions by addition of silica nanoparticles

An experimental investigation is described on the stability of magnetorheological fluids (MRFs) consisting of iron suspensions in silicone oil with a thixotropic agent (silica nanoparticles) as stabilizer. The rheological properties were investigated using a commercial rheometer with a parallel-plate measuring cell. Several kinds of experiments were performed in steady-state, oscillatory, and transient regimes. The effects of the volume fraction of magnetic particles, the concentration of silica, magnetic flux density, B, and waiting time after preshear on the rheology of the MRFs were considered. Steady-state measurements demonstrated that our systems only display plastic behavior, for which a yield stress, σy, is appreciable, for the highest iron concentrations and/or magnetic fields. The yield stress was found to be independent of the magnetic flux density when the concentration of silica particles was large enough (> ∼20 g/L). This is a manifestation of the entrapment of iron particles in the silica g...

Sedimentation and redispersion phenomena in iron-based magnetorheological fluids

In this work, the effect of three different additives (oleic acid, aluminum stearate, and silica nanoparticles) on the aggregation, sedimentation, and redispersibility of concentrated iron-based magnetorheological fluids was investigated. With this aim, the sedimentation behavior was analyzed using an electromagnetic induction method, which is suitable for studying the sedimentation of opaque magnetic suspensions. The redispersibility was studied, in a quantitative way, by means of rheological measurements, both in the presence and in the absence of external magnetic field. For this purpose, samples were subjected to a constant shear stress at different moments (steps) of the settling process. The time evolution of the corresponding shear rate was measured at each step. Interestingly, it was found that although the addition of oleic acid or aluminum stearate does not avoid particle settling, the redispersibility of the suspensions is considerably enhanced. On the contrary, silica nanoparticles behave as a...

Preparation of stable magnetorheological fluids based on extremely bimodal iron–magnetite suspensions

The high magneto-viscous response of magnetorheological fluids (MRFs) comes from the large size (≈1 μm) of the magnetic particles dispersed in the carrier liquid. Unfortunately, in the absence of a magnetic field, this large size constitutes the origin of some problems facing the technological applications of MRFs. These problems are (i) the instability of the suspensions caused by the fast settling of the high density magnetic particles used, and (ii) the poor redispersibility due to an irreversible aggregation. In this work, we used an electromagnetic induction method to study the stability of MRFs containing micron-sized iron particles dispersed in ferrofluids composed by oleate-covered magnetite nanoparticles dispersed in kerosene. Interestingly, we demonstrated that the sedimentation rate in iron/ferrofluid suspensions can be significantly lower than in iron/kerosene MRFs.

Magnetorheology of fiber suspensions. I. Experimental

This work reports a detailed study on the shear magnetorheology of suspensions of magnetic microfibers. The steady-state regime was investigated using a controlled-stress rheometer for different concentrations of particles and under the presence of a broad range of applied magnetic fields (up to 512kAm−1). The results were compared with those obtained for conventional magnetorheological fluids (suspensions of magnetic microspheres). It was found that the suspensions of magnetic fibers show an enhanced magnetorheological effect. We proposed the existence of field-dependent solid friction between fibers as the main physical reason for this enhancement. In order to ascertain the relevance of the interfiber solid friction, the microscopic structure of fiber suspensions was investigated using an optical microscope. In the absence of applied field, fibers form an entangled network with approximately isotropic orientation. Upon magnetic field application, the fiber network becomes deformed and approximately alig...

Magnetorheology for suspensions of solid particles dispersed in ferrofluids

In this work, the magnetorheological properties of suspensions of micron-sized iron particles dispersed in magnetite ferrofluids were studied. With this aim, the flow properties of the suspensions in the steady-state regime were investigated using a commercial magnetorheometer with a parallel-plate measuring cell. The effect of both magnetite and iron concentration on the magnitude of the yield stress was studied for a broad range of magnetic fields. In addition, the experimental values of the yield stress were compared with the predictions from the chain model. With this purpose the values of the yield stress were obtained by means of finite element calculations. Interestingly, it was found that the experimental yield stress increases with the concentration of magnetite nanoparticles in the ferrofluid. Unfortunately, this behaviour is not obtained from calculations based on the chain model, which predict just the opposite trend.

Dynamic characterization of extremely bidisperse magnetorheological fluids

In this work, we investigate the stability and redispersibility of magnetorheological fluids (MRFs). These are disperse systems where the solid is constituted by ferro- or ferri-magnetic microparticles. Upon the application of external magnetic field, they experience rapid and reversible increases in yield stress and viscosity. The problem considered here is first of all the determination of their stability against sedimentation, an essential issue in their practical application. Although this problem is typically faced through the addition of thixotropic agents to the liquid medium, in this work, we propose the investigation of the effect of magnetic nanoparticles addition, so that the dispersion medium is in reality a ferrofluid. It is found that a volume fraction of nanoparticles not higher than 3% is enough to provide a long-lasting stabilization to MRFs containing above 30% iron microparticles. In the, in fact unavoidable, event of settling, the important point is the ease of redispersion of the sediment. This is indirectly evaluated in the present investigation by measuring the penetration force in the suspension, using a standard hardness needle. Again, it is found that the nanoparticles addition produces soft sediments by avoiding short-range attractions between the large iron particles. Finally, the performance of the designed MRFs is evaluated by obtaining their steady-state rheograms for different volume fractions of magnetite and different magnetic field strengths. The yield stress is found to be strongly field-dependent, and it can achieve the high values expected in standard magnetorheological fluids but with improved stability and redispersibility.

New magnetorheological fluids based on magnetic fibers

In this work, we report the synthesis under the presence of external magnetic fields of two types of magnetic microfibers: (i) cobalt wires; and (ii) rigid filaments of chemically linked iron microspheres. Optical microscopy and scanning electron microscopy (SEM) were used for characterizing their shape and size. The effect of magnetic field intensity, mixing frequency and reagent concentration on the final size of the fibers was investigated. The particles were also characterized by means of energy dispersive X-ray spectroscopy (EDX). The magnetization curves of both the cobalt wires and the iron filaments were obtained and compared with those of cobalt spheres and iron spheres respectively. Finally, these magnetic fibers were used for the preparation of new smart composites by dispersing them in a liquid carrier. Interestingly, the suspensions of cobalt wires show a remarkable increase in the yield stress when compared to conventional (based on spherical particles) magnetorheological fluids.

Preparation and characterization of iron-based magnetorheological fluids stabilized by addition of organoclay particles.

Suspensions of micrometer-sized iron particles (10 vol %) dispersed in kerosene and stabilized by addition of organoclay particles were prepared. The magnetization curves of these suspensions were measured, and their sedimentation and redispersion behaviors were analyzed as a function of clay concentration by means of optical and rheological methods. Furthermore, their magnetorheological properties were investigated using a controlled rate magnetorheometer and the effect of clay concentration on these properties was also analyzed. These experiments showed that the addition of clay slows down iron particle settling and eases the redispersion of the iron-based suspensions without masking their magnetorheological properties. Two mechanisms were found to be involved in this behavior: (i) the formation of a clay gel network and (ii) the presence of heterogeneous iron-clay adhesion.

Colloids on the frontier of ferrofluids. Rheological properties.

This paper is devoted to the steady-state rheological properties of two new kinds of ferrofluids. One of these was constituted by CoNi nanospheres of 24 nm in diameter, whereas the other by CoNi nanofibers of 56 nm in length and 6.6 nm in width. These ferrofluids were subjected to shear rate ramps under the presence of magnetic fields of different intensity, and the corresponding shear stress values were measured. From the obtained rheograms (shear stress vs shear rate curves) the values of both the static and the dynamic yield stresses were obtained as a function of the magnetic field. The magnetoviscous effect was also obtained as a function of both the shear rate and the magnetic field. The experimental results demonstrate that upon magnetic field application these new ferrofluids develop yield stresses and magnetoviscous effects much greater than those of conventional ferrofluids, based on nanospheres of approximately 10 nm in diameter. Besides some expected differences, such as the stronger magnetorheological effect in the case of ferrofluids based on nanofibers, some intriguing differences are found between the rheological behaviors of nanofiber ferrofluids and nanosphere ferrofluid. First, upon field application the rheograms of nanofiber ferrofluids present N-shaped dependence of the shear stress on the shear rate. The decreasing part of the rheograms takes place at low shear rate. These regions of negative differential viscosity, and therefore, unstable flow is not observed in the case of nanosphere ferrofluids. The second intriguing difference concerns the curvature of the yield stress vs magnetic field curves. This curvature is negative in the case of nanosphere ferrofluid, giving rise to saturation of the yield stress at medium field, as expected. However, in the case of nanofiber ferrofluid this curvature is positive, which means a faster increase of the yield stress with the magnetic field the higher the magnitude of the latter. These interesting differences may be due to the existence of strong interparticle solid friction in the case of nanofiber ferrofluids. Finally, theoretical models for the static yield stress of the ferrofluids were developed. These models consider that upon field application the ferrofluid nanoparticles are condensed in drops of dense phase. These drops tend to be aligned along the field direction, opposing the flow of the ferrofluids and being responsible for the static quasielastic deformation and the yield-stress phenomena. By considering the existence of interparticle dry friction only in the case of nanofiber ferrofluids, the developed models predicted quite well not only the magnitude of the static yield stress but also the differences in curvature of the yield stress vs magnetic field curves.

Influence of particle shape on the magnetic and magnetorheological properties of nanoparticle suspensions

CoNi nanofibers (56 nm length, 6.6 nm width) and nanospheres (24 nm diameter) were synthesized by reduction of ions in a liquid polyol. These powders were then characterized by means of electron microscopy, N2 adsorption, energy dispersive X-ray spectroscopy, X-ray diffraction and magnetometry. Similar properties were found for nanofibers and nanospheres, with the exception of shape and the consequent differences in the magnetization of the compressed powders that arise from the strong effect of fiber orientation on the demagnetizing field. Finite element method simulation was used to check this last point. These powders were also employed for the preparation of magnetorheological (MR) suspensions and their steady-state and dynamic rheological properties were studied under application of magnetic fields of different intensities. Both kinds of rheological measurements showed that the MR effect was considerably strengthened when nanofibers were used as dispersed phase instead of nanospheres.