Ana Gómez-Ramírez

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.

Steady shear flow of magnetic fiber suspensions: theory and comparison with experiments

This paper is focused on the rheology of magnetic fiber suspensions in the presence of a magnetic field applied perpendicular to the flow. At low Mason numbers, Mn<0.1, the experimental flow curves show a steep initial section corresponding to the inclination and stretching of the gap-spanning aggregates formed upon magnetic field application. At higher Mason numbers, aggregates no longer stick to the walls and the flow curves reach a Bingham regime, with the dynamic yield stress growing with the magnetic field intensity. This yield stress appears to be about three times higher for the fiber suspensions than for the suspensions of spherical particles. Such difference, measured at relatively low magnetic field intensities, H0<30 kA/m, is explained in terms of the enhanced magnetic susceptibility of the aggregates composed of fibers compared to the aggregates composed of spherical particles. For weak magnetic fields, the forces of solid friction between fibers are expected to play a minor role on the stress...

Efficient synthesis of ammonia from N2 and H2 alone in a ferroelectric packed-bed DBD reactor

A detailed study of ammonia synthesis from hydrogen and nitrogen in a planar dielectric barrier discharge (DBD) reactor was carried out. Electrical parameters were systematically varied, including applied voltage and frequency, electrode gap, and type of ferroelectric material (BaTiO3 versus PZT). For selected operating conditions, power consumption and plasma electron density were estimated from Lissajous diagrams and by application of the Bolsig + model, respectively. Optical emission spectroscopy was used to follow the evolution of plasma species () as a function of applied voltage with both types of ferroelectric material. PZT gave both greater energy efficiency and higher ammonia yield than BaTiO3: 0.9 g NH3 kWh−1 and 2.7% single pass N2 conversion, respectively. This performance is substantially superior to previously published findings on DBD synthesis of NH3 from N2 and H2 alone. The influence of electrical working parameters, the beneficial effect of PZT and the importance of controlling reactant residence time are rationalized in a reaction model that takes account of the principal process variables

Stability of magnetorheological fluids in ionic liquids

In this work, magnetorheological (MR) fluids using ionic liquids (ILs) as carrier were prepared. With this aim, two different ILs, with low and high electrical conductivity, and two kinds of iron particles, either with silica coating or without it, were used. The viscosity of the suspensions was measured, and the results compared with the theoretical predictions of Batchelors equation. The deviations from Batchelors equation were used to make inferences on the aggregation state of the suspensions. Contact angle measurements on iron powder pellets were also performed in order to analyze the wetting of the particles by the ILs employed as carriers. In addition, sedimentation and redispersion experiments were carried out and discussed in view of the aggregation degrees of the suspensions inferred from the previously mentioned experiments. From all these experiments, it could be concluded that the suspension consisting of silica coated iron particles dispersed in the low conductivity ionic liquid presents the best stability and redispersibility properties. Finally, microscopic observations of diluted suspensions, carried out upon magnetic field application, showed that the most regular field-induced structure was also obtained for this suspension.

Effect of particle aggregation on the magnetic and magnetorheological properties of magnetic suspensions

In this work, the magnetorheological behavior of concentrated (10% solid volume fraction) iron-based magnetorheological fluids was studied. Two different compounds were added to stabilize the suspensions against aggregation and settling processes: A surfactant (aluminum stearate, AlSt), and a gel-forming agent (silica nanoparticles). The shear stress vs. shear rate flow curves of the suspensions were obtained in a wide range of applied magnetic fields with the aim of determining the intensity of the field-induced yield stress, that is, the strength of the so-called magnetorheological (MR) effect. The suspension stabilized against particle aggregation by surfactant addition reached the strongest MR response, while the opposite behavior (the weakest MR response) was found in the suspension that contained silica nanoparticles added as anti-settling agent. The scaling between the yield stress and the magnetic field strength (σy∼Hn) was calculated and compared with the predictions of theoretical structural mod...

Surface chemistry and germination improvement of Quinoa seeds subjected to plasma activation

Plasma treatment is recognized as a suitable technology to improve germination efficiency of numerous seeds. In this work Quinoa seeds have been subjected to air plasma treatments both at atmospheric and low pressure and improvements found in germination rate and percentage of success. Seed water uptake by exposure to water vapor, although slightly greater for plasma treated seeds, did not justify the observed germination improvement. To identify other possible factors contributing to germination, the chemical changes experienced by outer parts of the seed upon plasma exposure have been investigated by X-ray photoemission spectroscopy (XPS) and scanning electron microscopy (SEM-EDX). XPS revealed that the outer layers of the Quinoa plasma treated seeds were highly oxidized and appeared enriched in potassium ions and adsorbed nitrate species. Simultaneously, SEM-EDX showed that the enrichment in potassium and other mineral elements extended to the seed pericarp and closer zones. The disappearance from the surface of both potassium ions and nitrate species upon exposure of the plasma treated seeds to water vapor is proposed as a factor favoring germination. The use of XPS to study chemical changes at seed surfaces induced by plasma treatments is deemed very important to unravel the mechanisms contributing to germination improvement.

New Perspectives for Magnetic Fluid-Based Devices Using Novel Ionic Liquids as Carriers

© 2012 Lopez-Lopez et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. New Perspectives for Magnetic Fluid-Based Devices Using Novel Ionic Liquids as Carriers

RHEOLOGY OF MAGNETIC FIBER SUSPENSIONS

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 512 kA m−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 aligned with the field direction. Nonetheless, interfiber solid friction hinders a complete alignment of the fibers with the field, and the fiber network remains entangled.