Guillermo R. Iglesias

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.

Adsorption of Anionic and Cationic Surfactants on Anionic Colloids: Supercharging and Destabilization

We present herein a study on the adsorption of anionic (SDS), cationic (CTAB), and nonionic (C(12)E(5)) surfactants onto anionic silica nanoparticles. The effects of this adsorption are studied by means of the static structure factor, S(q), and the collective diffusion coefficient, D(c), obtained from small-angle X-ray scattering and dynamic light scattering measurements, respectively. The effective charge on the particles was determined also from classical electrophoresis and electroacoustic sonic-amplitude measurements. The surface tension of the sample was also investigated. Of particular note is the adsorption of SDS onto the silica nanoparticles, which leads to supercharging of the interface. This has interesting repercussions for structures obtained by the layer-by-layer (LbL) technique, because emulsions stabilized with supercharged and hydrophobized silica are perfect candidates for use in a multilayer system.

Predictions of the maximum energy extracted from salinity exchange inside porous electrodes

Capacitive energy extraction based on double layer expansion (CDLE) is the name of a new method devised for extracting energy from the exchange of fresh and salty water in porous electrodes. It is based on the change of the capacitance of electrical double layers (EDLs) at the electrode/solution interface when the concentration of the bulk electrolyte solution is modified. The use of porous electrodes provides huge amounts of surface area, but given the typically small pore size, the curvature of the interface and EDL overlap should affect the final result. This is the first aspect dealt with in this contribution: we envisage the electrode as a swarm of spherical particles, and from the knowledge of their EDL structure, we evaluate the stored charge, the differential capacitance and the extracted energy per CDLE cycle. In all cases, different pore radii and particle sizes and possible EDL overlap are taken into account. The second aspect is the consideration of finite ion size instead of the usual point-like ion model: given the size of the pores and the relatively high potentials that can be applied to the electrode, excluded volume effects can have a significant role. We find an extremely strong effect: the double layer capacitance is maximum for a certain value of the surface potential. This is a consequence of the limited ionic concentration at the particle-solution interface imposed by the finite size of ions, and leads to the presence of two potential ranges: for low electric potentials the capacitance increases with the ionic strength, while for large potentials we find the opposite trend. The consequences of these facts on the possibility of net energy extraction from porous electrodes, upon changing the solution in contact with them, are evaluated.

Multi-ionic effects on energy production based on double layer expansion by salinity exchange.

It has been recently shown that the free energy change upon salinity mixing in river mouths can be harvested taking advantage of the fact that the capacitance of charged solid/liquid interfaces (electrical double layers, EDLs) depends strongly on the ionic composition of the liquid medium. This has led to a new generation of techniques called Capmix technologies, one of them (CDLE or capacitive energy extraction based on DL expansion) based precisely on such dependence. Despite the solution composition playing a crucial role on the whole process, most of the research carried out so far has mainly focused on pure sodium chloride solutions. However, the effect of other species usually present in river and seawaters should be considered both theoretically and experimentally in order to succeed in optimizing a future device. In this paper, we analyse solutions of a more realistic composition from two points of view. Firstly, we find both experimentally and theoretically that the presence of ions other than sodium and chloride, even at low concentrations, may lead to a lower energy extraction in the process. Secondly, we experimentally consider the possible effects of other materials usually dispersed in natural water (mineral particles, microbes, shells, pollutants) by checking their accumulation in the carbon films used, after being exposed for a long period to natural sea water during CDLE cycles.

Temperature Effects on Energy Production by Salinity Exchange

In recent years, the capacitance of the interface between charged electrodes and ionic solutions (the electric double layer) has been investigated as a source of clean energy. Charge is placed on the electrodes either by means of ion-exchange membranes or of an external power source. In the latter method, net energy is produced by simple solution exchange in open circuit, due to the associated decrease in the capacitance of the electric double layer. In this work, we consider the change in capacitance associated with temperature variations: the former decreases when temperature is raised, and, hence, a cycle is possible in which some charge is put on the electrode at a certain potential and returned at a higher one. We demonstrate experimentally that it is thus viable to obtain energy from electric double layers if these are successively contacted with water at different temperatures. In addition, we show theoretically and experimentally that temperature and salinity variations can be conveniently combined to maximize the electrode potential increase. The resulting available energy is also estimated.

Description and performance of a fully automatic device for the study of the sedimentation of magnetic suspensions

In this paper we describe an experimental setup for the automatic determination of the sedimentation behavior of magnetic suspensions (i.e., disperse systems consisting on ferro- or ferri-magnetic particles in a suitable fluid) of arbitrary volume fraction of solids. The device is based on the evaluation of the inductance of a thin coil surrounding the test tube containing the sample. The inductance L is evaluated from the measurement of the resonant frequency of a parallel LC circuit constructed with the coil and a capacitor of known capacitance. The coil can be moved vertically along the tube at specified steps and time intervals, and from the knowledge of L as a function of the vertical position and time, one can get an image of the particle concentration profiles at given instants of time. The performance of the device is tested against suspensions of spherical iron particles in the micrometer size range dispersed in silicone oil, with various initial concentrations of solids. The sedimentation profiles are then compared with the predictions of existing models for the settling of disperse systems of non-interacting particles.

Effect of Solution Composition on the Energy Production by Capacitive Mixing in Membrane-Electrode Assembly

In this work, we consider the extent to which the presence of multivalent ions in solution modifies the equilibrium and dynamics of the energy production in a capacitive cell built with ion-exchange membranes in contact with high surface area electrodes. The cell potential in open circuit (OCV) is controlled by the difference between both membrane potentials, simulated as constant volume charge regions. A theoretical model is elaborated for steady state OCV, first in the case of monovalent solutions, as a reference. This is compared to the results in multi-ionic systems, containing divalent cations in concentrations similar to those in real seawater. It is found that the OCV is reduced by about 25% (as compared to the results in pure NaCl solutions) due to the presence of the divalent ions, even in low concentrations. Interestingly, this can be related to the “uphill” transport of such ions against their concentration gradients. On the contrary, their effect on the dynamics of the cell potential is negligible in the case of highly charged membranes. The comparison between model predictions and experimental results shows a very satisfactory agreement, and gives clues for the practical application of these recently introduced energy production methods.

Interactions between large colloids and surfactants

The interfacial adsorption of an anionic SDS and a nonionic surfactant C12E5, above the cmc onto submicron-sized, negatively charged silica particles in aqueous solution has been investigated by using electrokinetics, conductometry and static light scattering. It was found that both surfactants are prone to being adsorbed onto the silica/water interface. Addition of C12E5 to the silica dispersion leads to a decrease in mobility. This reduced surface charge causes a decrease in the stability of the silica particles. Surprisingly, the addition of SDS brings about an increase in the negative electrophoretic mobility of the anionic silica particles, leading to a super-charging effect. Subsequent addition of C12E5 gives rise to even higher negative electrophoretic mobilities. This unexpected hyper-charging effect can only be understood as a cooperative effect based on mixed micelles of C12E5 and non-adsorbed SDS. Not so unexpectedly, the sequence of surfactant addition was found to be decisive, as quite different results are obtained when C12E5 is added before SDS.

Stability and Flow Behavior of a Magnetorheological Lubricant in a Magnetic Shock Absorber

Interest in the use of field-responsive fluids in the automobile industry is growing in view of the increasing capabilities of electronic control in the vehicles dynamic response. This is the case of magnetorheological fluids or lubricants (MRF). These are capable to change in very short times their rheological behavior upon application of modest magnetic fields. In the present work we describe a new formulation of a stable magnetic fluid that is checked both in a laboratory rheometer and in a commercial shock absorber. It is found that the yield stress can increase by several orders of magnitude when the magnetic field strength reaches a few hundred mT. Furthermore, friction tests carried out in a damper test machine showed that the shock absorber behaved very regularly when charged with the fluid under very different conditions of speed and amplitude of the forced oscillations produced and for various applied magnetic fields.

Magnetic hyperthermia with magnetite nanoparticles: electrostatic and polymeric stabilization

In this work, magnetic hyperthermia, i.e., heating induced by an alternating magnetic field acting on a magnetic suspension, is considered in three main aspects. The first one regards the implementation of a simple device for producing AC magnetic fields. The second contribution concerns the comparison of the hyperthermia response (measured by the specific absorption rate (SAR)) of magnetite nanoparticles of two different sizes and of raw particles vs. polyelectrolyte-coated ones. An improvement is observed of the SAR values when the pH is fixed away from the isoelectric point or when the ionic strength is kept at low values. The addition of a polymer enhances significantly the stability of the suspensions and so does with the SAR values. Finally, we describe the implementation of a sort of magnetic hyperthermia applicator, avoiding the necessity of placing the magnetic sample inside the coil and making it of more practical use.