Jaime Rodríguez-López

Two-step yielding in magnetorheology

We use particle level dynamic simulations, ultrasonic characterization, and rheomicroscopy to investigate the yielding behavior of magnetorheological (MR) fluids under oscillatory shear in both dilute and concentrated regimes. Dilute suspensions exhibit a single peak in the elastic stress that is associated to the breaking of the field-induced structures at the flow point (G′ = G″). On the other hand, more concentrated suspensions demonstrate a two-step yielding that is associated to the existence of short-ranged attractions between the particles, possibly coming from remnant magnetization or van der Waals forces. This two-step yielding is demonstrated by introducing additives in the formulation of the MR fluids and performing particle level simulations that include R-shifted Lennard-Jones potentials of interaction.

Measuring the yield stress in magnetorheological fluids using ultrasounds

In this work, we propose a method to accurately determine the yield stress in magnetorheological (MR) fluids using ultrasounds. The setup is constructed, and experimental data are obtained on a model conventional MR fluid under steady shear stress ramp-up tests. By using video-microscopy, ultrasonic techniques, and rheometry simultaneously, it is possible to precisely determine the yield stress at experimentally accessible times.

Study of the effect of particle volume fraction on the microstructure of magnetorheological fluids using ultrasound: Transition between the strong-link to the weak-link regimes.

The effect of particle volume fraction on the microstructure of magnetorheological (MR) fluids has been studied using ultrasonic techniques. When no magnetic field is applied, they behave as slurry. However, when magnetic field is applied, important features regarding the change of the microstructure have been found with the help of ultrasonic waves propagating in the direction of the magnetic field. As the volume fraction increases, a rearrangement of particles which decrease the compressibility of the system is detected; nevertheless, the material behaves as a non-consolidated material. Three different particle volume fraction regions are found identifying a critical particle volume fraction predicted in the literature. Ultrasounds are confirmed as an interesting tool to study MR fluids in static conditions.

Sound attenuation in magnetorheological fluids

In this work, the attenuation of ultrasonic elastic waves propagating through magnetorheological (MR) fluids is analysed as a function of the particle volume fraction and the magnetic field intensity. Non-commercial MR fluids made with iron ferromagnetic particles and two different solvents (an olive oil based solution and an Araldite-epoxy) were used. Particle volume fractions of up to 0.25 were analysed. It is shown that the attenuation of sound depends strongly on the solvent used and the volume fraction. The influence of a magnetic field up to 212 mT was studied and it was found that the sound attenuation increases with the magnetic intensity until saturation is reached. A hysteretic effect is evident once the magnetic field is removed.

Using ultrasounds for the estimation of the misalignment in plate–plate torsional rheometry

A new method to accurately quantify the gap error arising from non-parallelism in plate?plate and cone?plate torsional rheometry is proposed. This method consists in monitoring the ultrasonic pulses sent by a transducer adapted to the rheometer bottom plate. The time of flight (TOF) of the echoes reflected by the rheometer upper plate, while it rotates at small angular velocity having a sample placed between the plates, is measured. As the sample acoustic velocity is constant and known, any variation in the TOF is correlated with changes in the distance between plates and thus, the misalignment is automatically determined.

Colloidal stability and magnetic field-induced ordering of magnetorheological fluids studied with a quartz crystal microbalance

This work proposes the use of quartz crystal microbalances (QCMs) as a method to analyze and characterize magnetorheological (MR) fluids. QCM devices are sensitive to changes in mass, surface interactions, and viscoelastic properties of the medium contacting its surface. These features make the QCM suitable to study MR fluids and their response to variable environmental conditions. MR fluids change their structure and viscoelastic properties under the action of an external magnetic field, this change being determined by the particle volume fraction, the magnetic field strength, and the presence of thixotropic agents among other factors. In this work, the measurement of the resonance parameters (resonance frequency and dissipation factor) of a QCM are used to analyze the behavior of MR fluids in static conditions (that is, in the absence of external mechanical stresses). The influence of sedimentation under gravity and the application of magnetic fields on the shifts of resonance frequency and dissipation factor were measured and discussed in the frame of the coupled resonance produced by particles touching the QCM surface. Furthermore, the MR-fluid/QCM system has a great potential for the study of high-frequency contact mechanics because the translational and rotational stiffness of the link between the surface and the particles can be tuned by the magnetic field.

On the yielding behaviour in magnetorheology using ultrasounds, shear and normal stresses, and optical microscopy

The yielding behaviour of magnetorheological fluids has been investigated by videomicroscopy, ultrasonic and rheometry techniques simultaneously. Particularly, the effect of different factors such as, the magnetic field strength, particle size, surface chemistry of the particles, particle concentration and carrier fluid viscosity has been studied. Special attention has been paid to correlate the yielding information obtained by acoustical, optical and mechanical techniques. As a general trend, independently of the particular field strength and suspension formulation, the steady shear flow curve exhibits three well differentiated regions. In the first region, at small stresses, field-induced structures remain quasistatic and all magnitudes remain constant. For larger stresses the number of aggregates decreases but their size increases. This is identified with the onset of flow, and corresponds to the classical static yield stress and a decrease in time-of-flight and normal stresses. For even larger stress values, the suspensions fully flow. This stress value corresponds to the classical dynamic yield stress and is associated to a minimum in the time-of-flight and normal stresses.

Magnetorheological fluid characterization using ultrasound measurements

In this work the variations of velocity of sound and attenuation in magnetorheological (MR) suspensions have been studied when the temperature and the intensity of magnetic field have been varied and, also, when the suspension is observed for a long period of time. It has been shown that the behaviour of the MR fluids depends strongly on the fluid used as solvent when temperature is varied. Regarding the sedimentation process, it has been proved that the application of an external magnetic field enhances the stabilization process. Analyzing the hysteretic behaviour it is seen that the system does not recover its initial state when the magnetic field is removed, because the ordered microstructure does not disappear completely. As ultrasound parameters are sensitive to changes in the temperature, in the structure and also in the volume fraction, they are a promising tool to characterize MR fluids in order to improve its performances.

Effect of particle volume fraction on the velocity of sound in magnetorheological fluids

In this work the velocity of sound in magnetorheological fluids as a function of the particle volume fraction is presented. The influence of the magnetic field on the sound speed and on the material microstructure is also analyzed. It is shown that particles in suspension interact to form complex microstructures which depend on the volume fraction. The range of particle volume fractions studied goes from 1% up to 10%. In the absence of magnetic field, there is a decrease in the velocity of sound as the particle volume fraction is increased, which agrees with the predictions of theoretical models. In an applied magnetic field, the microstructure passes from a suspension to an ordered structure, resulting in an increase velocity of sound. For low volume fractions a model of fiber suspensions predicts the microstructure formed in accordance to the experimental velocity of sound measured. On the other hand, for higher volume fractions, the microstructure can be considered as a porous material and the increase of sound velocity can be qualitatively explained from this theoretical point of view. These results are compared to microstructure images obtained using optical methods.

Ultrasonic velocity in magnetorheological fluids under fields induced by permanent magnets

The sound velocity in a magnetorheological fluid under the influence of magnetic fields is empirically studied in this work. The effect of changing the magnetic field both in geometry, intensity and uniformity using permanent magnets placed in different configurations is analyzed. It is shown that the acoustical properties of the fluid can be easily changed tuning them at our discretion.