F. Donado

Synthesis and characterization of micrometric ceramic powders for electro-rheological fluids

Abstract Micrometric lamellar ceramic powders of the displacive ferroelectric oxide Bi 4 Ti 3 O 12 were synthesized by co-precipitation of bismuth nitrate and ammonia titanyl solutions followed by a heat treatment. It was found that a complete thermal decomposition is reached at 1000 °C. Structural and thermal evolution of these ceramic powders were studied by X-ray diffraction, thermogravimetry and differential thermal analysis. The homogeneity in size and morphology of these ferroelectric particles are appropriate to prepare electro-rheological fluids. One of these fluids was prepared by dispersing the powders in silicone oil; the complex cluster structure formed by the particles, under an applied AC electric field, was observed.

Sound propagation in magneto-rheological suspensions

The propagation of elastic perturbations in magneto-rheological suspensions is studied theoretically and experimentally. Under the application of a magnetic field, these systems acquire a fibrillar fractal structure formed by clusters. In systems in that condition, two low-frequency sound propagation modes have been observed. In both of them, the speed of sound depends on the intensity of the applied field. We discuss the statistical fractal properties of the cluster structure and, on this basis, we calculate the speed of sound for both of the low-frequency modes. This theoretical approach provides a good quantitative agreement with the experimental results.

Fractal patterns and aggregation processes in rheological dispersions

Processes of pattern formation in rheological dispersions in the presence of an external field are studied. We found that the structure formed acquires multifractal characteristics revealed by three mass scaling ranges, which can be associated with different aggregation stages in the pattern formation process. The evaluation of the radial mass distribution allows us to test the self-affinity properties. We found that this multifractal feature is a common characteristic for rheological dispersions.

Thermal Properties of an Electro-Rheological Fluid Based on Bi 4 Ti 3 O 12 Ceramic Powders

Thermal diffusivity of a ferroelectric based electro-rheological fluid is studied. The rheological fluid is prepared by dispersing powders of Bi 4 Ti 3 O 12 in silicon oil. The thermal diffusivity of the sample, under different intensities of applied electric field, was measured by means of an open cell photo-acoustic technique. We have found that the main contribution to the measured photo-acoustic signal, comes from the thermo-elastic bending effect. It was clearly observed that thermal diffusivity increases with the intensity of the applied electric field, this fact is explained in terms of the formation of clusters of Bi 4 Ti 3 O 12 particles.

Dynamic elastic properties of magneto-rheological slurries

We study the propagation of elastic perturbations in magneto-rheological slurries of iron particles dispersed in glycerine. The complexity of these systems is revealed in the fibrillar structure acquired under the application of a magnetic field. Recently, it has been reported the observation of two different low frequency modes of propagation. One of these modes has been associated to the propagation of the perturbation through the fluid medium. The other one has been qualitatively explained as the propagation of the elastic perturbation through the suspended particles. This second mode appears when a magnetic field is applied to the slurry. The propagation speed for both modes depends on the field intensity and on the properties of the magnetic particles. Theoretically, we analyze these modes and calculate the sound velocity. We obtain a quantitative good agreement with the experimental results.

Nonvibrating granular model for a glass-forming liquid: Equilibration and aging

We studied experimentally a model of a glass-forming liquid on the basis of a nonvibrating magnetic granular system under an unsteady magnetic field. A sudden quenching was produced that drove the system from a liquid state to a different final state with lower temperature; the latter could be a liquid state or a solid state. We determined the mean-squared displacement in temporal windows to obtain the dynamic evolution of the system, and we determined the radial distribution function to obtain its structural characteristics. The results were analyzed using the intermediate scattering function and the effective potential between two particles. We observed that when quenching drives the system to a final state in the liquid phase far from the glass-transition temperature, equilibration occurs very quickly. When the final state has a temperature far below the glass-transition temperature, the system reaches its equilibrium state very quickly. In contrast, when the final state has an intermediate temperature but is below that corresponding to the glass transition, the system falls into a state that evolves slowly, presenting aging. The system evolves by an aging process toward more ordered states. However, after a waiting time, the dynamic behavior changes. It was observed that some particles get close enough to overpass the repulsive interactions and form small stable aggregates. In the effective potential curves, it was observed that the emergence of a second effective well due to the attraction quickly evolves and results in a deeper well than the initial effective well due to the repulsion. With the increase in time, more particles fall in the attractive well forming inhomogeneities, which produce a frustration in the aging process.

Reconstruction processes in electro and magneto rheological dispersions

The evolution of the structure of electro rheological as well as magneto rheological fluids in the presence of perturbation fields is studied. We have previously shown that the fibrous structure acquired by these dispersions in the presence of a static, electric or magnetic, field has multifractal characteristics. If in addition to the static field a perpendicular pulsed field is applied, under certain conditions it is possible to rearrange the structure into a crystalline-like one. Based on the measured mass fractal dimension and the radial correlation, we discuss these processes and other structural characteristics of the system.

Brownian motion in non-equilibrium systems and the Ornstein-Uhlenbeck stochastic process

The Ornstein-Uhlenbeck stochastic process is an exact mathematical model providing accurate representations of many real dynamic processes in systems in a stationary state. When applied to the description of random motion of particles such as that of Brownian particles, it provides exact predictions coinciding with those of the Langevin equation but not restricted to systems in thermal equilibrium but only conditioned to be stationary. Here, we investigate experimentally single particle motion in a two-dimensional granular system in a stationary state, consisting of 1 mm stainless balls on a plane circular surface. The motion of the particles is produced by an alternating magnetic field applied perpendicular to the surface of the container. The mean square displacement of the particles is measured for a range of low concentrations and it is found that following an appropriate scaling of length and time, the short-time experimental curves conform a master curve covering the range of particle motion from ballistic to diffusive in accordance with the description of the Ornstein-Uhlenbeck model.

Dynamical and structural properties of a granular model for a magnetorheological fluid

We study a two-dimensional nonvibrating granular system as a model of a magnetorheological fluid. The system is composed of magnetic steel particles on a horizontal plane under a vertical sinusoidal magnetic field and a horizontal static magnetic field. When the amplitude of the horizontal field is zero, we find that the motion of the particles has characteristics similar to those of Brownian particles. A slowing down of the dynamics is observed as the particle concentration increases or the magnitude of the vertical magnetic field decreases. When the amplitude of the horizontal field is nonzero, the particles interact through effective dipolar interactions. Above a threshold in the amplitude of the horizontal field, particles form chains that become longer and more stable as time increases. For some conditions, at short time intervals, the average chain length as a function of time exhibits scaling behavior. The chain length distribution at a given time is a decreasing exponential function. The behavior of this granular system is consistent with theoretical and experimental results for magnetorheological fluids.