M. J. Espin

Rheology of magnetofluidized fine powders: The role of interparticle contact forces

Usually, a bed of solid particles fluidized by a gas is inherently unstable. Gas bubbles are rapidly formed at the onset of fluidization, which hinders the efficiency of gas-solid contact. In the case of magnetizable particles, gas bubbles may be suppressed by means of an externally applied field that magnetizes the particles. In general, magnetized particles are assumed to behave as point dipoles that organize in chainlike structures oriented along field lines due to dipole-dipole attraction. The physical mechanism responsible for stabilization is, however, unclear. In particular, rheological characterization of magnetically stabilized beds (MSBs) has been a subject of controversy and there is no widely accepted explanation to the empirical fact that magnetofluidized beds (MFBs) can be stabilized by a horizontal field. Several experimental approaches have been used mainly aimed to observe the fluidity of MFBs. Generally, MFBs are reported to behave as a fluid up to a critical magnetic field strength at w...

Fluid to solid transition in magnetofluidized beds of fine powders

Experimental observations on the fluid to solid transition in beds of magnetized fine particles fluidized by gas are reported for different particle sizes (dp). Contrarily to stability analysis prediction, the fluidized bed is stabilized by a sufficiently strong magnetic field in the cross-flow configuration. As the strength H of the horizontally applied magnetic field is increased, particle chaining in the bubbling bed becomes apparent due to the induced attractive magnetostatic forces between the particles. In close analogy with magnetorheological fluids chain stability is determined by the balance between gas flow shear and the interparticle magnetostatic force. The jamming transition occurs at a gas velocity scaling proportionally to dp2H2 when the length of the stable chains reaches a critical size which is independent of particle size.Experimental observations on the fluid to solid transition in beds of magnetized fine particles fluidized by gas are reported for different particle sizes (dp). Contrarily to stability analysis prediction, the fluidized bed is stabilized by a sufficiently strong magnetic field in the cross-flow configuration. As the strength H of the horizontally applied magnetic field is increased, particle chaining in the bubbling bed becomes apparent due to the induced attractive magnetostatic forces between the particles. In close analogy with magnetorheological fluids chain stability is determined by the balance between gas flow shear and the interparticle magnetostatic force. The jamming transition occurs at a gas velocity scaling proportionally to dp2H2 when the length of the stable chains reaches a critical size which is independent of particle size.

Magnetic field induced inversion in the effect of particle size on powder cohesiveness

Experimental measurements are reported on the tensile yield stress of magnetofluidized beds of fine magnetic powders operated in the cross-flow configuration. In the absence of externally applied magnetic field the yield stress of the powder depends on particle size as expected, i.e., it increases as bead size is decreased. This trend is however inverted when an external magnetic field is applied. It is suggested that the average orientation of interparticle contacts relative to the direction of the field as affected by particle size plays a relevant role on the magnetic yield stress of these systems.

Tunable pattern structures in dielectric liquids under high dc electric fields

This work focuses on the abrupt changes that the application of large enough electric fields provokes in the internal structure of hematite/silicone oil suspensions. Experimental results reflect the existence of two well-defined structural patterns according to the strength of the field and the concentration of particles. At low electric fields, columns of particles between the electrodes can be observed when the concentration of solids exceeds a critical volume fraction. However, at higher fields, electrohydrodynamic convection and eventually electrophoretic migration take place, reflecting the relevance of the particle charge. A complete theoretical discussion is given to explain the origin of these so different behaviors. While the mismatch in the electrical properties (particularly, conductivity) of the solid and liquid phases, that is the Maxwell-Wagner polarization, can justify the chain-like structures of particles, it is necessary to take into an account the process of charge injection at the electrode/suspension interface to support the electrophoretic migration and deposition. The experimental conditions for which polarization or current effects predominate are elucidated in terms of the conductivity of the solid phase and the magnitude of the applied electric field

Mesoscopic structuring and yield stress of magnetofluidized fine particles

The fluidization behavior of a bed of fine magnetizable particles excited by an externally applied magnetic field is found to depend on the aggregative nature of the particles before the field was applied. Usually nonaggregated particles organize in quasivertical local linear chains when the field is applied. In contrast, naturally aggregated particles form large-scale branched structures when magnetized by an external field. As a consequence the yield stress of magnetically stabilized beds of naturally aggregated particles is relatively increased and the bed can be stabilized at smaller field intensities. As expected from the magnetic cohesive force between magnetized particles, the yield stress is proportional to the square of the magnetic-field intensity, with a proportionality constant that depends on the mesoscopic organization on the magnetic particles. Remarkably, it is found that quasivertical chainlike structures are stable in spite of the fact that the magnetic field is applied in the horizontal direction.

Alternating Field Electronanofluidization

The use of fluidized beds to remove submicron particles from gases has been investigated since 1949. High efficiency removal was achieved in the 1970’s by imposing an electric field on a fluidized bed of semi‐insulating granules that were able to collect the charged pollutant entrained in the fluidizing gas. In spite of their extended use nowadays, the collection efficiency of electrofluidized beds (EFB) is still hindered by gas bypassing associated to gas bubbling and the consequent requirement of too high gas flow and pressure drop. In this paper we report on the electromechanical behavior of an EFB of insulating nanoparticles. When fluidized by gas, these nanoparticles form extremely porous light agglomerates of size of the order of hundreds of microns that allow for a highly expanded nonbubbling fluidized state at reduced gas flow. It is found that fluidization uniformity and bed expansion are additionally enhanced by an imposed AC electric field for field oscillation frequencies of several tens of he...

Magneto-Stabilization of Fluidized Beds as due to Short Ranged Interparticle Forces

We present an experimental study on the stabilization of bubbling gas-fluidized beds of magnetic powders by interparticle forces induced by an externally applied magnetic field in the cross-flow configuration. The samples tested consist of magnetite and steel powders in a range of particle size dp between 35 and 110 microns, allowing us to investigate the effect of particle size and material properties on magnetic stabilization. According to our observations, the stabilization physical mechanism is ruled by the jamming of particle chains created due to attractive forces induced between the magnetized particles. Even in the case of the horizontally applied field, these chains are mechanically stable at orientations close to the gas flow direction in agreement with the prediction of a chain model based on the balance between gas flow shear and interparticle magnetic force fm. Since fm is increased as dp is increased, the critical gas velocity at marginal stability vc for a fixed field strength B is seen to increase with dp. The yield stress of the stabilized bed s increases steadily as the gas velocity v0 is decreased below vc. Thus, s is increased with dp for fixed v0 and B. It is inferred also from our results that natural aggregation of fine particles due to the universal van der Waals interaction enhances the yield stress of the magnetically stabilized bed. A main conclusion is that interparticle short ranged attractive forces play an essential role on magnetic stabilization of fluidized beds.

Particle structuring and yield stress in magnetofluidized beds

A novel experimental technique to measure the tensile yield stress of fluidized beds of magnetic powders stabilized by an externally applied cross‐flow magnetic field is shown. Basically, the tensile yield stress of the magnetically stabilized bed (MSB) is measured by means of the pressure drop of a gas flow that puts the bed under tension. A first relevant result is that the yield stress depends strongly on the field operation mode. In the H off/on operation mode, the bed was driven to bubbling by imposing a high gas velocity in the absence of magnetic field. Once the gas velocity was decreased below the bubbling onset and the bed was stabilized by the natural cohesive forces alone, the field was applied. The yield stress of the naturally stabilized bed is not essentially changed by application of the field a posteriori (H off/on), which can be attributed to the inability of the field to alter the arrangement of the particles once they were jammed in the stable fluidization state. In the H on/on mode, th...

CORRELATION BETWEEN MICROSTRUCTURE AND YIELD STRESS IN MAGNETICALLY STABILIZED FLUIDIZED BEDS

A magnetofluidized bed consists of a bed of magnetizable particles subjected to a gas flow in the presence of an externally applied magnetic field. In the absence of magnetic field, there is a given gas velocity at which naturally cohesive fine particles can form a network of permanent interparticle contacts capable of sustaining small stresses. This gas velocity marks the jamming transition of the fluidized bed. For gas velocities above the jamming transition, the bed resembles a liquid. Below the jamming transition, the bed behaves as a weak solid and it has a nonvanishing tensile strength. In the absence of magnetic field, the tensile strength of the solidlike stabilized bed has its only origin in nonmagnetic attractive forces acting between particles. In the presence of a magnetic field, the gas velocity at the jamming transition and the tensile strength of the bed depend on the field strength as a consequence of the magnetostatic attraction induced between the magnetized particles. In this work we present experimental measurements on the jamming transition and tensile strength of magnetofluidized beds of linearly magnetizable fine powders. It is shown that powders with similar magnetic susceptibility but different strength of the nonmagnetic forces show a different response to the magnetic field. This finding can be explained by the influence of the nonmagnetic natural forces on the network of contacts. Thus, our experimental results reported in this paper reinforce the role of short-ranged interparticle contact forces on the behavior of the system, which contrasts with the usual modeling approach in which the magnetofluidized bed is viewed as a continuum medium and a fundamental assumption is that the fields can be averaged over large distances as compared with particle size.

Magnetofluidization of Fine Magnetite Particles

In this study we investigate the behaviour of a fluidized bed of fine magnetite particles as affected by a cross‐flow uniform magnetic field. Due to the small particle size (35 microns), the fluidized system displays a typical Geldart A fluidization behaviour in the absence of an external field, i.e. natural van der Waals forces are able to stabilize fluidization in a short interval of gas velocities above the minimum fluidization velocity. The effect of the external field is to delay bed stability to higher gas velocities. Mechanical measurements on the magnetofluidized bed have been carried out using the Seville Powder Tester, which allows us for an accurate control of gas flow, and measurement of gas pressure drop across the bed and bed height. The tensile strength of the magnetically stabilized bed has been tested as a function of the gas velocity.