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.Ministerio de Ciencia y Tecnología de España-FIS2006-0364

Responsabilidad social y sostenibilidad corporativas: hacia una necesaria y urgente superación de ambos conceptos

Since its implementation, the CSR concept has undergone rapid evolution and, while maintaining its base, it expands as the world demands a greater field of action because the intrinsic philosophy of this concept and its related ones is revealed as the most suitable to fight against the already immediate danger that global society and its environment are running, which, if we do not begin to stop it effectively and quickly, will soon lead to the extinction of life in its various manifestations. For this reason, there has been talk of SSC for some time and, from there, a symbiosis of both terms leads us, simply, to sustainability in all its manifestations, if there is still time. But the damage caused by old energy sources on the one hand and news technologys availables to anyone, a runaway advance in telecommunications and globalization itself in its most negative aspect on the other, just a great change at all levels in principles, values, standards and methods of human beings, agreed and coordinated in a national, international and supranational legal framework will be able to avert them.Desde su implantación, el concepto RSC ha sufrido una rápida evolución y, conservando su base, se amplía a medida que el mundo demanda un mayor campo de actuación porque la filosofía intrínseca de ese concepto y sus afines se revela como la más idónea para luchar contra el peligro ya inmediato que la sociedad global y su medio ambiente están corriendo, que, de no comenzar a detenerlo con eficacia y rapidez, llevará pronto a la extinción de la vida en sus diversas manifestaciones. Por eso se habla hace algún tiempo ya de SSC y, de ahí se marcha a una simbiosis de ambos términos que nos lleve, simplemente, a sostenibilidad en todas sus manifestaciones, si es que aún hay tiempo. Pero los daños que ha causado las viejas fuentes de energía de un lado y la nueva tecnología al alcance de cualquiera, un avance desbocado de las telecomunicaciones y la propia globalización en su aspecto más negativo de otro, sólo un gran cambio a todos los niveles en los principios, los valores, las normas y los métodos de los seres humanos, acordado y coordinado en un marco jurídico nacional, internacional y supranacional podrá conjurarlos

Physics of Compaction of Fine Cohesive Particles

Fluidized fractal clusters of fine particles display critical-like dynamics at the jamming transition, characterized by a power law relating consolidation stress with volume fraction increment. At a critical stress clusters are disrupted and there is a crossover to a logarithmic law resembling the phenomenology of soils. We measure _ _ _@__1=__=@ log_^ c / Bo0:2 g , where Bog is the ratio of interparticle attractive force (in the fluidlike regime) to particle weight. This law suggests that compaction is ruled by the internal packing structure of the jammed clusters at nearly zero consolidation

Self-Diffusion in a Gas-Fluidized Bed of Fine Powder

We have investigated the self-diffusion in a stable gas-fluidized bed of fine powder. Two regimes have been observed: for gas velocities yg above the minimum fluidization velocity ym and below a critical gas velocity yc smaller than the minimum bubbling velocity yb the powder does not mix. Experimental measurements show the existence of yield stresses in this regime which are responsible for the static behavior of the bed. For yg . yc the yield stress vanishes; the bed behaves like a fluid and displays a diffusive dynamics. In this region we have found that the diffusion coefficient D increases with gas velocity until the bed expansion approaches its maximum value

Fine cohesive powders in rotating drums: Transition from rigid-plastic flow to gas-fluidized regime

We investigate the dynamics of fine cohesive powders inside rotating drums. We show that these powders may be fluidized due to entrapment of ambient gas, and we determine the onset of fluidization. Experimental measurements on the bed expansion as a function of the rotation velocity have been performed. Drums of different diameters and fine powders of varying cohesiveness have been tested. We show that (i) fine powders transit directly from a rigid-plastic state to a gas-fluidized state in accordance with the flow regime boundaries predicted elsewhere [A. Castellanos et al., Phys. Rev. Lett. 82, 1156 ~1999], (ii) the onset of fluidization in the rotating drum is determined by the ratio of the powder kinetic energy per unit volume to its tensile strength, and ~iii! once the powder is completely fluidized the average interstitial gas velocity increases proportionally to the rotation velocity. The last two results imply that the required velocity to fluidize a powder, vR (v angular velocity, R radius of the drum), must increase as the square root of its tensile strength, and this has been confirmed by independent measurements and estimations

Jamming Threshold of Dry Fine Powders

We report a novel experimental study on the jamming transition of dry fine powders with controlled attractive energy and particle size. Like in attractive colloids dry fine particles experience diffusionlimited clustering in the fluidlike regime. At the jamming threshold fractal clusters crowd in a metastable state at volume fractions depending on attractive energy and close to the volume fraction of hard nonattractive spheres at jamming. Near the phase transition the stress-(volume fraction) relationship can be fitted to a critical-like functional form for a small range of applied stresses J) as measured on foams, emulsions, and colloidal systems and predicted by numerical simulations on hard spheres

Correlation between bulk stresses and interparticle contact forces in fine powders

We present measurements of the tensile strength as a function of the consolidation stress for a set of fine cohesive powders ~xerographic toners! of 12.7 mm particle size and with a range of concentration of submicron fumed silica as flow control additive. This additive is well known for its ability to control interparticle adhesion force. Parallel measurements using an atomic force microscope have been carried out on the adhesion force between two individual grains as a function of a controlled previous load force. The effect of the additive on the tensile strength and adhesion force is analyzed. We have found a good correlation between bulk stresses and adhesion forces between individual particles. This correlation is compatible with the existence of a subnetwork of force chains

Granular avalanches: Deterministic, correlated and decorrelated dynamics

A statistical analysis of granular avalanches in a half-filled and slowly rotated drum is presented. For large-sized grains the classical coherent oscillation is reproduced, i.e. we observe a quasi-periodic succession of regularly sized avalanches. As particle size is decreased, we see a crossover to a new complex dynamics characterized by long-range time correlations of local avalanches without a typical size, although the size distribution is not scale invariant. In the limit of large values of the time lag avalanches turn gradually to be decorrelated. The trend observed in the system dynamics as particle size is decreased is ascribed to the increase of cohesiveness which promotes bulk disorder. We argue that our experimental findings can be qualitatively predicted by theoretical models with adjustable parameters such as unquenched disorder, random critical slope, fluidization length, inertia and dissipation.Ministerio de Ciencia y Tecnología BFM2003-0173

Aggregation and sedimentation in gas-fluidized beds of cohesive powders

We present measurements on the settling velocity of gas-fluidized beds of fine cohesive powders. In the solidlike regime ~solid volume fraction f.fc) particles are static, sustained by enduring contacts. The settling is hindered by interparticle contacts and is a very slow process. In the fluidlike regime (f,fc) permanent contacts no longer exist, and the bed displays a diffusive dynamics. The interparticle adhesive force leads to the formation of particle aggregates, and for this reason the sedimentation velocity exceeds the predicted value by empirical or theoretical laws on the settling of individual particles. We use an extension of the Richardson-Zaki empirical law for the settling of aggregates in the fluidlike regime to fit the experimental data. Aggregates are characterized by the number of aggregated particles N and by an effective radius R. The trend followed by these parameters with particle size is confirmed by direct visualization of the aggregates, and shows that cohesive effects become important when the adhesion force between particles is above particle weight. Results show that aggregates form open structures with a fractal dimension close to the predicted one in the diffusionlimited- aggregation model (D52.5)

Effect of vibration on the stability of a gas-fluidized bed of fine powder

We have investigated the effect of vibrations on the stability of gas-fluidized beds of fine powders (particle size ~10 mm). The powder is uniformly fluidized by an adjustable gas flow that enables us to control the average solid volume fraction f0. The fluidized bed is then subjected to a vertical oscillatory motion of controlled amplitude and frequency. The response of the fluidized bed depends essentially on the value of f0. For f0.0.28 the fluidized bed is in a weak solidlike regime, it has a mechanical strength, and particles are static. In this regime vibration causes compaction of the loosely packed bed. For f0,0.28 the mechanical strength vanishes and stresses are carried by interstitial gas and collisions. In this fluidlike regime the fluidized bed displays a diffusive dynamics and particles aggregate due to the strong interparticle van der Waals forces. When vibration is applied the powder expands due to the partial disruption of aggregates. However at a critical value of the vibration amplitude A5Ac either surface ~sloshing! or flow ~bubbling! instabilities develop. The nucleation of gas bubbles has been correlated to the saturation in particle diffusivity measured elsewhere. The size of the bubbles increases as A is further increased above Ac and as the vibration frequency is reduced. Moreover, as it should be expected from the predictions of hydrodynamic models, Ac is independent of cohesivity for particles of the same size and density