Rheology of magnetofluidized fine powders

Abstract

Usually a bed of solid particles °uidized by a gas is inherently unstable. Gas bubbles are rapidly formed at the onset of °uidization, which hinders the e±ciency of gas-solid contact. In the case of magnetizable particles, gas bubbles may be suppressed by means of an externally applied ¯eld that magnetizes the particles. In general, magnetized particles are assumed to behave as point dipoles that organize in chainlike structures oriented along ¯eld 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 not a widely accepted explanation to the empirical fact that magneto°uidized beds can be stabilized by a horizontal ¯eld. Several experimental approaches have been used mainly aimed to observe the °uidity of MSBs. Generally, MSBs are reported to behave as a °uid up to a critical magnetic ¯eld strength at which the bed is frozen and there appears an appreciable yield stress. Most of these techniques are invasive, which sheds doubts on the mechanism responsible for the apparition of the yield stress. In this work, we have measured the yield stress of MSBs of ¯ne magnetic powders by means of a noninvasive technique that uses gas °ow to put the bed under tension. It is shown that the MSB behave as a plastic solid. The yield stress of the stabilized bed, which is developed just at marginal stability, arises as a consequence of the existence of the magnetic attraction between particles at contact. Fine magnetic powders of di®erent aggregative nature have been used in our work. Direct visual observation of mesoscopic structures have revealed that naturally nonaggregated particles organize in quasivertical local linear chains when the ¯eld