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

Abstract

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 which there appears an appreciable yield stress. Most of these techniques are however invasive, which sheds doubts on the mechanism responsible for the appearance of the yield stress. In this work, we have measured the yield stress of MFBs of fine magnetic powders by means of a noninvasive technique that uses gas flow to put the bed under tension. It is shown that the MFB behaves as a plastic solid. The yield stress of the MFB, which is developed just at marginal stability, arises as a consequence of the magnetic attraction between particles at contact. Fine magnetic powders of different aggregative nature in the absence of applied magnetic field have been used in our work. It has been seen that the yield stress of MSBs of naturally aggregated particles is significantly larger than the yield stress of MSBs of naturally nonaggregated particles. Moreover, the MFB is stabilized at smaller field intensities in the former case. Noninvasive visualization of the MFB surface shows that quasivertical chainlike structures are stable despite that the magnetic field is applied in the horizontal direction, which sheds doubts on the validity of the widely used dipolar assumption.