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Effect of vibration on flow properties of fine glass beads



An estimation of cohesion C and angle of internal friction φ of fine glass beads, which have been previously subjected to small intensity vertical vibrations of controlled frequency and amplitude for a fixed period of time, is presented. In the range of vibration intensities tested previbration causes powder compaction. Experimental measurements to obtain C and φ have been performed by means of a centrifuge powder tester in which the previbrated powder bed is rotated around its vertical axis. At a critical rotation velocity the shear stress induced by rotation is large enough to drive material avalanches. From a theoretical analysis of these avalanches, based on the Coulomb's method of wedges, C and φ are derived. We have found a significant increase of cohesion, and a slight decrease of the angle of internal friction as the effective consolidation stress due to previbration is increased. The estimated interparticle cohesion force ft from the experimental measurements, and using a Rumpf's averaging equation, is in agreement with the calculated interparticle attractive force due to van der Waals and capillary interaction for the unconsolidated beds. Moreover ft increases as the effective consolidation stress is increased in agreement with estimations from theoretical models based on contact plasticity. This result suggests that, at the microscopic level of asperities, compaction due to previbration is able to induce plastic deformation of the contacts. © 2008 American Institute of Chemical Engineers AIChE J, 2008