Avalanches in moistened beds of glass beads

In this work we have investigated the onset of avalanches in moistened glass beads within a range of sizes from 0.4 mm to 4 mm. The bed of glass beads is first subjected to a controlled flow of humidified air by using a fluidized bed setup. Water moisture is added to the fluidizing air by bubbling it through water. The relative humidity and dew point of the air are monitored on-line by using humidity sensors upstream and downstream of the bed. After the stationary condition is reached, shear stress is generated by slowly tilting the bed until the material avalanches. Generally, it is seen that the avalanche angle of the moistened beads decreases as bead size is increased and becomes much larger than the angle of internal friction, which is about the avalanche angle of the dry beads. The tensile strength of the moistened beads are calculated from the Rumpf equation and using the interparticle capillary force theoretically expected in the pendular state. From these calculations, it is shown that the avalanche angle predicted by a simple Coulombʹs wedge model fits reasonably well to the experimental data.