The transient dynamics of dilation waves in granular phase transitions during silo discharge

Abstract

Granular material has the unique ability to transition between solid and liquid-like phases, but quantitative observations of the dynamics involved in this process remain rare. We hypothesize that granular packing of the solid phase has a leading control on this transition. To test this, we visualize the flow transitions that occur during discharge from a grain-filled silo. X-ray fluoroscopy and high-speed video analysis are used to detect and characterize the kinematics of dilation waves that trigger the phase transitions. Wave velocities are shown to vary by an order of magnitude with strong dependence on the packing density of the initially static bed. The speed of dilation waves exceeds any granular flow velocity in the system, and a simple model based upon conservation of mass is presented to describe this phenomenon. Our results have major implications for the quantitative description and prediction of granular system behaviour in natural and industrial applications, particularly with regards to the onset of avalanche motion and the handling of powders and grains.