Clogging-flowing transition of granular media in a two-dimensional vertical pipe

We experimentally and numerically investigate the clogging behavior of granular materials in a two-dimensional vertical pipe. The nonmonotonicity of the clogging probability found in a cylindrical vertical pipe [López et al., Phys. Rev. E 102, 010902(R) (2020)2470-004510.1103/PhysRevE.102.010902] is also observed in the two-dimensional case. We numerically demonstrate that the clogging probability strongly correlates with the friction coefficient μ in addition to the pipe-to-particle diameter ratio D/d. We thus construct a clogging-flowing D/d-μ phase diagram within the 2<D/d<3 range. Finally, by analyzing the geometrical arrangements of particles and using a simple analysis of forces and torques, we are able to predict the clogging-flowing transition in the D/d-μ phase diagram and explain the mechanism of the observed counterintuitive nonmonotonicity in more detail. We demonstrate that for pipe clogging, friction-mediated force and torque equilibrium are essential for arch formation.