  We experimentally investigate the clogging process of granular materials in a
two-dimensional hopper, and present a self-consistent physical mechanism of
clogging based on preformed dynamic chain structures in the flow. We found that
these chain structures follow a specific modified restricted random walk, and
clogging occurs when they are mechanically stable enough to withstand the flow
fluctuations, resulting in the formation of an arch at the outlet. We introduce
a simple model which can explain the clogging probability by incorporating an
analytical expression for chain formation and its transition into an arch. Our
results provide insight into the microscopic mechanism of clogging in hopper
flow.
