Quicksand, Landslides, and flows in Hourglass: an integrated view
Could we produce and explain the three vastly different scenarios of granular flow above, in one single experiment? Recently, by carefully observing the shear flow of soft frictional particles immersed in fluid, JC Tsai and co-workers at Institute of Physics, Academia Sinica in Taipei identify a new dimensionless parameter that explicitly takes into account the speed dependence of inter-particle friction, and demonstrate a “dangerous zone” of shear rates with prominent force fluctuations mimicking landslides or earthquakes in nature. This zone also serves as a signature that sits between fully-lubricated flow (as a "fluid", like quicksand) and quasi-static "solid" (in an hourglass).
In the past two decades, paradigms have been established in a few idealized contexts, where changes with driving rates are successfully described by dimensionless parameters such as Inertial Number [1] or Viscous Number [2]. However, in our daily lives, behaviors of frictional particles remain largely unexplained. The recent findings by this research team in Taiwan piece together states of "solid", "liquid", and "suspension" of granular flows, "provides a new anchoring point in the climb towards a complete understanding of disordered media flows and should certainly inspire new modeling approaches"--- as remarked by one of the reviewers.
WebSite: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.128001