Understanding empirical powder flowability criteria scaled by Hausner ratio or Carr index with the analogous viscosity concept

The viscosity concept is introduced to granular powders after the analogous temperature is defined consistently with thermodynamics, and the corresponding viscosity equation is obtained with the aid of Eyring's rate process theory and free volume concept. The popular empirical powder flowability criteria scaled with the Hausner ratio or Carr index are theoretically found only to work at special conditions and more universal criteria correlated with shearing conditions and particle physical properties are presented. Our results resolve a long time controversial mystery related to those two empirical indexes on powder flowability, and has a broad impact in many industrial areas. The work presented in this article may lay a foundation to scale powder flowability in a more fundamental and consistent manner.