On Time Scales and Rheology of Dry and Wet Granular Materials

This paper results from an ongoing investigation of the rheology of dry and wet granular materials. We describe the different time scales involved and the related dimensionless numbers that govern dry as well as wet granular flows, with a focus to the dimensionless numbers relevant for flow in the quasistatic wet system at low confining stress. The macroscopic friction coefficient and the apparent viscosity of granular materials are the two vital parameters that can also be deduced from experiments. While the time scales are coupled to each other in the intermediate bulk of the materials and interplay among each other, they get decoupled at the extremes of the free surface and the depth of the material due to one time scale being way smaller in magnitude than the other. In the decoupled state, the smaller time scale influences the rheology in different ways, the role of the larger time scale being negligibly small. Furthermore, the capillary force related time scale results in an increase in viscosity of the materials with cohesion, especially close to the free surface.