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 on 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 two vital parameters that can also be deduced from experiments. While various time scales, as related to the ongoing mechanisms in the sheared bulk of the material, can interfere, they also can get decoupled, in the extremes of the free surface or in the depth of the material, if one time scale gets way smaller in magnitude than the other. In the decoupled state, the smaller time scale influences the rheology in a dominant way, while the role of the larger time scale is negligibly small. For example, the capillary force results in an increase of the Bond number (due to liquid-bridge cohesion) and thus the shear-viscosity of the material, especially close to the free surface. 1. INTRODUCTION