Simple Interaction Model for Partially Wet Granular Materials

It is well known that just adding some liquid to dry granular materials changes their behaviors very much. Liquid forms bridges between grains, and the bridge induces cohesion between grains due to the surface tension. When the liquid content is small, the liquid forms a bridge at each contact point (the pendular state), which induces two-body cohesive force. As the liquid content increases, some liquid bridges merge, and more than two grains interact through a single liquid cluster (the funicular state). We propose a simple phenomenological model for wet granular media to take into account many particle interaction through a liquid cluster in the funicular state as well as two-body cohesive force by a liquid bridge in the pendular state. In our model, the cohesive force acts among the grains connected by a liquid-gas interface. As the liquid content is increased, the number of grains that interact through the liquid increase, but the liquid-gas interface may decrease when liquid clusters are formed. Due to this competition, the shear stress shows a maximum as a function of the liquid-content.