Drag in wet granular materials

We have studied experimentally the effect of the water content on the drag force applied by a wet granular system on a probe moving through it at constant velocity. Similar to what has been observed for dry granular systems, the drag force is found to be independent of the velocity of the probe. By comparing our result with the dry case, we have shown that three main physical mechanisms occur: cohesion (consequence of capillary forces), contact lubrication and dilatancy induced by cohesion. The influence of the latter mechanism prevails in our system and the relative influence of the two former ones is found to depend on grains' characteristics. At important water contents, the granular system may become heterogeneous and its structure is modified by the motion of the probe. Our analysis is strengthened by shear cell experiments which also point out the importance of the geometry of the system on the effect of the water content.