Depinning and creeplike motion of wetting fronts in weakly vibrated granular media.

We study the effect of weak vibrations on the imbibition of water in granular media. In our experiments, we have observed that as soon as the vibration is applied, an initially pinned wetting front advances in the direction of imbibition. We found that the front motion is governed by the avalanches of localized intermittent advances directed at 45° to the imbibition direction. When the rescaled gravitational acceleration of vertical vibrations is in the range of 0.81≤G≤0.95, we observed an almost steady motion of wetting front with a constant velocity v(cr)(G)∝exp(-1/G) during more than 20 min, whereas at lower accelerations (0.5≤G≤0.8) the front velocity decreases in time as v∝t(-δ). We suggest that the steady motion of an imbibition front in a weakly vibrated granular medium can be treated as a creep motion associated with nonthermal temporal fluctuations of packing density in a weakly vibrated granular medium.