Critical angle of wet sandpiles.

We measure the increase in the maximum stable angle of a sandpile, theta(c), with the volume fraction, phi of a liquid added to cause cohesion between the grains. For two different liquids, tan theta(c) does not apparently scale with the air-liquid surface tension at low phi, whereas it does at higher phi. This suggests that the liquid forms menisci at asperities on the surfaces of the grains before filling cohesive menisci at intergrain contact points. In this cohesive limit, theta(c)(phi) agrees with a surface roughness theory. Electron and fluorescence microscopy of the dry and wet surfaces of the grains support this model.