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 sc ale with the air-liquid surface tension at low phi whereas it does at highe r phi This suggests that the liquid forms menisci at asperities on the surf aces of the grains before filling cohesive menisci at intergrain contact po ints. In this cohesive limit, theta(c)(phi) agrees with a surface roughness theory. Electron and fluorescence microscopy of the dry and wet surfaces o f the grains support this model. [S1063-651X(99)50411-X].