From sugar poured into a bowl to tons of grains discharge processes in industry, a peculiar feature of granular materials emerges : the formation of a pile. When the grains are not cohesive, only the friction and the grain geometry determine the shape of the assembly which resembles a conical pyramid. As known by any sandcastle architect, the addition of some liquid induces cohesion between the grains due to the surface tension and capillary effects. During the last years, different groups studied the mechanical properties of wet granular piles, finding a remarkably insensitivity on the liquid content. This unexpected result is explained by a particular organization of the liquid between the grains. Although a significant progress has been achieved, all these experiments only consider wet granular piles with a low percentage of liquid. In our last paper, we report what occurs when dry sand is poured on a highly humid granular bed. Above a threshold humidity, an interesting phenomenon appears: instead of a wet pile, stable sand towers emerge. The impacting grains have a non-zero probability to stick on the wet grains, due to instantaneous liquid bridges created during their impact. The growth velocity of this self-assembled structures is determined by the flux of grains and the liquid content of the bed. We found that the higher the humidity the greater the probability of the grains to stick, but the smaller the final height of the tower, which falls when the cohesive stress at its base is surpassed. Beyond an artistic technique to sculpt sandcastles, this experimental method represents a new alternative to study the mechanical properties of wet granular materials.
F.Pacheco-Vazquez, F.Moreau, N.Vandewalle and S.Dorbolo, Sculpting sandcastles grain by grain: Self-assembled sand towers, Phys. Rev. E 86, 051303 (2012) – PDF