When sandcastles collapse: the effect of interparticle cohesion on collapsing granular columns*

Cohesive effects can drastically change the behavior of granular flows. For instance, powders are challenging to handle, and one can make a sandcastle with wet grains. We report experimental results for collapsing columns of cohesive grains, falling under their own weight in air. Two different geometries are considered: (i) a confined bi-dimensional rectangular column that flows and (ii) a 3D cylindrical column that spreads axisymmetrically on a horizontal surface. Two different physical sources of interparticle cohesion are considered, either through the addition of a small amount of water in initially dry grains such that they are in the pendular state or by the application of an artificial polymer coating. These are both rationalized together showing the general effects of cohesion on the morphology of the final deposit across these geometries. To this end, a characterization of the cohesive strength of the granular materials is obtained independently. Altogether, a dimensionless number relating the cohesive strength and the grain size is shown to capture the effects of cohesion. Despite different sources of cohesion at the grain scale, this allows for a unified macroscopic approach for its effects on collapses. With the explicit contribution of cohesion accounted, the cohesive granular collapse can be rationalized through the established final morphology for relaxed cohesionless piles.